diff --git a/.github/workflows/coverage.yml b/.github/workflows/coverage.yml
index 75bdf83..6dd973b 100644
--- a/.github/workflows/coverage.yml
+++ b/.github/workflows/coverage.yml
@@ -48,6 +48,7 @@
         ./python -m venv .venv
         source ./.venv/bin/activate
         python -m pip install -U coverage
+        python -m pip install -r Misc/requirements-test.txt
         python -m test.pythoninfo
     - name: 'Tests with coverage'
       run: >
diff --git a/.travis.yml b/.travis.yml
index 3c2fb4b..133385f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -87,6 +87,7 @@
         # Need a venv that can parse covered code.
         - ./python -m venv venv
         - ./venv/bin/python -m pip install -U coverage
+        - ./venv/bin/python -m pip install -r Misc/requirements-test.txt
         - ./venv/bin/python -m test.pythoninfo
       script:
         # Skip tests that re-run the entire test suite.
diff --git a/Lib/sysconfig.py b/Lib/sysconfig.py
index eaee837..bf04ac5 100644
--- a/Lib/sysconfig.py
+++ b/Lib/sysconfig.py
@@ -546,6 +546,7 @@
 
         if os.name == 'nt':
             _init_non_posix(_CONFIG_VARS)
+            _CONFIG_VARS['TZPATH'] = ''
         if os.name == 'posix':
             _init_posix(_CONFIG_VARS)
         # For backward compatibility, see issue19555
diff --git a/Lib/test/test_zoneinfo/__init__.py b/Lib/test/test_zoneinfo/__init__.py
new file mode 100644
index 0000000..98cc441
--- /dev/null
+++ b/Lib/test/test_zoneinfo/__init__.py
@@ -0,0 +1 @@
+from .test_zoneinfo import *
diff --git a/Lib/test/test_zoneinfo/__main__.py b/Lib/test/test_zoneinfo/__main__.py
new file mode 100644
index 0000000..5cc4e05
--- /dev/null
+++ b/Lib/test/test_zoneinfo/__main__.py
@@ -0,0 +1,3 @@
+import unittest
+
+unittest.main('test.test_zoneinfo')
diff --git a/Lib/test/test_zoneinfo/_support.py b/Lib/test/test_zoneinfo/_support.py
new file mode 100644
index 0000000..6bd8d8d
--- /dev/null
+++ b/Lib/test/test_zoneinfo/_support.py
@@ -0,0 +1,76 @@
+import contextlib
+import functools
+import sys
+import threading
+import unittest
+from test.support import import_fresh_module
+
+OS_ENV_LOCK = threading.Lock()
+TZPATH_LOCK = threading.Lock()
+TZPATH_TEST_LOCK = threading.Lock()
+
+
+def call_once(f):
+    """Decorator that ensures a function is only ever called once."""
+    lock = threading.Lock()
+    cached = functools.lru_cache(None)(f)
+
+    @functools.wraps(f)
+    def inner():
+        with lock:
+            return cached()
+
+    return inner
+
+
+@call_once
+def get_modules():
+    """Retrieve two copies of zoneinfo: pure Python and C accelerated.
+
+    Because this function manipulates the import system in a way that might
+    be fragile or do unexpected things if it is run many times, it uses a
+    `call_once` decorator to ensure that this is only ever called exactly
+    one time — in other words, when using this function you will only ever
+    get one copy of each module rather than a fresh import each time.
+    """
+    import zoneinfo as c_module
+
+    py_module = import_fresh_module("zoneinfo", blocked=["_zoneinfo"])
+
+    return py_module, c_module
+
+
+@contextlib.contextmanager
+def set_zoneinfo_module(module):
+    """Make sure sys.modules["zoneinfo"] refers to `module`.
+
+    This is necessary because `pickle` will refuse to serialize
+    an type calling itself `zoneinfo.ZoneInfo` unless `zoneinfo.ZoneInfo`
+    refers to the same object.
+    """
+
+    NOT_PRESENT = object()
+    old_zoneinfo = sys.modules.get("zoneinfo", NOT_PRESENT)
+    sys.modules["zoneinfo"] = module
+    yield
+    if old_zoneinfo is not NOT_PRESENT:
+        sys.modules["zoneinfo"] = old_zoneinfo
+    else:  # pragma: nocover
+        sys.modules.pop("zoneinfo")
+
+
+class ZoneInfoTestBase(unittest.TestCase):
+    @classmethod
+    def setUpClass(cls):
+        cls.klass = cls.module.ZoneInfo
+        super().setUpClass()
+
+    @contextlib.contextmanager
+    def tzpath_context(self, tzpath, lock=TZPATH_LOCK):
+        with lock:
+            old_path = self.module.TZPATH
+            try:
+                self.module.reset_tzpath(tzpath)
+                yield
+            finally:
+                self.module.reset_tzpath(old_path)
diff --git a/Lib/test/test_zoneinfo/data/update_test_data.py b/Lib/test/test_zoneinfo/data/update_test_data.py
new file mode 100644
index 0000000..f531ab3
--- /dev/null
+++ b/Lib/test/test_zoneinfo/data/update_test_data.py
@@ -0,0 +1,122 @@
+"""
+Script to automatically generate a JSON file containing time zone information.
+
+This is done to allow "pinning" a small subset of the tzdata in the tests,
+since we are testing properties of a file that may be subject to change. For
+example, the behavior in the far future of any given zone is likely to change,
+but "does this give the right answer for this file in 2040" is still an
+important property to test.
+
+This must be run from a computer with zoneinfo data installed.
+"""
+from __future__ import annotations
+
+import base64
+import functools
+import json
+import lzma
+import pathlib
+import textwrap
+import typing
+
+import zoneinfo
+
+KEYS = [
+    "Africa/Abidjan",
+    "Africa/Casablanca",
+    "America/Los_Angeles",
+    "America/Santiago",
+    "Asia/Tokyo",
+    "Australia/Sydney",
+    "Europe/Dublin",
+    "Europe/Lisbon",
+    "Europe/London",
+    "Pacific/Kiritimati",
+    "UTC",
+]
+
+TEST_DATA_LOC = pathlib.Path(__file__).parent
+
+
+@functools.lru_cache(maxsize=None)
+def get_zoneinfo_path() -> pathlib.Path:
+    """Get the first zoneinfo directory on TZPATH containing the "UTC" zone."""
+    key = "UTC"
+    for path in map(pathlib.Path, zoneinfo.TZPATH):
+        if (path / key).exists():
+            return path
+    else:
+        raise OSError("Cannot find time zone data.")
+
+
+def get_zoneinfo_metadata() -> typing.Dict[str, str]:
+    path = get_zoneinfo_path()
+
+    tzdata_zi = path / "tzdata.zi"
+    if not tzdata_zi.exists():
+        # tzdata.zi is necessary to get the version information
+        raise OSError("Time zone data does not include tzdata.zi.")
+
+    with open(tzdata_zi, "r") as f:
+        version_line = next(f)
+
+    _, version = version_line.strip().rsplit(" ", 1)
+
+    if (
+        not version[0:4].isdigit()
+        or len(version) < 5
+        or not version[4:].isalpha()
+    ):
+        raise ValueError(
+            "Version string should be YYYYx, "
+            + "where YYYY is the year and x is a letter; "
+            + f"found: {version}"
+        )
+
+    return {"version": version}
+
+
+def get_zoneinfo(key: str) -> bytes:
+    path = get_zoneinfo_path()
+
+    with open(path / key, "rb") as f:
+        return f.read()
+
+
+def encode_compressed(data: bytes) -> typing.List[str]:
+    compressed_zone = lzma.compress(data)
+    raw = base64.b85encode(compressed_zone)
+
+    raw_data_str = raw.decode("utf-8")
+
+    data_str = textwrap.wrap(raw_data_str, width=70)
+    return data_str
+
+
+def load_compressed_keys() -> typing.Dict[str, typing.List[str]]:
+    output = {key: encode_compressed(get_zoneinfo(key)) for key in KEYS}
+
+    return output
+
+
+def update_test_data(fname: str = "zoneinfo_data.json") -> None:
+    TEST_DATA_LOC.mkdir(exist_ok=True, parents=True)
+
+    # Annotation required: https://github.com/python/mypy/issues/8772
+    json_kwargs: typing.Dict[str, typing.Any] = dict(
+        indent=2, sort_keys=True,
+    )
+
+    compressed_keys = load_compressed_keys()
+    metadata = get_zoneinfo_metadata()
+    output = {
+        "metadata": metadata,
+        "data": compressed_keys,
+    }
+
+    with open(TEST_DATA_LOC / fname, "w") as f:
+        json.dump(output, f, **json_kwargs)
+
+
+if __name__ == "__main__":
+    update_test_data()
diff --git a/Lib/test/test_zoneinfo/data/zoneinfo_data.json b/Lib/test/test_zoneinfo/data/zoneinfo_data.json
new file mode 100644
index 0000000..ec4414a
--- /dev/null
+++ b/Lib/test/test_zoneinfo/data/zoneinfo_data.json
@@ -0,0 +1,190 @@
+{
+  "data": {
+    "Africa/Abidjan": [
+      "{Wp48S^xk9=GL@E0stWa761SMbT8$j-~f{VGF<>F7KxBg5R*{Ksocg8-YYVul=v7vZzaHN",
+      "uC=da5UI2rH18c!OnjV{y4u(+A!!VBKmY&$ORw>7UO^(500B;v0RR91bXh%WvBYQl0ssI2",
+      "00dcD"
+    ],
+    "Africa/Casablanca": [
+      "{Wp48S^xk9=GL@E0stWa761SMbT8$j;0b&Kz+C_;7KxBg5R*{N&yjMUR~;C-fDaSOU;q-~",
+      "FqW+4{YBjbcw}`a!dW>b)R2-0a+uwf`P3{_Y@HuCz}S$J$ZJ>R_V<~|Fk>sgX4=%0vUrh-",
+      "lt@YP^Wrus;j?`Th#xRPzf<<~Hp4DH^gZX>d{+WOp~HNu8!{uWu}&XphAd{j1;rB4|9?R!",
+      "pqruAFUMt8#*WcrVS{;kLlY(cJRV$w?d2car%R<ALOSO?^`4;ZZtI)%f^^G^>s>q9BgTU4",
+      "Ht-tQKZ7Z`9QqOb?R#b%z?rk>!CkH7jy3wja4NG2q)H}fNRKg8v{);Em;K3Cncf4C6&Oaj",
+      "V+DbX%o4+)CV3+e!Lm6dutu(0BQpH1T?W(~cQtKV*^_Pdx!LirjpTs?Bmt@vktjLq4;)O!",
+      "rrly=c*rwTwMJFd0I57`hgkc?=nyI4RZf9W$6DCWugmf&)wk^tWH17owj=#PGH7Xv-?9$j",
+      "njwDlkOE+BFNR9YXEmBpO;rqEw=e2IR-8^(W;8ma?M3JVd($2T>IW+0tk|Gm8>ftukRQ9J",
+      "8k3brzqMnVyjsLI-CKneFa)Lxvp_a<CkQEd#(pMA^rr}rBNElGA=*!M)puBdoErR9{kWL@",
+      "w=svMc6eZ^-(vQZrV<u^PY#nOIUDJ8%A&;BUVlY9=;@i2j2J1_`P>q40f}0J3VVoWL5rox",
+      "`Kptivcp}o5xA^@>qNI%?zo=Yj4AMV?kbAA)j(1%)+Pp)bSn+7Yk`M{oE}L-Z!G6<Dgq&*",
+      "(C-mFJfbEGDH5M^vBr65rcnsx*~|Em_GeU#B)(+T!|MG-nxj0@IPbp-nHejH3~>OMr5G+h",
+      "p)$3Lg{ono{4cN>Vr&>L4kXH;_VnBL5U!LgzqE%P7QQ*<E!guRW2SE@ayq@)G2nXqA2tGo",
+      "QIgc6>tue}O`3(TZ0`aKn&~8trOQ-rBXCp)f@P6RMO4l0+;b|5-pk9_ryNh}Zc*v%mvz_#",
+      "yd<xXt%~gT90dn4e{Ac<baL-)Y{L7&5G($I$>6fjB0g9{MmMnu8bG%#C~ugXK^S^k@?ab#",
+      "O|aE>dDTt4s4n69(~@t~!wniV%g<uWQat_i6>7khFx~I*4>Y|V$4j5%KPF*-FyKIi@!Ho&",
+      "x8QQsksYt8)D+W)Ni!=G`ogSu^vLL-l#7A7=iIAKL2SuZk9F}NfNk86VI)9WZE?%2wC-ya",
+      "F~z#Qsq)LH0|_D8^5fU8X%GeQ4TB>R-dlziA&tZe&1ada208!$nk`7bOFO2S00G<w{Sp8G",
+      "{cR_IvBYQl0ssI200dcD"
+    ],
+    "America/Los_Angeles": [
+      "{Wp48S^xk9=GL@E0stWa761SMbT8$j;0qH3OkDsf7KxBg5R*;z{h&-RlhRYu$%jt%!jv+I",
+      "JxhE=%W1?wYb!37Rb?(rgwFIAQI{L#8r*zy!$TMtER_1(vn(Zix^{AVB1(jwr$iL6h0Z!2",
+      "8Gb~UW@0~e512{Z%8}Qzdnjl~wJ1{c2>`Z@1A~t&lyL{p{eM{5)QGf7Mo5FW9==mlyXJt2",
+      "UwpntR7H0eSq!(aYq#aqUz&RM*tvuMI)AsM?K3-dV3-TT{t)!Iy#JTo=tXkzAM9~j2YbiO",
+      "ls3(H8Dc>Y|D1aqL51vjLbpYG;GvGTQB4bXuJ%mA;(B4eUpu$$@zv2vVcq-Y)VKbzp^tei",
+      "uzy}R{Luv<C;_cPe*n$Z<jeC9ogWF9=1mvvUYXS>DjpuVb`79O+CBmg{Wx!bvx$eu4zRE&",
+      "PehMb=&G<9$>iZ|bFE)0=4I?KLFGBC0I(0_svgw0%FiMsT%koo*!nEYc6GY@QnU}&4Isg;",
+      "l=|khi(!VaiSE2=Ny`&&tpi~~;{$u<GHlsr3Ze!iYsU205RFKsLnrXwOL?Mq08xffgS{6h",
+      "E|figx+&N%wbO}re@|}$l;g_6J-Wl%j|qev8A<T?NJ)`;2neGi_DHE4ET*W!c*ggPAgU+L",
+      "E9=bH7;maCUikw^R)UM;TdVvNkQ;FGgN=yQER`SZ1nOgPXr0LCebLety&}kVdmVmB=8eSg",
+      "td!1%p=a2wooIL!Da}OPXvKBfRo?YxqS>N}%f|7mBhAy;<Er2&_LfND#qXN~Mkgf!@4VFA",
+      "Hr%$c)wrKA2cJYWK2>s3YT^sy!$eG~?`9mNJC9@4Bac_p^BZh)Yd_rWW5qh-?tKY(>5VHO",
+      "L*iT8P@wCavLj^yYbnDR+4ukhS+xPrpl)iqB?u)bj9a2aW==g6G3lCJd>(+Blf<d4CF%7u",
+      "tlBUDki}J-!_Dy}5S(MrxSXy~$Z+hgH3P^<<w7D72L7I-R%H3(xm&q_DXxkp$owLTS6Wzk",
+      "hc3nn;laROa3)6hl&gH#)2Lif8fZe$@CdeJ-Zn&*>r)~^40F4f>cRZ^UF;RibfZ>0m73hR",
+      "C{$vTfC(STN`g7(B<=Z2556{}0`?p&|Akkst!4Xy4OT;A@c$XTUI3FRRjy*KA7uC56FD)z",
+      "^X{WV*sr(w!c$W357o!&eLO2wTDNOyw@gf(&R<<LF_3URI4=Ei`-%dM3T66j#9!aG7&b_@",
+      "g1-9vo?DzXZ5vGaf~w__p_@_X?OdvQ_r5bvy2hpESTf+{p?jL+!~!{g8-<-5$@d8EZV&-5",
+      "@a|;^1gB*R-~{EHFA-td_G2bt;~Y}>t;=-Tu1TV{>%8ZVATC9tjD8|(&`$9YHvZ9bVe#>w",
+      "|8c;Tg|xE&)`*}LwM*E}q}q8^Qja%p`_U)*5DdLI9O@!e=3jFjOCrCq28b_bb;s>%D#iJB",
+      "CWJi{JH!Js;6nfayos$kq^OEX00HO-lokL0!mqm{vBYQl0ssI200dcD"
+    ],
+    "America/Santiago": [
+      "{Wp48S^xk9=GL@E0stWa761SMbT8$j;0fRZ<6QtM7KxBg84(fsEAUJ$J{f-TXlPEUec5Ee",
+      "n+hsD4lC(QYax=JdSpoyje8%VM`GW}<Unz6IOY4=y66tfqG2X4E8xIJQ(~?r{`L~T!sI~o",
+      "VBl7Ao!R1A76Y8P6Y<TfwVHf@sl@S-D4OuAy5mq0MKJZ>{bJ8@y$A8O&*$pw{(f~Os#}2w",
+      "eX6^Rgi$IT%n^V^85L>$_c7{cB^#ogV=rHBJGiz-RQNFGK?gdPi|q)j`&8)}KJ{qo6dixa",
+      "9@yYyVg+%lo0nO+Tw0-w2hJ%mafy<Co(;L+24CYl&?rN0mrh90nxG?%1&Ed@za`Yd>WL)|",
+      ")<o0dZL-*?RFtH7dAv%G*O%l?qvq!0F5C?K#_ZoT{P$77IMoj3&8w3f&n36zquu~s`s0T)",
+      ";>?W6Bi%FWuGPA1Dru$XR4SZANsAthU2EoKH<MU4wYvUTlZGcLIDR+hSik>F6oEtKq`rwP",
+      "(VNegnI_NI%;ma$)wj{k!@KFB30Yo)IOr<QX7IQ@TBq9d;e3QAtYU?$PS-WoaiqwFrg4PR",
+      "A->l>)$)D|+(5h&+%2vuwGuy^@S8FT^s21V5};>VA9Iu;?8bHz#r<;JtfZDI1(FT@edh0#",
+      "MYW$A1qkMGIwTZqqdYNE3gl#zp&NbL9Mp=voqN|;?gqR&4$)1`znddtEyuKS*^nMMD=0^>",
+      "7^z6-C4P67UWOXuMBubP>j6i~03aR@jD^-Y`JSYu#Yp0P8dLLJ0QOPE8=BoiuRX59YW7xg",
+      "WiexjHX%&0?`ZQCdxCdL^qd1v@kOjQKaWo2Y1++~LcA%FTq?5o<?(jL(_Uo}I}k_Fwflcr",
+      "aovwSR_(ILA6li<iBLPQ0#rEet;W-*54kj#sZEGK*tAF{)HNkn#&Hc5`#eaRF;N#$<xQU?",
+      "E%zm?2+b5Ho>%}fX1-RIvlB)1#iTNomGnUL=nM!>Ix|AGtON7!F1O?53kqlC2o-`ZGw*+s",
+      "NM$^9znsIJMwlgscE`|O3|;BRgsQMYm~`uv+nvuv`nigRa}X=BX=A5Sw$)WEklF7&c>_~$",
+      "zJ(m--bqXgiN^w-U=BJH9C0Qro(x90zo@rK;&TJ$nI@&k$ORgOb2<MjjIhYfr;pFUGdMd!",
+      "0d&bOvyq3AZPCez8E(XSg2hBu2A&^k?w|1u8v3JE>s%gWbc}ok_27)Eoku~Fq|B-Ps+4J_",
+      "HPJMLJ2^_)cOU$p&3kNAlrV!)%~6r$BJ>OOi~=-<6byle{?zd4J{NG}o8tw|+#ZNLcpNwk",
+      "TuPE~sbJB8_RZb2DopStO+Wwux~F#S59zm%00I98;S&G=b(j+6vBYQl0ssI200dcD"
+    ],
+    "Asia/Tokyo": [
+      "{Wp48S^xk9=GL@E0stWa761SMbT8$j-~luMgIxeB7KxBg5R*;y?l4Rl4neXH3cv!OtfK@h",
+      "KZzauI)S!FSDREPhhBS6Fb$&Vv#7%;?Te|>pF^0HBr&z_Tk<%vMW_QqjevRZOp8XVFgP<8",
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+      "dIxbZ00Ex?wE_SDJu@vkvBYQl0ssI200dcD"
+    ],
+    "Australia/Sydney": [
+      "{Wp48S^xk9=GL@E0stWa761SMbT8$j;0T)o7+nA=7KxBg5R*_t6jS5T`_Ull(nK1_YY;k%",
+      ";_YdTuU3*!K)eKg@^kzjAtbo@Jd|KGai=Q%%sX5FI?*?LG!|m9cKH5~IEwI=PAr_Yc}w35",
+      ">}hOdk<>TdUa07R(LPI6@!GU$ty4=mwqHG-XVe*n(Yvgdlr+FqIU18!osi)48t~eWX8)&L",
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+      "%RRC~q_D87;UyN{nLRu!fEqKeRR*U$vs>f9Y72K~o-T%}D&z%}#7g<qim`EbfhF7ntyAiP",
+      "%LFNc&!$@Kv)Olyf&Y9%(#SkM+%yI}S%b+@ZM2dH7DpmndGMIda<(`#E9q|?H(HzClx+l;",
+      "M?IEz1eF}r?}ay!V9?9rKD^-ayjE@wUMD$2kC!iwH`n=eVrJPmJyNKaW`LdJ68&u;2nF1K",
+      "kZjKCY_A<>2br?oH6ZiYH^%>J3D)TPKV(JY*bwjuw5=DsPB@~CrR<E_U_fJTF9ufU%!cXK",
+      "_4uM#!%%Q1e1G~{E}~vGVE0{Kxecm^NjtJM`c8EFHFTiUIVl@YUD8F+s!u8jz~6hte@oa|",
+      "qayb*^Lwd(etNmBro;aXQjkY8g(*`_JQ0%{V3QP2l!GGQ7D+v&k_PK0F(?f{GziU5>OZeN",
+      "x>A*H&CHrWt0`EP`m!F%waepl#|w#&`XgVc?~2M3uw$fGX~tf_Il!q#Aa<*8xlzQ2+7r6Z",
+      "^;Laa9F(WB_O&Dy2r>~@kSi16W{=6+i5GV=Uq~KX*~&HUN4oz7*O(gXIr}sDVcD`Ikgw#|",
+      "50ssal8s)Qy;?YGCf;*UKKKN!T4!Kqy_G;7<gSrPK{)5#a>PfQapugqvVBKy12v3TVH^L2",
+      "0?#5*VP~MOYfe$h`*L!7@tiW|_^X1N%<}`7YahiUYtMu5XwmOf3?dr+@zXHwW`z}ZDqZlT",
+      "<2Cs(<1%M!i6o&VK89BY0J7HPIo;O62s=|IbV^@y$N&#<x=a876<(U>=>i^F00FcHoDl#3",
+      "Mdv&xvBYQl0ssI200dcD"
+    ],
+    "Europe/Dublin": [
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+    ],
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+      "Z2tpNvBYQl0ssI200dcD"
+    ],
+    "Europe/London": [
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+      "$3P+3^&|~i28@=d_u6Cgthe(Lq(wxKpdSDL|7X6Un<nrt00Gwuz#ISo`BbmvvBYQl0ssI2",
+      "00dcD"
+    ],
+    "Pacific/Kiritimati": [
+      "{Wp48S^xk9=GL@E0stWa761SMbT8$j-~jCaVO;<!7KxBg5R*{K!`A|q%C5j6({{dSEy5>+",
+      "NF2>iK{8KMUf+)<-)VxXbLxD(alL}N$AT-ogNbJSMMYeX+Z{jS)b8TK^PB=FxyBxzfmFto",
+      "eo0R`a(%NO?#aEH9|?Cv00000NIsFh6BW2800DjO0RR918Pu^`vBYQl0ssI200dcD"
+    ],
+    "UTC": [
+      "{Wp48S^xk9=GL@E0stWa761SMbT8$j-~e#|9bEt_7KxBg5R*|3h1|xhHLji!C57qW6L*|H",
+      "pEErm00000ygu;I+>V)?00B92fhY-(AGY&-0RR9100dcD"
+    ]
+  },
+  "metadata": {
+    "version": "2020a"
+  }
+}
\ No newline at end of file
diff --git a/Lib/test/test_zoneinfo/test_zoneinfo.py b/Lib/test/test_zoneinfo/test_zoneinfo.py
new file mode 100644
index 0000000..05db03a
--- /dev/null
+++ b/Lib/test/test_zoneinfo/test_zoneinfo.py
@@ -0,0 +1,1994 @@
+from __future__ import annotations
+
+import base64
+import contextlib
+import dataclasses
+import importlib.metadata
+import io
+import json
+import lzma
+import os
+import pathlib
+import pickle
+import re
+import shutil
+import struct
+import tempfile
+import unittest
+from datetime import date, datetime, time, timedelta, timezone
+
+from . import _support as test_support
+from ._support import (
+    OS_ENV_LOCK,
+    TZPATH_LOCK,
+    TZPATH_TEST_LOCK,
+    ZoneInfoTestBase,
+)
+
+py_zoneinfo, c_zoneinfo = test_support.get_modules()
+
+try:
+    importlib.metadata.metadata("tzdata")
+    HAS_TZDATA_PKG = True
+except importlib.metadata.PackageNotFoundError:
+    HAS_TZDATA_PKG = False
+
+ZONEINFO_DATA = None
+ZONEINFO_DATA_V1 = None
+TEMP_DIR = None
+DATA_DIR = pathlib.Path(__file__).parent / "data"
+ZONEINFO_JSON = DATA_DIR / "zoneinfo_data.json"
+
+# Useful constants
+ZERO = timedelta(0)
+ONE_H = timedelta(hours=1)
+
+
+def setUpModule():
+    global TEMP_DIR
+    global ZONEINFO_DATA
+    global ZONEINFO_DATA_V1
+
+    TEMP_DIR = pathlib.Path(tempfile.mkdtemp(prefix="zoneinfo"))
+    ZONEINFO_DATA = ZoneInfoData(ZONEINFO_JSON, TEMP_DIR / "v2")
+    ZONEINFO_DATA_V1 = ZoneInfoData(ZONEINFO_JSON, TEMP_DIR / "v1", v1=True)
+
+
+def tearDownModule():
+    shutil.rmtree(TEMP_DIR)
+
+
+class TzPathUserMixin:
+    """
+    Adds a setUp() and tearDown() to make TZPATH manipulations thread-safe.
+
+    Any tests that require manipulation of the TZPATH global are necessarily
+    thread unsafe, so we will acquire a lock and reset the TZPATH variable
+    to the default state before each test and release the lock after the test
+    is through.
+    """
+
+    @property
+    def tzpath(self):  # pragma: nocover
+        return None
+
+    def setUp(self):
+        with contextlib.ExitStack() as stack:
+            stack.enter_context(
+                self.tzpath_context(self.tzpath, lock=TZPATH_TEST_LOCK)
+            )
+            self.addCleanup(stack.pop_all().close)
+
+        super().setUp()
+
+
+class DatetimeSubclassMixin:
+    """
+    Replaces all ZoneTransition transition dates with a datetime subclass.
+    """
+
+    class DatetimeSubclass(datetime):
+        @classmethod
+        def from_datetime(cls, dt):
+            return cls(
+                dt.year,
+                dt.month,
+                dt.day,
+                dt.hour,
+                dt.minute,
+                dt.second,
+                dt.microsecond,
+                tzinfo=dt.tzinfo,
+                fold=dt.fold,
+            )
+
+    def load_transition_examples(self, key):
+        transition_examples = super().load_transition_examples(key)
+        for zt in transition_examples:
+            dt = zt.transition
+            new_dt = self.DatetimeSubclass.from_datetime(dt)
+            new_zt = dataclasses.replace(zt, transition=new_dt)
+            yield new_zt
+
+
+class ZoneInfoTest(TzPathUserMixin, ZoneInfoTestBase):
+    module = py_zoneinfo
+    class_name = "ZoneInfo"
+
+    def setUp(self):
+        super().setUp()
+
+        # This is necessary because various subclasses pull from different
+        # data sources (e.g. tzdata, V1 files, etc).
+        self.klass.clear_cache()
+
+    @property
+    def zoneinfo_data(self):
+        return ZONEINFO_DATA
+
+    @property
+    def tzpath(self):
+        return [self.zoneinfo_data.tzpath]
+
+    def zone_from_key(self, key):
+        return self.klass(key)
+
+    def zones(self):
+        return ZoneDumpData.transition_keys()
+
+    def fixed_offset_zones(self):
+        return ZoneDumpData.fixed_offset_zones()
+
+    def load_transition_examples(self, key):
+        return ZoneDumpData.load_transition_examples(key)
+
+    def test_str(self):
+        # Zones constructed with a key must have str(zone) == key
+        for key in self.zones():
+            with self.subTest(key):
+                zi = self.zone_from_key(key)
+
+                self.assertEqual(str(zi), key)
+
+        # Zones with no key constructed should have str(zone) == repr(zone)
+        file_key = self.zoneinfo_data.keys[0]
+        file_path = self.zoneinfo_data.path_from_key(file_key)
+
+        with open(file_path, "rb") as f:
+            with self.subTest(test_name="Repr test", path=file_path):
+                zi_ff = self.klass.from_file(f)
+                self.assertEqual(str(zi_ff), repr(zi_ff))
+
+    def test_repr(self):
+        # The repr is not guaranteed, but I think we can insist that it at
+        # least contain the name of the class.
+        key = next(iter(self.zones()))
+
+        zi = self.klass(key)
+        class_name = self.class_name
+        with self.subTest(name="from key"):
+            self.assertRegex(repr(zi), class_name)
+
+        file_key = self.zoneinfo_data.keys[0]
+        file_path = self.zoneinfo_data.path_from_key(file_key)
+        with open(file_path, "rb") as f:
+            zi_ff = self.klass.from_file(f, key=file_key)
+
+        with self.subTest(name="from file with key"):
+            self.assertRegex(repr(zi_ff), class_name)
+
+        with open(file_path, "rb") as f:
+            zi_ff_nk = self.klass.from_file(f)
+
+        with self.subTest(name="from file without key"):
+            self.assertRegex(repr(zi_ff_nk), class_name)
+
+    def test_key_attribute(self):
+        key = next(iter(self.zones()))
+
+        def from_file_nokey(key):
+            with open(self.zoneinfo_data.path_from_key(key), "rb") as f:
+                return self.klass.from_file(f)
+
+        constructors = (
+            ("Primary constructor", self.klass, key),
+            ("no_cache", self.klass.no_cache, key),
+            ("from_file", from_file_nokey, None),
+        )
+
+        for msg, constructor, expected in constructors:
+            zi = constructor(key)
+
+            # Ensure that the key attribute is set to the input to ``key``
+            with self.subTest(msg):
+                self.assertEqual(zi.key, expected)
+
+            # Ensure that the key attribute is read-only
+            with self.subTest(f"{msg}: readonly"):
+                with self.assertRaises(AttributeError):
+                    zi.key = "Some/Value"
+
+    def test_bad_keys(self):
+        bad_keys = [
+            "Eurasia/Badzone",  # Plausible but does not exist
+            "BZQ",
+            "America.Los_Angeles",
+            "🇨🇦",  # Non-ascii
+            "America/New\ud800York",  # Contains surrogate character
+        ]
+
+        for bad_key in bad_keys:
+            with self.assertRaises(self.module.ZoneInfoNotFoundError):
+                self.klass(bad_key)
+
+    def test_bad_keys_paths(self):
+        bad_keys = [
+            "/America/Los_Angeles",  # Absolute path
+            "America/Los_Angeles/",  # Trailing slash - not normalized
+            "../zoneinfo/America/Los_Angeles",  # Traverses above TZPATH
+            "America/../America/Los_Angeles",  # Not normalized
+            "America/./Los_Angeles",
+        ]
+
+        for bad_key in bad_keys:
+            with self.assertRaises(ValueError):
+                self.klass(bad_key)
+
+    def test_bad_zones(self):
+        bad_zones = [
+            b"",  # Empty file
+            b"AAAA3" + b" " * 15,  # Bad magic
+        ]
+
+        for bad_zone in bad_zones:
+            fobj = io.BytesIO(bad_zone)
+            with self.assertRaises(ValueError):
+                self.klass.from_file(fobj)
+
+    def test_fromutc_errors(self):
+        key = next(iter(self.zones()))
+        zone = self.zone_from_key(key)
+
+        bad_values = [
+            (datetime(2019, 1, 1, tzinfo=timezone.utc), ValueError),
+            (datetime(2019, 1, 1), ValueError),
+            (date(2019, 1, 1), TypeError),
+            (time(0), TypeError),
+            (0, TypeError),
+            ("2019-01-01", TypeError),
+        ]
+
+        for val, exc_type in bad_values:
+            with self.subTest(val=val):
+                with self.assertRaises(exc_type):
+                    zone.fromutc(val)
+
+    def test_utc(self):
+        zi = self.klass("UTC")
+        dt = datetime(2020, 1, 1, tzinfo=zi)
+
+        self.assertEqual(dt.utcoffset(), ZERO)
+        self.assertEqual(dt.dst(), ZERO)
+        self.assertEqual(dt.tzname(), "UTC")
+
+    def test_unambiguous(self):
+        test_cases = []
+        for key in self.zones():
+            for zone_transition in self.load_transition_examples(key):
+                test_cases.append(
+                    (
+                        key,
+                        zone_transition.transition - timedelta(days=2),
+                        zone_transition.offset_before,
+                    )
+                )
+
+                test_cases.append(
+                    (
+                        key,
+                        zone_transition.transition + timedelta(days=2),
+                        zone_transition.offset_after,
+                    )
+                )
+
+        for key, dt, offset in test_cases:
+            with self.subTest(key=key, dt=dt, offset=offset):
+                tzi = self.zone_from_key(key)
+                dt = dt.replace(tzinfo=tzi)
+
+                self.assertEqual(dt.tzname(), offset.tzname, dt)
+                self.assertEqual(dt.utcoffset(), offset.utcoffset, dt)
+                self.assertEqual(dt.dst(), offset.dst, dt)
+
+    def test_folds_and_gaps(self):
+        test_cases = []
+        for key in self.zones():
+            tests = {"folds": [], "gaps": []}
+            for zt in self.load_transition_examples(key):
+                if zt.fold:
+                    test_group = tests["folds"]
+                elif zt.gap:
+                    test_group = tests["gaps"]
+                else:
+                    # Assign a random variable here to disable the peephole
+                    # optimizer so that coverage can see this line.
+                    # See bpo-2506 for more information.
+                    no_peephole_opt = None
+                    continue
+
+                # Cases are of the form key, dt, fold, offset
+                dt = zt.anomaly_start - timedelta(seconds=1)
+                test_group.append((dt, 0, zt.offset_before))
+                test_group.append((dt, 1, zt.offset_before))
+
+                dt = zt.anomaly_start
+                test_group.append((dt, 0, zt.offset_before))
+                test_group.append((dt, 1, zt.offset_after))
+
+                dt = zt.anomaly_start + timedelta(seconds=1)
+                test_group.append((dt, 0, zt.offset_before))
+                test_group.append((dt, 1, zt.offset_after))
+
+                dt = zt.anomaly_end - timedelta(seconds=1)
+                test_group.append((dt, 0, zt.offset_before))
+                test_group.append((dt, 1, zt.offset_after))
+
+                dt = zt.anomaly_end
+                test_group.append((dt, 0, zt.offset_after))
+                test_group.append((dt, 1, zt.offset_after))
+
+                dt = zt.anomaly_end + timedelta(seconds=1)
+                test_group.append((dt, 0, zt.offset_after))
+                test_group.append((dt, 1, zt.offset_after))
+
+            for grp, test_group in tests.items():
+                test_cases.append(((key, grp), test_group))
+
+        for (key, grp), tests in test_cases:
+            with self.subTest(key=key, grp=grp):
+                tzi = self.zone_from_key(key)
+
+                for dt, fold, offset in tests:
+                    dt = dt.replace(fold=fold, tzinfo=tzi)
+
+                    self.assertEqual(dt.tzname(), offset.tzname, dt)
+                    self.assertEqual(dt.utcoffset(), offset.utcoffset, dt)
+                    self.assertEqual(dt.dst(), offset.dst, dt)
+
+    def test_folds_from_utc(self):
+        tests = []
+        for key in self.zones():
+            zi = self.zone_from_key(key)
+            with self.subTest(key=key):
+                for zt in self.load_transition_examples(key):
+                    if not zt.fold:
+                        continue
+
+                    dt_utc = zt.transition_utc
+                    dt_before_utc = dt_utc - timedelta(seconds=1)
+                    dt_after_utc = dt_utc + timedelta(seconds=1)
+
+                    dt_before = dt_before_utc.astimezone(zi)
+                    self.assertEqual(dt_before.fold, 0, (dt_before, dt_utc))
+
+                    dt_after = dt_after_utc.astimezone(zi)
+                    self.assertEqual(dt_after.fold, 1, (dt_after, dt_utc))
+
+    def test_time_variable_offset(self):
+        # self.zones() only ever returns variable-offset zones
+        for key in self.zones():
+            zi = self.zone_from_key(key)
+            t = time(11, 15, 1, 34471, tzinfo=zi)
+
+            with self.subTest(key=key):
+                self.assertIs(t.tzname(), None)
+                self.assertIs(t.utcoffset(), None)
+                self.assertIs(t.dst(), None)
+
+    def test_time_fixed_offset(self):
+        for key, offset in self.fixed_offset_zones():
+            zi = self.zone_from_key(key)
+
+            t = time(11, 15, 1, 34471, tzinfo=zi)
+
+            with self.subTest(key=key):
+                self.assertEqual(t.tzname(), offset.tzname)
+                self.assertEqual(t.utcoffset(), offset.utcoffset)
+                self.assertEqual(t.dst(), offset.dst)
+
+
+class CZoneInfoTest(ZoneInfoTest):
+    module = c_zoneinfo
+
+    def test_fold_mutate(self):
+        """Test that fold isn't mutated when no change is necessary.
+
+        The underlying C API is capable of mutating datetime objects, and
+        may rely on the fact that addition of a datetime object returns a
+        new datetime; this test ensures that the input datetime to fromutc
+        is not mutated.
+        """
+
+        def to_subclass(dt):
+            class SameAddSubclass(type(dt)):
+                def __add__(self, other):
+                    if other == timedelta(0):
+                        return self
+
+                    return super().__add__(other)  # pragma: nocover
+
+            return SameAddSubclass(
+                dt.year,
+                dt.month,
+                dt.day,
+                dt.hour,
+                dt.minute,
+                dt.second,
+                dt.microsecond,
+                fold=dt.fold,
+                tzinfo=dt.tzinfo,
+            )
+
+        subclass = [False, True]
+
+        key = "Europe/London"
+        zi = self.zone_from_key(key)
+        for zt in self.load_transition_examples(key):
+            if zt.fold and zt.offset_after.utcoffset == ZERO:
+                example = zt.transition_utc.replace(tzinfo=zi)
+                break
+
+        for subclass in [False, True]:
+            if subclass:
+                dt = to_subclass(example)
+            else:
+                dt = example
+
+            with self.subTest(subclass=subclass):
+                dt_fromutc = zi.fromutc(dt)
+
+                self.assertEqual(dt_fromutc.fold, 1)
+                self.assertEqual(dt.fold, 0)
+
+
+class ZoneInfoDatetimeSubclassTest(DatetimeSubclassMixin, ZoneInfoTest):
+    pass
+
+
+class CZoneInfoDatetimeSubclassTest(DatetimeSubclassMixin, CZoneInfoTest):
+    pass
+
+
+class ZoneInfoTestSubclass(ZoneInfoTest):
+    @classmethod
+    def setUpClass(cls):
+        super().setUpClass()
+
+        class ZISubclass(cls.klass):
+            pass
+
+        cls.class_name = "ZISubclass"
+        cls.parent_klass = cls.klass
+        cls.klass = ZISubclass
+
+    def test_subclass_own_cache(self):
+        base_obj = self.parent_klass("Europe/London")
+        sub_obj = self.klass("Europe/London")
+
+        self.assertIsNot(base_obj, sub_obj)
+        self.assertIsInstance(base_obj, self.parent_klass)
+        self.assertIsInstance(sub_obj, self.klass)
+
+
+class CZoneInfoTestSubclass(ZoneInfoTest):
+    module = c_zoneinfo
+
+
+class ZoneInfoV1Test(ZoneInfoTest):
+    @property
+    def zoneinfo_data(self):
+        return ZONEINFO_DATA_V1
+
+    def load_transition_examples(self, key):
+        # We will discard zdump examples outside the range epoch +/- 2**31,
+        # because they are not well-supported in Version 1 files.
+        epoch = datetime(1970, 1, 1)
+        max_offset_32 = timedelta(seconds=2 ** 31)
+        min_dt = epoch - max_offset_32
+        max_dt = epoch + max_offset_32
+
+        for zt in ZoneDumpData.load_transition_examples(key):
+            if min_dt <= zt.transition <= max_dt:
+                yield zt
+
+
+class CZoneInfoV1Test(ZoneInfoV1Test):
+    module = c_zoneinfo
+
+
+@unittest.skipIf(
+    not HAS_TZDATA_PKG, "Skipping tzdata-specific tests: tzdata not installed"
+)
+class TZDataTests(ZoneInfoTest):
+    """
+    Runs all the ZoneInfoTest tests, but against the tzdata package
+
+    NOTE: The ZoneDumpData has frozen test data, but tzdata will update, so
+    some of the tests (particularly those related to the far future) may break
+    in the event that the time zone policies in the relevant time zones change.
+    """
+
+    @property
+    def tzpath(self):
+        return []
+
+    def zone_from_key(self, key):
+        return self.klass(key=key)
+
+
+@unittest.skipIf(
+    not HAS_TZDATA_PKG, "Skipping tzdata-specific tests: tzdata not installed"
+)
+class CTZDataTests(TZDataTests):
+    module = c_zoneinfo
+
+
+class WeirdZoneTest(ZoneInfoTestBase):
+    module = py_zoneinfo
+
+    def test_one_transition(self):
+        LMT = ZoneOffset("LMT", -timedelta(hours=6, minutes=31, seconds=2))
+        STD = ZoneOffset("STD", -timedelta(hours=6))
+
+        transitions = [
+            ZoneTransition(datetime(1883, 6, 9, 14), LMT, STD),
+        ]
+
+        after = "STD6"
+
+        zf = self.construct_zone(transitions, after)
+        zi = self.klass.from_file(zf)
+
+        dt0 = datetime(1883, 6, 9, 1, tzinfo=zi)
+        dt1 = datetime(1883, 6, 10, 1, tzinfo=zi)
+
+        for dt, offset in [(dt0, LMT), (dt1, STD)]:
+            with self.subTest(name="local", dt=dt):
+                self.assertEqual(dt.tzname(), offset.tzname)
+                self.assertEqual(dt.utcoffset(), offset.utcoffset)
+                self.assertEqual(dt.dst(), offset.dst)
+
+        dts = [
+            (
+                datetime(1883, 6, 9, 1, tzinfo=zi),
+                datetime(1883, 6, 9, 7, 31, 2, tzinfo=timezone.utc),
+            ),
+            (
+                datetime(2010, 4, 1, 12, tzinfo=zi),
+                datetime(2010, 4, 1, 18, tzinfo=timezone.utc),
+            ),
+        ]
+
+        for dt_local, dt_utc in dts:
+            with self.subTest(name="fromutc", dt=dt_local):
+                dt_actual = dt_utc.astimezone(zi)
+                self.assertEqual(dt_actual, dt_local)
+
+                dt_utc_actual = dt_local.astimezone(timezone.utc)
+                self.assertEqual(dt_utc_actual, dt_utc)
+
+    def test_one_zone_dst(self):
+        DST = ZoneOffset("DST", ONE_H, ONE_H)
+        transitions = [
+            ZoneTransition(datetime(1970, 1, 1), DST, DST),
+        ]
+
+        after = "STD0DST-1,0/0,J365/25"
+
+        zf = self.construct_zone(transitions, after)
+        zi = self.klass.from_file(zf)
+
+        dts = [
+            datetime(1900, 3, 1),
+            datetime(1965, 9, 12),
+            datetime(1970, 1, 1),
+            datetime(2010, 11, 3),
+            datetime(2040, 1, 1),
+        ]
+
+        for dt in dts:
+            dt = dt.replace(tzinfo=zi)
+            with self.subTest(dt=dt):
+                self.assertEqual(dt.tzname(), DST.tzname)
+                self.assertEqual(dt.utcoffset(), DST.utcoffset)
+                self.assertEqual(dt.dst(), DST.dst)
+
+    def test_no_tz_str(self):
+        STD = ZoneOffset("STD", ONE_H, ZERO)
+        DST = ZoneOffset("DST", 2 * ONE_H, ONE_H)
+
+        transitions = []
+        for year in range(1996, 2000):
+            transitions.append(
+                ZoneTransition(datetime(year, 3, 1, 2), STD, DST)
+            )
+            transitions.append(
+                ZoneTransition(datetime(year, 11, 1, 2), DST, STD)
+            )
+
+        after = ""
+
+        zf = self.construct_zone(transitions, after)
+
+        # According to RFC 8536, local times after the last transition time
+        # with an empty TZ string are unspecified. We will go with "hold the
+        # last transition", but the most we should promise is "doesn't crash."
+        zi = self.klass.from_file(zf)
+
+        cases = [
+            (datetime(1995, 1, 1), STD),
+            (datetime(1996, 4, 1), DST),
+            (datetime(1996, 11, 2), STD),
+            (datetime(2001, 1, 1), STD),
+        ]
+
+        for dt, offset in cases:
+            dt = dt.replace(tzinfo=zi)
+            with self.subTest(dt=dt):
+                self.assertEqual(dt.tzname(), offset.tzname)
+                self.assertEqual(dt.utcoffset(), offset.utcoffset)
+                self.assertEqual(dt.dst(), offset.dst)
+
+        # Test that offsets return None when using a datetime.time
+        t = time(0, tzinfo=zi)
+        with self.subTest("Testing datetime.time"):
+            self.assertIs(t.tzname(), None)
+            self.assertIs(t.utcoffset(), None)
+            self.assertIs(t.dst(), None)
+
+    def test_tz_before_only(self):
+        # From RFC 8536 Section 3.2:
+        #
+        #   If there are no transitions, local time for all timestamps is
+        #   specified by the TZ string in the footer if present and nonempty;
+        #   otherwise, it is specified by time type 0.
+
+        offsets = [
+            ZoneOffset("STD", ZERO, ZERO),
+            ZoneOffset("DST", ONE_H, ONE_H),
+        ]
+
+        for offset in offsets:
+            # Phantom transition to set time type 0.
+            transitions = [
+                ZoneTransition(None, offset, offset),
+            ]
+
+            after = ""
+
+            zf = self.construct_zone(transitions, after)
+            zi = self.klass.from_file(zf)
+
+            dts = [
+                datetime(1900, 1, 1),
+                datetime(1970, 1, 1),
+                datetime(2000, 1, 1),
+            ]
+
+            for dt in dts:
+                dt = dt.replace(tzinfo=zi)
+                with self.subTest(offset=offset, dt=dt):
+                    self.assertEqual(dt.tzname(), offset.tzname)
+                    self.assertEqual(dt.utcoffset(), offset.utcoffset)
+                    self.assertEqual(dt.dst(), offset.dst)
+
+    def test_empty_zone(self):
+        zf = self.construct_zone([], "")
+
+        with self.assertRaises(ValueError):
+            self.klass.from_file(zf)
+
+    def test_zone_very_large_timestamp(self):
+        """Test when a transition is in the far past or future.
+
+        Particularly, this is a concern if something:
+
+            1. Attempts to call ``datetime.timestamp`` for a datetime outside
+               of ``[datetime.min, datetime.max]``.
+            2. Attempts to construct a timedelta outside of
+               ``[timedelta.min, timedelta.max]``.
+
+        This actually occurs "in the wild", as some time zones on Ubuntu (at
+        least as of 2020) have an initial transition added at ``-2**58``.
+        """
+
+        LMT = ZoneOffset("LMT", timedelta(seconds=-968))
+        GMT = ZoneOffset("GMT", ZERO)
+
+        transitions = [
+            (-(1 << 62), LMT, LMT),
+            ZoneTransition(datetime(1912, 1, 1), LMT, GMT),
+            ((1 << 62), GMT, GMT),
+        ]
+
+        after = "GMT0"
+
+        zf = self.construct_zone(transitions, after)
+        zi = self.klass.from_file(zf, key="Africa/Abidjan")
+
+        offset_cases = [
+            (datetime.min, LMT),
+            (datetime.max, GMT),
+            (datetime(1911, 12, 31), LMT),
+            (datetime(1912, 1, 2), GMT),
+        ]
+
+        for dt_naive, offset in offset_cases:
+            dt = dt_naive.replace(tzinfo=zi)
+            with self.subTest(name="offset", dt=dt, offset=offset):
+                self.assertEqual(dt.tzname(), offset.tzname)
+                self.assertEqual(dt.utcoffset(), offset.utcoffset)
+                self.assertEqual(dt.dst(), offset.dst)
+
+        utc_cases = [
+            (datetime.min, datetime.min + timedelta(seconds=968)),
+            (datetime(1898, 12, 31, 23, 43, 52), datetime(1899, 1, 1)),
+            (
+                datetime(1911, 12, 31, 23, 59, 59, 999999),
+                datetime(1912, 1, 1, 0, 16, 7, 999999),
+            ),
+            (datetime(1912, 1, 1, 0, 16, 8), datetime(1912, 1, 1, 0, 16, 8)),
+            (datetime(1970, 1, 1), datetime(1970, 1, 1)),
+            (datetime.max, datetime.max),
+        ]
+
+        for naive_dt, naive_dt_utc in utc_cases:
+            dt = naive_dt.replace(tzinfo=zi)
+            dt_utc = naive_dt_utc.replace(tzinfo=timezone.utc)
+
+            self.assertEqual(dt_utc.astimezone(zi), dt)
+            self.assertEqual(dt, dt_utc)
+
+    def test_fixed_offset_phantom_transition(self):
+        UTC = ZoneOffset("UTC", ZERO, ZERO)
+
+        transitions = [ZoneTransition(datetime(1970, 1, 1), UTC, UTC)]
+
+        after = "UTC0"
+        zf = self.construct_zone(transitions, after)
+        zi = self.klass.from_file(zf, key="UTC")
+
+        dt = datetime(2020, 1, 1, tzinfo=zi)
+        with self.subTest("datetime.datetime"):
+            self.assertEqual(dt.tzname(), UTC.tzname)
+            self.assertEqual(dt.utcoffset(), UTC.utcoffset)
+            self.assertEqual(dt.dst(), UTC.dst)
+
+        t = time(0, tzinfo=zi)
+        with self.subTest("datetime.time"):
+            self.assertEqual(t.tzname(), UTC.tzname)
+            self.assertEqual(t.utcoffset(), UTC.utcoffset)
+            self.assertEqual(t.dst(), UTC.dst)
+
+    def construct_zone(self, transitions, after=None, version=3):
+        # These are not used for anything, so we're not going to include
+        # them for now.
+        isutc = []
+        isstd = []
+        leap_seconds = []
+
+        offset_lists = [[], []]
+        trans_times_lists = [[], []]
+        trans_idx_lists = [[], []]
+
+        v1_range = (-(2 ** 31), 2 ** 31)
+        v2_range = (-(2 ** 63), 2 ** 63)
+        ranges = [v1_range, v2_range]
+
+        def zt_as_tuple(zt):
+            # zt may be a tuple (timestamp, offset_before, offset_after) or
+            # a ZoneTransition object — this is to allow the timestamp to be
+            # values that are outside the valid range for datetimes but still
+            # valid 64-bit timestamps.
+            if isinstance(zt, tuple):
+                return zt
+
+            if zt.transition:
+                trans_time = int(zt.transition_utc.timestamp())
+            else:
+                trans_time = None
+
+            return (trans_time, zt.offset_before, zt.offset_after)
+
+        transitions = sorted(map(zt_as_tuple, transitions), key=lambda x: x[0])
+
+        for zt in transitions:
+            trans_time, offset_before, offset_after = zt
+
+            for v, (dt_min, dt_max) in enumerate(ranges):
+                offsets = offset_lists[v]
+                trans_times = trans_times_lists[v]
+                trans_idx = trans_idx_lists[v]
+
+                if trans_time is not None and not (
+                    dt_min <= trans_time <= dt_max
+                ):
+                    continue
+
+                if offset_before not in offsets:
+                    offsets.append(offset_before)
+
+                if offset_after not in offsets:
+                    offsets.append(offset_after)
+
+                if trans_time is not None:
+                    trans_times.append(trans_time)
+                    trans_idx.append(offsets.index(offset_after))
+
+        isutcnt = len(isutc)
+        isstdcnt = len(isstd)
+        leapcnt = len(leap_seconds)
+
+        zonefile = io.BytesIO()
+
+        time_types = ("l", "q")
+        for v in range(min((version, 2))):
+            offsets = offset_lists[v]
+            trans_times = trans_times_lists[v]
+            trans_idx = trans_idx_lists[v]
+            time_type = time_types[v]
+
+            # Translate the offsets into something closer to the C values
+            abbrstr = bytearray()
+            ttinfos = []
+
+            for offset in offsets:
+                utcoff = int(offset.utcoffset.total_seconds())
+                isdst = bool(offset.dst)
+                abbrind = len(abbrstr)
+
+                ttinfos.append((utcoff, isdst, abbrind))
+                abbrstr += offset.tzname.encode("ascii") + b"\x00"
+            abbrstr = bytes(abbrstr)
+
+            typecnt = len(offsets)
+            timecnt = len(trans_times)
+            charcnt = len(abbrstr)
+
+            # Write the header
+            zonefile.write(b"TZif")
+            zonefile.write(b"%d" % version)
+            zonefile.write(b" " * 15)
+            zonefile.write(
+                struct.pack(
+                    ">6l", isutcnt, isstdcnt, leapcnt, timecnt, typecnt, charcnt
+                )
+            )
+
+            # Now the transition data
+            zonefile.write(struct.pack(f">{timecnt}{time_type}", *trans_times))
+            zonefile.write(struct.pack(f">{timecnt}B", *trans_idx))
+
+            for ttinfo in ttinfos:
+                zonefile.write(struct.pack(">lbb", *ttinfo))
+
+            zonefile.write(bytes(abbrstr))
+
+            # Now the metadata and leap seconds
+            zonefile.write(struct.pack(f"{isutcnt}b", *isutc))
+            zonefile.write(struct.pack(f"{isstdcnt}b", *isstd))
+            zonefile.write(struct.pack(f">{leapcnt}l", *leap_seconds))
+
+            # Finally we write the TZ string if we're writing a Version 2+ file
+            if v > 0:
+                zonefile.write(b"\x0A")
+                zonefile.write(after.encode("ascii"))
+                zonefile.write(b"\x0A")
+
+        zonefile.seek(0)
+        return zonefile
+
+
+class CWeirdZoneTest(WeirdZoneTest):
+    module = c_zoneinfo
+
+
+class TZStrTest(ZoneInfoTestBase):
+    module = py_zoneinfo
+
+    NORMAL = 0
+    FOLD = 1
+    GAP = 2
+
+    @classmethod
+    def setUpClass(cls):
+        super().setUpClass()
+
+        cls._populate_test_cases()
+        cls.populate_tzstr_header()
+
+    @classmethod
+    def populate_tzstr_header(cls):
+        out = bytearray()
+        # The TZif format always starts with a Version 1 file followed by
+        # the Version 2+ file. In this case, we have no transitions, just
+        # the tzstr in the footer, so up to the footer, the files are
+        # identical and we can just write the same file twice in a row.
+        for i in range(2):
+            out += b"TZif"  # Magic value
+            out += b"3"  # Version
+            out += b" " * 15  # Reserved
+
+            # We will not write any of the manual transition parts
+            out += struct.pack(">6l", 0, 0, 0, 0, 0, 0)
+
+        cls._tzif_header = bytes(out)
+
+    def zone_from_tzstr(self, tzstr):
+        """Creates a zoneinfo file following a POSIX rule."""
+        zonefile = io.BytesIO(self._tzif_header)
+        zonefile.seek(0, 2)
+
+        # Write the footer
+        zonefile.write(b"\x0A")
+        zonefile.write(tzstr.encode("ascii"))
+        zonefile.write(b"\x0A")
+
+        zonefile.seek(0)
+
+        return self.klass.from_file(zonefile, key=tzstr)
+
+    def test_tzstr_localized(self):
+        i = 0
+        for tzstr, cases in self.test_cases.items():
+            with self.subTest(tzstr=tzstr):
+                zi = self.zone_from_tzstr(tzstr)
+
+            for dt_naive, offset, _ in cases:
+                dt = dt_naive.replace(tzinfo=zi)
+
+                with self.subTest(tzstr=tzstr, dt=dt, offset=offset):
+                    self.assertEqual(dt.tzname(), offset.tzname)
+                    self.assertEqual(dt.utcoffset(), offset.utcoffset)
+                    self.assertEqual(dt.dst(), offset.dst)
+
+    def test_tzstr_from_utc(self):
+        for tzstr, cases in self.test_cases.items():
+            with self.subTest(tzstr=tzstr):
+                zi = self.zone_from_tzstr(tzstr)
+
+            for dt_naive, offset, dt_type in cases:
+                if dt_type == self.GAP:
+                    continue  # Cannot create a gap from UTC
+
+                dt_utc = (dt_naive - offset.utcoffset).replace(
+                    tzinfo=timezone.utc
+                )
+
+                # Check that we can go UTC -> Our zone
+                dt_act = dt_utc.astimezone(zi)
+                dt_exp = dt_naive.replace(tzinfo=zi)
+
+                self.assertEqual(dt_act, dt_exp)
+
+                if dt_type == self.FOLD:
+                    self.assertEqual(dt_act.fold, dt_naive.fold, dt_naive)
+                else:
+                    self.assertEqual(dt_act.fold, 0)
+
+                # Now check that we can go our zone -> UTC
+                dt_act = dt_exp.astimezone(timezone.utc)
+
+                self.assertEqual(dt_act, dt_utc)
+
+    def test_invalid_tzstr(self):
+        invalid_tzstrs = [
+            "PST8PDT",  # DST but no transition specified
+            "+11",  # Unquoted alphanumeric
+            "GMT,M3.2.0/2,M11.1.0/3",  # Transition rule but no DST
+            "GMT0+11,M3.2.0/2,M11.1.0/3",  # Unquoted alphanumeric in DST
+            "PST8PDT,M3.2.0/2",  # Only one transition rule
+            # Invalid offsets
+            "STD+25",
+            "STD-25",
+            "STD+374",
+            "STD+374DST,M3.2.0/2,M11.1.0/3",
+            "STD+23DST+25,M3.2.0/2,M11.1.0/3",
+            "STD-23DST-25,M3.2.0/2,M11.1.0/3",
+            # Completely invalid dates
+            "AAA4BBB,M1443339,M11.1.0/3",
+            "AAA4BBB,M3.2.0/2,0349309483959c",
+            # Invalid months
+            "AAA4BBB,M13.1.1/2,M1.1.1/2",
+            "AAA4BBB,M1.1.1/2,M13.1.1/2",
+            "AAA4BBB,M0.1.1/2,M1.1.1/2",
+            "AAA4BBB,M1.1.1/2,M0.1.1/2",
+            # Invalid weeks
+            "AAA4BBB,M1.6.1/2,M1.1.1/2",
+            "AAA4BBB,M1.1.1/2,M1.6.1/2",
+            # Invalid weekday
+            "AAA4BBB,M1.1.7/2,M2.1.1/2",
+            "AAA4BBB,M1.1.1/2,M2.1.7/2",
+            # Invalid numeric offset
+            "AAA4BBB,-1/2,20/2",
+            "AAA4BBB,1/2,-1/2",
+            "AAA4BBB,367,20/2",
+            "AAA4BBB,1/2,367/2",
+            # Invalid julian offset
+            "AAA4BBB,J0/2,J20/2",
+            "AAA4BBB,J20/2,J366/2",
+        ]
+
+        for invalid_tzstr in invalid_tzstrs:
+            with self.subTest(tzstr=invalid_tzstr):
+                # Not necessarily a guaranteed property, but we should show
+                # the problematic TZ string if that's the cause of failure.
+                tzstr_regex = re.escape(invalid_tzstr)
+                with self.assertRaisesRegex(ValueError, tzstr_regex):
+                    self.zone_from_tzstr(invalid_tzstr)
+
+    @classmethod
+    def _populate_test_cases(cls):
+        # This method uses a somewhat unusual style in that it populates the
+        # test cases for each tzstr by using a decorator to automatically call
+        # a function that mutates the current dictionary of test cases.
+        #
+        # The population of the test cases is done in individual functions to
+        # give each set of test cases its own namespace in which to define
+        # its offsets (this way we don't have to worry about variable reuse
+        # causing problems if someone makes a typo).
+        #
+        # The decorator for calling is used to make it more obvious that each
+        # function is actually called (if it's not decorated, it's not called).
+        def call(f):
+            """Decorator to call the addition methods.
+
+            This will call a function which adds at least one new entry into
+            the `cases` dictionary. The decorator will also assert that
+            something was added to the dictionary.
+            """
+            prev_len = len(cases)
+            f()
+            assert len(cases) > prev_len, "Function did not add a test case!"
+
+        NORMAL = cls.NORMAL
+        FOLD = cls.FOLD
+        GAP = cls.GAP
+
+        cases = {}
+
+        @call
+        def _add():
+            # Transition to EDT on the 2nd Sunday in March at 4 AM, and
+            # transition back on the first Sunday in November at 3AM
+            tzstr = "EST5EDT,M3.2.0/4:00,M11.1.0/3:00"
+
+            EST = ZoneOffset("EST", timedelta(hours=-5), ZERO)
+            EDT = ZoneOffset("EDT", timedelta(hours=-4), ONE_H)
+
+            cases[tzstr] = (
+                (datetime(2019, 3, 9), EST, NORMAL),
+                (datetime(2019, 3, 10, 3, 59), EST, NORMAL),
+                (datetime(2019, 3, 10, 4, 0, fold=0), EST, GAP),
+                (datetime(2019, 3, 10, 4, 0, fold=1), EDT, GAP),
+                (datetime(2019, 3, 10, 4, 1, fold=0), EST, GAP),
+                (datetime(2019, 3, 10, 4, 1, fold=1), EDT, GAP),
+                (datetime(2019, 11, 2), EDT, NORMAL),
+                (datetime(2019, 11, 3, 1, 59, fold=1), EDT, NORMAL),
+                (datetime(2019, 11, 3, 2, 0, fold=0), EDT, FOLD),
+                (datetime(2019, 11, 3, 2, 0, fold=1), EST, FOLD),
+                (datetime(2020, 3, 8, 3, 59), EST, NORMAL),
+                (datetime(2020, 3, 8, 4, 0, fold=0), EST, GAP),
+                (datetime(2020, 3, 8, 4, 0, fold=1), EDT, GAP),
+                (datetime(2020, 11, 1, 1, 59, fold=1), EDT, NORMAL),
+                (datetime(2020, 11, 1, 2, 0, fold=0), EDT, FOLD),
+                (datetime(2020, 11, 1, 2, 0, fold=1), EST, FOLD),
+            )
+
+        @call
+        def _add():
+            # Transition to BST happens on the last Sunday in March at 1 AM GMT
+            # and the transition back happens the last Sunday in October at 2AM BST
+            tzstr = "GMT0BST-1,M3.5.0/1:00,M10.5.0/2:00"
+
+            GMT = ZoneOffset("GMT", ZERO, ZERO)
+            BST = ZoneOffset("BST", ONE_H, ONE_H)
+
+            cases[tzstr] = (
+                (datetime(2019, 3, 30), GMT, NORMAL),
+                (datetime(2019, 3, 31, 0, 59), GMT, NORMAL),
+                (datetime(2019, 3, 31, 2, 0), BST, NORMAL),
+                (datetime(2019, 10, 26), BST, NORMAL),
+                (datetime(2019, 10, 27, 0, 59, fold=1), BST, NORMAL),
+                (datetime(2019, 10, 27, 1, 0, fold=0), BST, GAP),
+                (datetime(2019, 10, 27, 2, 0, fold=1), GMT, GAP),
+                (datetime(2020, 3, 29, 0, 59), GMT, NORMAL),
+                (datetime(2020, 3, 29, 2, 0), BST, NORMAL),
+                (datetime(2020, 10, 25, 0, 59, fold=1), BST, NORMAL),
+                (datetime(2020, 10, 25, 1, 0, fold=0), BST, FOLD),
+                (datetime(2020, 10, 25, 2, 0, fold=1), GMT, NORMAL),
+            )
+
+        @call
+        def _add():
+            # Austrialian time zone - DST start is chronologically first
+            tzstr = "AEST-10AEDT,M10.1.0/2,M4.1.0/3"
+
+            AEST = ZoneOffset("AEST", timedelta(hours=10), ZERO)
+            AEDT = ZoneOffset("AEDT", timedelta(hours=11), ONE_H)
+
+            cases[tzstr] = (
+                (datetime(2019, 4, 6), AEDT, NORMAL),
+                (datetime(2019, 4, 7, 1, 59), AEDT, NORMAL),
+                (datetime(2019, 4, 7, 1, 59, fold=1), AEDT, NORMAL),
+                (datetime(2019, 4, 7, 2, 0, fold=0), AEDT, FOLD),
+                (datetime(2019, 4, 7, 2, 1, fold=0), AEDT, FOLD),
+                (datetime(2019, 4, 7, 2, 0, fold=1), AEST, FOLD),
+                (datetime(2019, 4, 7, 2, 1, fold=1), AEST, FOLD),
+                (datetime(2019, 4, 7, 3, 0, fold=0), AEST, NORMAL),
+                (datetime(2019, 4, 7, 3, 0, fold=1), AEST, NORMAL),
+                (datetime(2019, 10, 5, 0), AEST, NORMAL),
+                (datetime(2019, 10, 6, 1, 59), AEST, NORMAL),
+                (datetime(2019, 10, 6, 2, 0, fold=0), AEST, GAP),
+                (datetime(2019, 10, 6, 2, 0, fold=1), AEDT, GAP),
+                (datetime(2019, 10, 6, 3, 0), AEDT, NORMAL),
+            )
+
+        @call
+        def _add():
+            # Irish time zone - negative DST
+            tzstr = "IST-1GMT0,M10.5.0,M3.5.0/1"
+
+            GMT = ZoneOffset("GMT", ZERO, -ONE_H)
+            IST = ZoneOffset("IST", ONE_H, ZERO)
+
+            cases[tzstr] = (
+                (datetime(2019, 3, 30), GMT, NORMAL),
+                (datetime(2019, 3, 31, 0, 59), GMT, NORMAL),
+                (datetime(2019, 3, 31, 2, 0), IST, NORMAL),
+                (datetime(2019, 10, 26), IST, NORMAL),
+                (datetime(2019, 10, 27, 0, 59, fold=1), IST, NORMAL),
+                (datetime(2019, 10, 27, 1, 0, fold=0), IST, FOLD),
+                (datetime(2019, 10, 27, 1, 0, fold=1), GMT, FOLD),
+                (datetime(2019, 10, 27, 2, 0, fold=1), GMT, NORMAL),
+                (datetime(2020, 3, 29, 0, 59), GMT, NORMAL),
+                (datetime(2020, 3, 29, 2, 0), IST, NORMAL),
+                (datetime(2020, 10, 25, 0, 59, fold=1), IST, NORMAL),
+                (datetime(2020, 10, 25, 1, 0, fold=0), IST, FOLD),
+                (datetime(2020, 10, 25, 2, 0, fold=1), GMT, NORMAL),
+            )
+
+        @call
+        def _add():
+            # Pacific/Kosrae: Fixed offset zone with a quoted numerical tzname
+            tzstr = "<+11>-11"
+
+            cases[tzstr] = (
+                (
+                    datetime(2020, 1, 1),
+                    ZoneOffset("+11", timedelta(hours=11)),
+                    NORMAL,
+                ),
+            )
+
+        @call
+        def _add():
+            # Quoted STD and DST, transitions at 24:00
+            tzstr = "<-04>4<-03>,M9.1.6/24,M4.1.6/24"
+
+            M04 = ZoneOffset("-04", timedelta(hours=-4))
+            M03 = ZoneOffset("-03", timedelta(hours=-3), ONE_H)
+
+            cases[tzstr] = (
+                (datetime(2020, 5, 1), M04, NORMAL),
+                (datetime(2020, 11, 1), M03, NORMAL),
+            )
+
+        @call
+        def _add():
+            # Permanent daylight saving time is modeled with transitions at 0/0
+            # and J365/25, as mentioned in RFC 8536 Section 3.3.1
+            tzstr = "EST5EDT,0/0,J365/25"
+
+            EDT = ZoneOffset("EDT", timedelta(hours=-4), ONE_H)
+
+            cases[tzstr] = (
+                (datetime(2019, 1, 1), EDT, NORMAL),
+                (datetime(2019, 6, 1), EDT, NORMAL),
+                (datetime(2019, 12, 31, 23, 59, 59, 999999), EDT, NORMAL),
+                (datetime(2020, 1, 1), EDT, NORMAL),
+                (datetime(2020, 3, 1), EDT, NORMAL),
+                (datetime(2020, 6, 1), EDT, NORMAL),
+                (datetime(2020, 12, 31, 23, 59, 59, 999999), EDT, NORMAL),
+                (datetime(2400, 1, 1), EDT, NORMAL),
+                (datetime(2400, 3, 1), EDT, NORMAL),
+                (datetime(2400, 12, 31, 23, 59, 59, 999999), EDT, NORMAL),
+            )
+
+        @call
+        def _add():
+            # Transitions on March 1st and November 1st of each year
+            tzstr = "AAA3BBB,J60/12,J305/12"
+
+            AAA = ZoneOffset("AAA", timedelta(hours=-3))
+            BBB = ZoneOffset("BBB", timedelta(hours=-2), ONE_H)
+
+            cases[tzstr] = (
+                (datetime(2019, 1, 1), AAA, NORMAL),
+                (datetime(2019, 2, 28), AAA, NORMAL),
+                (datetime(2019, 3, 1, 11, 59), AAA, NORMAL),
+                (datetime(2019, 3, 1, 12, fold=0), AAA, GAP),
+                (datetime(2019, 3, 1, 12, fold=1), BBB, GAP),
+                (datetime(2019, 3, 1, 13), BBB, NORMAL),
+                (datetime(2019, 11, 1, 10, 59), BBB, NORMAL),
+                (datetime(2019, 11, 1, 11, fold=0), BBB, FOLD),
+                (datetime(2019, 11, 1, 11, fold=1), AAA, FOLD),
+                (datetime(2019, 11, 1, 12), AAA, NORMAL),
+                (datetime(2019, 12, 31, 23, 59, 59, 999999), AAA, NORMAL),
+                (datetime(2020, 1, 1), AAA, NORMAL),
+                (datetime(2020, 2, 29), AAA, NORMAL),
+                (datetime(2020, 3, 1, 11, 59), AAA, NORMAL),
+                (datetime(2020, 3, 1, 12, fold=0), AAA, GAP),
+                (datetime(2020, 3, 1, 12, fold=1), BBB, GAP),
+                (datetime(2020, 3, 1, 13), BBB, NORMAL),
+                (datetime(2020, 11, 1, 10, 59), BBB, NORMAL),
+                (datetime(2020, 11, 1, 11, fold=0), BBB, FOLD),
+                (datetime(2020, 11, 1, 11, fold=1), AAA, FOLD),
+                (datetime(2020, 11, 1, 12), AAA, NORMAL),
+                (datetime(2020, 12, 31, 23, 59, 59, 999999), AAA, NORMAL),
+            )
+
+        @call
+        def _add():
+            # Taken from America/Godthab, this rule has a transition on the
+            # Saturday before the last Sunday of March and October, at 22:00
+            # and 23:00, respectively. This is encoded with negative start
+            # and end transition times.
+            tzstr = "<-03>3<-02>,M3.5.0/-2,M10.5.0/-1"
+
+            N03 = ZoneOffset("-03", timedelta(hours=-3))
+            N02 = ZoneOffset("-02", timedelta(hours=-2), ONE_H)
+
+            cases[tzstr] = (
+                (datetime(2020, 3, 27), N03, NORMAL),
+                (datetime(2020, 3, 28, 21, 59, 59), N03, NORMAL),
+                (datetime(2020, 3, 28, 22, fold=0), N03, GAP),
+                (datetime(2020, 3, 28, 22, fold=1), N02, GAP),
+                (datetime(2020, 3, 28, 23), N02, NORMAL),
+                (datetime(2020, 10, 24, 21), N02, NORMAL),
+                (datetime(2020, 10, 24, 22, fold=0), N02, FOLD),
+                (datetime(2020, 10, 24, 22, fold=1), N03, FOLD),
+                (datetime(2020, 10, 24, 23), N03, NORMAL),
+            )
+
+        @call
+        def _add():
+            # Transition times with minutes and seconds
+            tzstr = "AAA3BBB,M3.2.0/01:30,M11.1.0/02:15:45"
+
+            AAA = ZoneOffset("AAA", timedelta(hours=-3))
+            BBB = ZoneOffset("BBB", timedelta(hours=-2), ONE_H)
+
+            cases[tzstr] = (
+                (datetime(2012, 3, 11, 1, 0), AAA, NORMAL),
+                (datetime(2012, 3, 11, 1, 30, fold=0), AAA, GAP),
+                (datetime(2012, 3, 11, 1, 30, fold=1), BBB, GAP),
+                (datetime(2012, 3, 11, 2, 30), BBB, NORMAL),
+                (datetime(2012, 11, 4, 1, 15, 44, 999999), BBB, NORMAL),
+                (datetime(2012, 11, 4, 1, 15, 45, fold=0), BBB, FOLD),
+                (datetime(2012, 11, 4, 1, 15, 45, fold=1), AAA, FOLD),
+                (datetime(2012, 11, 4, 2, 15, 45), AAA, NORMAL),
+            )
+
+        cls.test_cases = cases
+
+
+class CTZStrTest(TZStrTest):
+    module = c_zoneinfo
+
+
+class ZoneInfoCacheTest(TzPathUserMixin, ZoneInfoTestBase):
+    module = py_zoneinfo
+
+    def setUp(self):
+        self.klass.clear_cache()
+        super().setUp()
+
+    @property
+    def zoneinfo_data(self):
+        return ZONEINFO_DATA
+
+    @property
+    def tzpath(self):
+        return [self.zoneinfo_data.tzpath]
+
+    def test_ephemeral_zones(self):
+        self.assertIs(
+            self.klass("America/Los_Angeles"), self.klass("America/Los_Angeles")
+        )
+
+    def test_strong_refs(self):
+        tz0 = self.klass("Australia/Sydney")
+        tz1 = self.klass("Australia/Sydney")
+
+        self.assertIs(tz0, tz1)
+
+    def test_no_cache(self):
+
+        tz0 = self.klass("Europe/Lisbon")
+        tz1 = self.klass.no_cache("Europe/Lisbon")
+
+        self.assertIsNot(tz0, tz1)
+
+    def test_cache_reset_tzpath(self):
+        """Test that the cache persists when tzpath has been changed.
+
+        The PEP specifies that as long as a reference exists to one zone
+        with a given key, the primary constructor must continue to return
+        the same object.
+        """
+        zi0 = self.klass("America/Los_Angeles")
+        with self.tzpath_context([]):
+            zi1 = self.klass("America/Los_Angeles")
+
+        self.assertIs(zi0, zi1)
+
+    def test_clear_cache_explicit_none(self):
+        la0 = self.klass("America/Los_Angeles")
+        self.klass.clear_cache(only_keys=None)
+        la1 = self.klass("America/Los_Angeles")
+
+        self.assertIsNot(la0, la1)
+
+    def test_clear_cache_one_key(self):
+        """Tests that you can clear a single key from the cache."""
+        la0 = self.klass("America/Los_Angeles")
+        dub0 = self.klass("Europe/Dublin")
+
+        self.klass.clear_cache(only_keys=["America/Los_Angeles"])
+
+        la1 = self.klass("America/Los_Angeles")
+        dub1 = self.klass("Europe/Dublin")
+
+        self.assertIsNot(la0, la1)
+        self.assertIs(dub0, dub1)
+
+    def test_clear_cache_two_keys(self):
+        la0 = self.klass("America/Los_Angeles")
+        dub0 = self.klass("Europe/Dublin")
+        tok0 = self.klass("Asia/Tokyo")
+
+        self.klass.clear_cache(
+            only_keys=["America/Los_Angeles", "Europe/Dublin"]
+        )
+
+        la1 = self.klass("America/Los_Angeles")
+        dub1 = self.klass("Europe/Dublin")
+        tok1 = self.klass("Asia/Tokyo")
+
+        self.assertIsNot(la0, la1)
+        self.assertIsNot(dub0, dub1)
+        self.assertIs(tok0, tok1)
+
+
+class CZoneInfoCacheTest(ZoneInfoCacheTest):
+    module = c_zoneinfo
+
+
+class ZoneInfoPickleTest(TzPathUserMixin, ZoneInfoTestBase):
+    module = py_zoneinfo
+
+    def setUp(self):
+        self.klass.clear_cache()
+
+        with contextlib.ExitStack() as stack:
+            stack.enter_context(test_support.set_zoneinfo_module(self.module))
+            self.addCleanup(stack.pop_all().close)
+
+        super().setUp()
+
+    @property
+    def zoneinfo_data(self):
+        return ZONEINFO_DATA
+
+    @property
+    def tzpath(self):
+        return [self.zoneinfo_data.tzpath]
+
+    def test_cache_hit(self):
+        zi_in = self.klass("Europe/Dublin")
+        pkl = pickle.dumps(zi_in)
+        zi_rt = pickle.loads(pkl)
+
+        with self.subTest(test="Is non-pickled ZoneInfo"):
+            self.assertIs(zi_in, zi_rt)
+
+        zi_rt2 = pickle.loads(pkl)
+        with self.subTest(test="Is unpickled ZoneInfo"):
+            self.assertIs(zi_rt, zi_rt2)
+
+    def test_cache_miss(self):
+        zi_in = self.klass("Europe/Dublin")
+        pkl = pickle.dumps(zi_in)
+
+        del zi_in
+        self.klass.clear_cache()  # Induce a cache miss
+        zi_rt = pickle.loads(pkl)
+        zi_rt2 = pickle.loads(pkl)
+
+        self.assertIs(zi_rt, zi_rt2)
+
+    def test_no_cache(self):
+        zi_no_cache = self.klass.no_cache("Europe/Dublin")
+
+        pkl = pickle.dumps(zi_no_cache)
+        zi_rt = pickle.loads(pkl)
+
+        with self.subTest(test="Not the pickled object"):
+            self.assertIsNot(zi_rt, zi_no_cache)
+
+        zi_rt2 = pickle.loads(pkl)
+        with self.subTest(test="Not a second unpickled object"):
+            self.assertIsNot(zi_rt, zi_rt2)
+
+        zi_cache = self.klass("Europe/Dublin")
+        with self.subTest(test="Not a cached object"):
+            self.assertIsNot(zi_rt, zi_cache)
+
+    def test_from_file(self):
+        key = "Europe/Dublin"
+        with open(self.zoneinfo_data.path_from_key(key), "rb") as f:
+            zi_nokey = self.klass.from_file(f)
+
+            f.seek(0)
+            zi_key = self.klass.from_file(f, key=key)
+
+        test_cases = [
+            (zi_key, "ZoneInfo with key"),
+            (zi_nokey, "ZoneInfo without key"),
+        ]
+
+        for zi, test_name in test_cases:
+            with self.subTest(test_name=test_name):
+                with self.assertRaises(pickle.PicklingError):
+                    pickle.dumps(zi)
+
+    def test_pickle_after_from_file(self):
+        # This may be a bit of paranoia, but this test is to ensure that no
+        # global state is maintained in order to handle the pickle cache and
+        # from_file behavior, and that it is possible to interweave the
+        # constructors of each of these and pickling/unpickling without issues.
+        key = "Europe/Dublin"
+        zi = self.klass(key)
+
+        pkl_0 = pickle.dumps(zi)
+        zi_rt_0 = pickle.loads(pkl_0)
+        self.assertIs(zi, zi_rt_0)
+
+        with open(self.zoneinfo_data.path_from_key(key), "rb") as f:
+            zi_ff = self.klass.from_file(f, key=key)
+
+        pkl_1 = pickle.dumps(zi)
+        zi_rt_1 = pickle.loads(pkl_1)
+        self.assertIs(zi, zi_rt_1)
+
+        with self.assertRaises(pickle.PicklingError):
+            pickle.dumps(zi_ff)
+
+        pkl_2 = pickle.dumps(zi)
+        zi_rt_2 = pickle.loads(pkl_2)
+        self.assertIs(zi, zi_rt_2)
+
+
+class CZoneInfoPickleTest(ZoneInfoPickleTest):
+    module = c_zoneinfo
+
+
+class CallingConventionTest(ZoneInfoTestBase):
+    """Tests for functions with restricted calling conventions."""
+
+    module = py_zoneinfo
+
+    @property
+    def zoneinfo_data(self):
+        return ZONEINFO_DATA
+
+    def test_from_file(self):
+        with open(self.zoneinfo_data.path_from_key("UTC"), "rb") as f:
+            with self.assertRaises(TypeError):
+                self.klass.from_file(fobj=f)
+
+    def test_clear_cache(self):
+        with self.assertRaises(TypeError):
+            self.klass.clear_cache(["UTC"])
+
+
+class CCallingConventionTest(CallingConventionTest):
+    module = c_zoneinfo
+
+
+class TzPathTest(TzPathUserMixin, ZoneInfoTestBase):
+    module = py_zoneinfo
+
+    @staticmethod
+    @contextlib.contextmanager
+    def python_tzpath_context(value):
+        path_var = "PYTHONTZPATH"
+        try:
+            with OS_ENV_LOCK:
+                old_env = os.environ.get(path_var, None)
+                os.environ[path_var] = value
+                yield
+        finally:
+            if old_env is None:
+                del os.environ[path_var]
+            else:
+                os.environ[path_var] = old_env  # pragma: nocover
+
+    def test_env_variable(self):
+        """Tests that the environment variable works with reset_tzpath."""
+        new_paths = [
+            ("", []),
+            ("/etc/zoneinfo", ["/etc/zoneinfo"]),
+            (f"/a/b/c{os.pathsep}/d/e/f", ["/a/b/c", "/d/e/f"]),
+        ]
+
+        for new_path_var, expected_result in new_paths:
+            with self.python_tzpath_context(new_path_var):
+                with self.subTest(tzpath=new_path_var):
+                    self.module.reset_tzpath()
+                    tzpath = self.module.TZPATH
+                    self.assertSequenceEqual(tzpath, expected_result)
+
+    def test_env_variable_relative_paths(self):
+        test_cases = [
+            [("path/to/somewhere",), ()],
+            [
+                ("/usr/share/zoneinfo", "path/to/somewhere",),
+                ("/usr/share/zoneinfo",),
+            ],
+            [("../relative/path",), ()],
+            [
+                ("/usr/share/zoneinfo", "../relative/path",),
+                ("/usr/share/zoneinfo",),
+            ],
+            [("path/to/somewhere", "../relative/path",), ()],
+            [
+                (
+                    "/usr/share/zoneinfo",
+                    "path/to/somewhere",
+                    "../relative/path",
+                ),
+                ("/usr/share/zoneinfo",),
+            ],
+        ]
+
+        for input_paths, expected_paths in test_cases:
+            path_var = os.pathsep.join(input_paths)
+            with self.python_tzpath_context(path_var):
+                with self.subTest("warning", path_var=path_var):
+                    # Note: Per PEP 615 the warning is implementation-defined
+                    # behavior, other implementations need not warn.
+                    with self.assertWarns(self.module.InvalidTZPathWarning):
+                        self.module.reset_tzpath()
+
+                tzpath = self.module.TZPATH
+                with self.subTest("filtered", path_var=path_var):
+                    self.assertSequenceEqual(tzpath, expected_paths)
+
+    def test_reset_tzpath_kwarg(self):
+        self.module.reset_tzpath(to=["/a/b/c"])
+
+        self.assertSequenceEqual(self.module.TZPATH, ("/a/b/c",))
+
+    def test_reset_tzpath_relative_paths(self):
+        bad_values = [
+            ("path/to/somewhere",),
+            ("/usr/share/zoneinfo", "path/to/somewhere",),
+            ("../relative/path",),
+            ("/usr/share/zoneinfo", "../relative/path",),
+            ("path/to/somewhere", "../relative/path",),
+            ("/usr/share/zoneinfo", "path/to/somewhere", "../relative/path",),
+        ]
+        for input_paths in bad_values:
+            with self.subTest(input_paths=input_paths):
+                with self.assertRaises(ValueError):
+                    self.module.reset_tzpath(to=input_paths)
+
+    def test_tzpath_type_error(self):
+        bad_values = [
+            "/etc/zoneinfo:/usr/share/zoneinfo",
+            b"/etc/zoneinfo:/usr/share/zoneinfo",
+            0,
+        ]
+
+        for bad_value in bad_values:
+            with self.subTest(value=bad_value):
+                with self.assertRaises(TypeError):
+                    self.module.reset_tzpath(bad_value)
+
+    def test_tzpath_attribute(self):
+        tzpath_0 = ["/one", "/two"]
+        tzpath_1 = ["/three"]
+
+        with self.tzpath_context(tzpath_0):
+            query_0 = self.module.TZPATH
+
+        with self.tzpath_context(tzpath_1):
+            query_1 = self.module.TZPATH
+
+        self.assertSequenceEqual(tzpath_0, query_0)
+        self.assertSequenceEqual(tzpath_1, query_1)
+
+
+class CTzPathTest(TzPathTest):
+    module = c_zoneinfo
+
+
+class TestModule(ZoneInfoTestBase):
+    module = py_zoneinfo
+
+    def test_getattr_error(self):
+        with self.assertRaises(AttributeError):
+            self.module.NOATTRIBUTE
+
+    def test_dir_contains_all(self):
+        """dir(self.module) should at least contain everything in __all__."""
+        module_all_set = set(self.module.__all__)
+        module_dir_set = set(dir(self.module))
+
+        difference = module_all_set - module_dir_set
+
+        self.assertFalse(difference)
+
+    def test_dir_unique(self):
+        """Test that there are no duplicates in dir(self.module)"""
+        module_dir = dir(self.module)
+        module_unique = set(module_dir)
+
+        self.assertCountEqual(module_dir, module_unique)
+
+
+class CTestModule(TestModule):
+    module = c_zoneinfo
+
+
+class ExtensionBuiltTest(unittest.TestCase):
+    """Smoke test to ensure that the C and Python extensions are both tested.
+
+    Because the intention is for the Python and C versions of ZoneInfo to
+    behave identically, these tests necessarily rely on implementation details,
+    so the tests may need to be adjusted if the implementations change. Do not
+    rely on these tests as an indication of stable properties of these classes.
+    """
+
+    def test_cache_location(self):
+        # The pure Python version stores caches on attributes, but the C
+        # extension stores them in C globals (at least for now)
+        self.assertFalse(hasattr(c_zoneinfo.ZoneInfo, "_weak_cache"))
+        self.assertTrue(hasattr(py_zoneinfo.ZoneInfo, "_weak_cache"))
+
+    def test_gc_tracked(self):
+        # The pure Python version is tracked by the GC but (for now) the C
+        # version is not.
+        import gc
+
+        self.assertTrue(gc.is_tracked(py_zoneinfo.ZoneInfo))
+        self.assertFalse(gc.is_tracked(c_zoneinfo.ZoneInfo))
+
+
+@dataclasses.dataclass(frozen=True)
+class ZoneOffset:
+    tzname: str
+    utcoffset: timedelta
+    dst: timedelta = ZERO
+
+
+@dataclasses.dataclass(frozen=True)
+class ZoneTransition:
+    transition: datetime
+    offset_before: ZoneOffset
+    offset_after: ZoneOffset
+
+    @property
+    def transition_utc(self):
+        return (self.transition - self.offset_before.utcoffset).replace(
+            tzinfo=timezone.utc
+        )
+
+    @property
+    def fold(self):
+        """Whether this introduces a fold"""
+        return self.offset_before.utcoffset > self.offset_after.utcoffset
+
+    @property
+    def gap(self):
+        """Whether this introduces a gap"""
+        return self.offset_before.utcoffset < self.offset_after.utcoffset
+
+    @property
+    def delta(self):
+        return self.offset_after.utcoffset - self.offset_before.utcoffset
+
+    @property
+    def anomaly_start(self):
+        if self.fold:
+            return self.transition + self.delta
+        else:
+            return self.transition
+
+    @property
+    def anomaly_end(self):
+        if not self.fold:
+            return self.transition + self.delta
+        else:
+            return self.transition
+
+
+class ZoneInfoData:
+    def __init__(self, source_json, tzpath, v1=False):
+        self.tzpath = pathlib.Path(tzpath)
+        self.keys = []
+        self.v1 = v1
+        self._populate_tzpath(source_json)
+
+    def path_from_key(self, key):
+        return self.tzpath / key
+
+    def _populate_tzpath(self, source_json):
+        with open(source_json, "rb") as f:
+            zoneinfo_dict = json.load(f)
+
+        zoneinfo_data = zoneinfo_dict["data"]
+
+        for key, value in zoneinfo_data.items():
+            self.keys.append(key)
+            raw_data = self._decode_text(value)
+
+            if self.v1:
+                data = self._convert_to_v1(raw_data)
+            else:
+                data = raw_data
+
+            destination = self.path_from_key(key)
+            destination.parent.mkdir(exist_ok=True, parents=True)
+            with open(destination, "wb") as f:
+                f.write(data)
+
+    def _decode_text(self, contents):
+        raw_data = b"".join(map(str.encode, contents))
+        decoded = base64.b85decode(raw_data)
+
+        return lzma.decompress(decoded)
+
+    def _convert_to_v1(self, contents):
+        assert contents[0:4] == b"TZif", "Invalid TZif data found!"
+        version = int(contents[4:5])
+
+        header_start = 4 + 16
+        header_end = header_start + 24  # 6l == 24 bytes
+        assert version >= 2, "Version 1 file found: no conversion necessary"
+        isutcnt, isstdcnt, leapcnt, timecnt, typecnt, charcnt = struct.unpack(
+            ">6l", contents[header_start:header_end]
+        )
+
+        file_size = (
+            timecnt * 5
+            + typecnt * 6
+            + charcnt
+            + leapcnt * 8
+            + isstdcnt
+            + isutcnt
+        )
+        file_size += header_end
+        out = b"TZif" + b"\x00" + contents[5:file_size]
+
+        assert (
+            contents[file_size : (file_size + 4)] == b"TZif"
+        ), "Version 2 file not truncated at Version 2 header"
+
+        return out
+
+
+class ZoneDumpData:
+    @classmethod
+    def transition_keys(cls):
+        return cls._get_zonedump().keys()
+
+    @classmethod
+    def load_transition_examples(cls, key):
+        return cls._get_zonedump()[key]
+
+    @classmethod
+    def fixed_offset_zones(cls):
+        if not cls._FIXED_OFFSET_ZONES:
+            cls._populate_fixed_offsets()
+
+        return cls._FIXED_OFFSET_ZONES.items()
+
+    @classmethod
+    def _get_zonedump(cls):
+        if not cls._ZONEDUMP_DATA:
+            cls._populate_zonedump_data()
+        return cls._ZONEDUMP_DATA
+
+    @classmethod
+    def _populate_fixed_offsets(cls):
+        cls._FIXED_OFFSET_ZONES = {
+            "UTC": ZoneOffset("UTC", ZERO, ZERO),
+        }
+
+    @classmethod
+    def _populate_zonedump_data(cls):
+        def _Africa_Abidjan():
+            LMT = ZoneOffset("LMT", timedelta(seconds=-968))
+            GMT = ZoneOffset("GMT", ZERO)
+
+            return [
+                ZoneTransition(datetime(1912, 1, 1), LMT, GMT),
+            ]
+
+        def _Africa_Casablanca():
+            P00_s = ZoneOffset("+00", ZERO, ZERO)
+            P01_d = ZoneOffset("+01", ONE_H, ONE_H)
+            P00_d = ZoneOffset("+00", ZERO, -ONE_H)
+            P01_s = ZoneOffset("+01", ONE_H, ZERO)
+
+            return [
+                # Morocco sometimes pauses DST during Ramadan
+                ZoneTransition(datetime(2018, 3, 25, 2), P00_s, P01_d),
+                ZoneTransition(datetime(2018, 5, 13, 3), P01_d, P00_s),
+                ZoneTransition(datetime(2018, 6, 17, 2), P00_s, P01_d),
+                # On October 28th Morocco set standard time to +01,
+                # with negative DST only during Ramadan
+                ZoneTransition(datetime(2018, 10, 28, 3), P01_d, P01_s),
+                ZoneTransition(datetime(2019, 5, 5, 3), P01_s, P00_d),
+                ZoneTransition(datetime(2019, 6, 9, 2), P00_d, P01_s),
+            ]
+
+        def _America_Los_Angeles():
+            LMT = ZoneOffset("LMT", timedelta(seconds=-28378), ZERO)
+            PST = ZoneOffset("PST", timedelta(hours=-8), ZERO)
+            PDT = ZoneOffset("PDT", timedelta(hours=-7), ONE_H)
+            PWT = ZoneOffset("PWT", timedelta(hours=-7), ONE_H)
+            PPT = ZoneOffset("PPT", timedelta(hours=-7), ONE_H)
+
+            return [
+                ZoneTransition(datetime(1883, 11, 18, 12, 7, 2), LMT, PST),
+                ZoneTransition(datetime(1918, 3, 31, 2), PST, PDT),
+                ZoneTransition(datetime(1918, 3, 31, 2), PST, PDT),
+                ZoneTransition(datetime(1918, 10, 27, 2), PDT, PST),
+                # Transition to Pacific War Time
+                ZoneTransition(datetime(1942, 2, 9, 2), PST, PWT),
+                # Transition from Pacific War Time to Pacific Peace Time
+                ZoneTransition(datetime(1945, 8, 14, 16), PWT, PPT),
+                ZoneTransition(datetime(1945, 9, 30, 2), PPT, PST),
+                ZoneTransition(datetime(2015, 3, 8, 2), PST, PDT),
+                ZoneTransition(datetime(2015, 11, 1, 2), PDT, PST),
+                # After 2038: Rules continue indefinitely
+                ZoneTransition(datetime(2450, 3, 13, 2), PST, PDT),
+                ZoneTransition(datetime(2450, 11, 6, 2), PDT, PST),
+            ]
+
+        def _America_Santiago():
+            LMT = ZoneOffset("LMT", timedelta(seconds=-16966), ZERO)
+            SMT = ZoneOffset("SMT", timedelta(seconds=-16966), ZERO)
+            N05 = ZoneOffset("-05", timedelta(seconds=-18000), ZERO)
+            N04 = ZoneOffset("-04", timedelta(seconds=-14400), ZERO)
+            N03 = ZoneOffset("-03", timedelta(seconds=-10800), ONE_H)
+
+            return [
+                ZoneTransition(datetime(1890, 1, 1), LMT, SMT),
+                ZoneTransition(datetime(1910, 1, 10), SMT, N05),
+                ZoneTransition(datetime(1916, 7, 1), N05, SMT),
+                ZoneTransition(datetime(2008, 3, 30), N03, N04),
+                ZoneTransition(datetime(2008, 10, 12), N04, N03),
+                ZoneTransition(datetime(2040, 4, 8), N03, N04),
+                ZoneTransition(datetime(2040, 9, 2), N04, N03),
+            ]
+
+        def _Asia_Tokyo():
+            JST = ZoneOffset("JST", timedelta(seconds=32400), ZERO)
+            JDT = ZoneOffset("JDT", timedelta(seconds=36000), ONE_H)
+
+            # Japan had DST from 1948 to 1951, and it was unusual in that
+            # the transition from DST to STD occurred at 25:00, and is
+            # denominated as such in the time zone database
+            return [
+                ZoneTransition(datetime(1948, 5, 2), JST, JDT),
+                ZoneTransition(datetime(1948, 9, 12, 1), JDT, JST),
+                ZoneTransition(datetime(1951, 9, 9, 1), JDT, JST),
+            ]
+
+        def _Australia_Sydney():
+            LMT = ZoneOffset("LMT", timedelta(seconds=36292), ZERO)
+            AEST = ZoneOffset("AEST", timedelta(seconds=36000), ZERO)
+            AEDT = ZoneOffset("AEDT", timedelta(seconds=39600), ONE_H)
+
+            return [
+                ZoneTransition(datetime(1895, 2, 1), LMT, AEST),
+                ZoneTransition(datetime(1917, 1, 1, 0, 1), AEST, AEDT),
+                ZoneTransition(datetime(1917, 3, 25, 2), AEDT, AEST),
+                ZoneTransition(datetime(2012, 4, 1, 3), AEDT, AEST),
+                ZoneTransition(datetime(2012, 10, 7, 2), AEST, AEDT),
+                ZoneTransition(datetime(2040, 4, 1, 3), AEDT, AEST),
+                ZoneTransition(datetime(2040, 10, 7, 2), AEST, AEDT),
+            ]
+
+        def _Europe_Dublin():
+            LMT = ZoneOffset("LMT", timedelta(seconds=-1500), ZERO)
+            DMT = ZoneOffset("DMT", timedelta(seconds=-1521), ZERO)
+            IST_0 = ZoneOffset("IST", timedelta(seconds=2079), ONE_H)
+            GMT_0 = ZoneOffset("GMT", ZERO, ZERO)
+            BST = ZoneOffset("BST", ONE_H, ONE_H)
+            GMT_1 = ZoneOffset("GMT", ZERO, -ONE_H)
+            IST_1 = ZoneOffset("IST", ONE_H, ZERO)
+
+            return [
+                ZoneTransition(datetime(1880, 8, 2, 0), LMT, DMT),
+                ZoneTransition(datetime(1916, 5, 21, 2), DMT, IST_0),
+                ZoneTransition(datetime(1916, 10, 1, 3), IST_0, GMT_0),
+                ZoneTransition(datetime(1917, 4, 8, 2), GMT_0, BST),
+                ZoneTransition(datetime(2016, 3, 27, 1), GMT_1, IST_1),
+                ZoneTransition(datetime(2016, 10, 30, 2), IST_1, GMT_1),
+                ZoneTransition(datetime(2487, 3, 30, 1), GMT_1, IST_1),
+                ZoneTransition(datetime(2487, 10, 26, 2), IST_1, GMT_1),
+            ]
+
+        def _Europe_Lisbon():
+            WET = ZoneOffset("WET", ZERO, ZERO)
+            WEST = ZoneOffset("WEST", ONE_H, ONE_H)
+            CET = ZoneOffset("CET", ONE_H, ZERO)
+            CEST = ZoneOffset("CEST", timedelta(seconds=7200), ONE_H)
+
+            return [
+                ZoneTransition(datetime(1992, 3, 29, 1), WET, WEST),
+                ZoneTransition(datetime(1992, 9, 27, 2), WEST, CET),
+                ZoneTransition(datetime(1993, 3, 28, 2), CET, CEST),
+                ZoneTransition(datetime(1993, 9, 26, 3), CEST, CET),
+                ZoneTransition(datetime(1996, 3, 31, 2), CET, WEST),
+                ZoneTransition(datetime(1996, 10, 27, 2), WEST, WET),
+            ]
+
+        def _Europe_London():
+            LMT = ZoneOffset("LMT", timedelta(seconds=-75), ZERO)
+            GMT = ZoneOffset("GMT", ZERO, ZERO)
+            BST = ZoneOffset("BST", ONE_H, ONE_H)
+
+            return [
+                ZoneTransition(datetime(1847, 12, 1), LMT, GMT),
+                ZoneTransition(datetime(2005, 3, 27, 1), GMT, BST),
+                ZoneTransition(datetime(2005, 10, 30, 2), BST, GMT),
+                ZoneTransition(datetime(2043, 3, 29, 1), GMT, BST),
+                ZoneTransition(datetime(2043, 10, 25, 2), BST, GMT),
+            ]
+
+        def _Pacific_Kiritimati():
+            LMT = ZoneOffset("LMT", timedelta(seconds=-37760), ZERO)
+            N1040 = ZoneOffset("-1040", timedelta(seconds=-38400), ZERO)
+            N10 = ZoneOffset("-10", timedelta(seconds=-36000), ZERO)
+            P14 = ZoneOffset("+14", timedelta(seconds=50400), ZERO)
+
+            # This is literally every transition in Christmas Island history
+            return [
+                ZoneTransition(datetime(1901, 1, 1), LMT, N1040),
+                ZoneTransition(datetime(1979, 10, 1), N1040, N10),
+                # They skipped December 31, 1994
+                ZoneTransition(datetime(1994, 12, 31), N10, P14),
+            ]
+
+        cls._ZONEDUMP_DATA = {
+            "Africa/Abidjan": _Africa_Abidjan(),
+            "Africa/Casablanca": _Africa_Casablanca(),
+            "America/Los_Angeles": _America_Los_Angeles(),
+            "America/Santiago": _America_Santiago(),
+            "Australia/Sydney": _Australia_Sydney(),
+            "Asia/Tokyo": _Asia_Tokyo(),
+            "Europe/Dublin": _Europe_Dublin(),
+            "Europe/Lisbon": _Europe_Lisbon(),
+            "Europe/London": _Europe_London(),
+            "Pacific/Kiritimati": _Pacific_Kiritimati(),
+        }
+
+    _ZONEDUMP_DATA = None
+    _FIXED_OFFSET_ZONES = None
diff --git a/Lib/zoneinfo/__init__.py b/Lib/zoneinfo/__init__.py
new file mode 100644
index 0000000..81a2d5e
--- /dev/null
+++ b/Lib/zoneinfo/__init__.py
@@ -0,0 +1,29 @@
+__all__ = [
+    "ZoneInfo",
+    "reset_tzpath",
+    "TZPATH",
+    "ZoneInfoNotFoundError",
+    "InvalidTZPathWarning",
+]
+
+from . import _tzpath
+from ._common import ZoneInfoNotFoundError
+
+try:
+    from _zoneinfo import ZoneInfo
+except ImportError:  # pragma: nocover
+    from ._zoneinfo import ZoneInfo
+
+reset_tzpath = _tzpath.reset_tzpath
+InvalidTZPathWarning = _tzpath.InvalidTZPathWarning
+
+
+def __getattr__(name):
+    if name == "TZPATH":
+        return _tzpath.TZPATH
+    else:
+        raise AttributeError(f"module {__name__!r} has no attribute {name!r}")
+
+
+def __dir__():
+    return sorted(list(globals()) + ["TZPATH"])
diff --git a/Lib/zoneinfo/_common.py b/Lib/zoneinfo/_common.py
new file mode 100644
index 0000000..3d35d4f
--- /dev/null
+++ b/Lib/zoneinfo/_common.py
@@ -0,0 +1,166 @@
+import struct
+
+
+def load_tzdata(key):
+    import importlib.resources
+
+    components = key.split("/")
+    package_name = ".".join(["tzdata.zoneinfo"] + components[:-1])
+    resource_name = components[-1]
+
+    try:
+        return importlib.resources.open_binary(package_name, resource_name)
+    except (ImportError, FileNotFoundError, UnicodeEncodeError):
+        # There are three types of exception that can be raised that all amount
+        # to "we cannot find this key":
+        #
+        # ImportError: If package_name doesn't exist (e.g. if tzdata is not
+        #   installed, or if there's an error in the folder name like
+        #   Amrica/New_York)
+        # FileNotFoundError: If resource_name doesn't exist in the package
+        #   (e.g. Europe/Krasnoy)
+        # UnicodeEncodeError: If package_name or resource_name are not UTF-8,
+        #   such as keys containing a surrogate character.
+        raise ZoneInfoNotFoundError(f"No time zone found with key {key}")
+
+
+def load_data(fobj):
+    header = _TZifHeader.from_file(fobj)
+
+    if header.version == 1:
+        time_size = 4
+        time_type = "l"
+    else:
+        # Version 2+ has 64-bit integer transition times
+        time_size = 8
+        time_type = "q"
+
+        # Version 2+ also starts with a Version 1 header and data, which
+        # we need to skip now
+        skip_bytes = (
+            header.timecnt * 5  # Transition times and types
+            + header.typecnt * 6  # Local time type records
+            + header.charcnt  # Time zone designations
+            + header.leapcnt * 8  # Leap second records
+            + header.isstdcnt  # Standard/wall indicators
+            + header.isutcnt  # UT/local indicators
+        )
+
+        fobj.seek(skip_bytes, 1)
+
+        # Now we need to read the second header, which is not the same
+        # as the first
+        header = _TZifHeader.from_file(fobj)
+
+    typecnt = header.typecnt
+    timecnt = header.timecnt
+    charcnt = header.charcnt
+
+    # The data portion starts with timecnt transitions and indices
+    if timecnt:
+        trans_list_utc = struct.unpack(
+            f">{timecnt}{time_type}", fobj.read(timecnt * time_size)
+        )
+        trans_idx = struct.unpack(f">{timecnt}B", fobj.read(timecnt))
+    else:
+        trans_list_utc = ()
+        trans_idx = ()
+
+    # Read the ttinfo struct, (utoff, isdst, abbrind)
+    if typecnt:
+        utcoff, isdst, abbrind = zip(
+            *(struct.unpack(">lbb", fobj.read(6)) for i in range(typecnt))
+        )
+    else:
+        utcoff = ()
+        isdst = ()
+        abbrind = ()
+
+    # Now read the abbreviations. They are null-terminated strings, indexed
+    # not by position in the array but by position in the unsplit
+    # abbreviation string. I suppose this makes more sense in C, which uses
+    # null to terminate the strings, but it's inconvenient here...
+    char_total = 0
+    abbr_vals = {}
+    abbr_chars = fobj.read(charcnt)
+
+    def get_abbr(idx):
+        # Gets a string starting at idx and running until the next \x00
+        #
+        # We cannot pre-populate abbr_vals by splitting on \x00 because there
+        # are some zones that use subsets of longer abbreviations, like so:
+        #
+        #  LMT\x00AHST\x00HDT\x00
+        #
+        # Where the idx to abbr mapping should be:
+        #
+        # {0: "LMT", 4: "AHST", 5: "HST", 9: "HDT"}
+        if idx not in abbr_vals:
+            span_end = abbr_chars.find(b"\x00", idx)
+            abbr_vals[idx] = abbr_chars[idx:span_end].decode()
+
+        return abbr_vals[idx]
+
+    abbr = tuple(get_abbr(idx) for idx in abbrind)
+
+    # The remainder of the file consists of leap seconds (currently unused) and
+    # the standard/wall and ut/local indicators, which are metadata we don't need.
+    # In version 2 files, we need to skip the unnecessary data to get at the TZ string:
+    if header.version >= 2:
+        # Each leap second record has size (time_size + 4)
+        skip_bytes = header.isutcnt + header.isstdcnt + header.leapcnt * 12
+        fobj.seek(skip_bytes, 1)
+
+        c = fobj.read(1)  # Should be \n
+        assert c == b"\n", c
+
+        tz_bytes = b""
+        while (c := fobj.read(1)) != b"\n":
+            tz_bytes += c
+
+        tz_str = tz_bytes
+    else:
+        tz_str = None
+
+    return trans_idx, trans_list_utc, utcoff, isdst, abbr, tz_str
+
+
+class _TZifHeader:
+    __slots__ = [
+        "version",
+        "isutcnt",
+        "isstdcnt",
+        "leapcnt",
+        "timecnt",
+        "typecnt",
+        "charcnt",
+    ]
+
+    def __init__(self, *args):
+        assert len(self.__slots__) == len(args)
+        for attr, val in zip(self.__slots__, args):
+            setattr(self, attr, val)
+
+    @classmethod
+    def from_file(cls, stream):
+        # The header starts with a 4-byte "magic" value
+        if stream.read(4) != b"TZif":
+            raise ValueError("Invalid TZif file: magic not found")
+
+        _version = stream.read(1)
+        if _version == b"\x00":
+            version = 1
+        else:
+            version = int(_version)
+        stream.read(15)
+
+        args = (version,)
+
+        # Slots are defined in the order that the bytes are arranged
+        args = args + struct.unpack(">6l", stream.read(24))
+
+        return cls(*args)
+
+
+class ZoneInfoNotFoundError(KeyError):
+    """Exception raised when a ZoneInfo key is not found."""
diff --git a/Lib/zoneinfo/_tzpath.py b/Lib/zoneinfo/_tzpath.py
new file mode 100644
index 0000000..8cff0b1
--- /dev/null
+++ b/Lib/zoneinfo/_tzpath.py
@@ -0,0 +1,110 @@
+import os
+import sys
+import sysconfig
+
+
+def reset_tzpath(to=None):
+    global TZPATH
+
+    tzpaths = to
+    if tzpaths is not None:
+        if isinstance(tzpaths, (str, bytes)):
+            raise TypeError(
+                f"tzpaths must be a list or tuple, "
+                + f"not {type(tzpaths)}: {tzpaths!r}"
+            )
+        elif not all(map(os.path.isabs, tzpaths)):
+            raise ValueError(_get_invalid_paths_message(tzpaths))
+        base_tzpath = tzpaths
+    else:
+        env_var = os.environ.get("PYTHONTZPATH", None)
+        if env_var is not None:
+            base_tzpath = _parse_python_tzpath(env_var)
+        else:
+            base_tzpath = _parse_python_tzpath(
+                sysconfig.get_config_var("TZPATH")
+            )
+
+    TZPATH = tuple(base_tzpath)
+
+
+def _parse_python_tzpath(env_var):
+    if not env_var:
+        return ()
+
+    raw_tzpath = env_var.split(os.pathsep)
+    new_tzpath = tuple(filter(os.path.isabs, raw_tzpath))
+
+    # If anything has been filtered out, we will warn about it
+    if len(new_tzpath) != len(raw_tzpath):
+        import warnings
+
+        msg = _get_invalid_paths_message(raw_tzpath)
+
+        warnings.warn(
+            "Invalid paths specified in PYTHONTZPATH environment variable."
+            + msg,
+            InvalidTZPathWarning,
+        )
+
+    return new_tzpath
+
+
+def _get_invalid_paths_message(tzpaths):
+    invalid_paths = (path for path in tzpaths if not os.path.isabs(path))
+
+    prefix = "\n    "
+    indented_str = prefix + prefix.join(invalid_paths)
+
+    return (
+        "Paths should be absolute but found the following relative paths:"
+        + indented_str
+    )
+
+
+def find_tzfile(key):
+    """Retrieve the path to a TZif file from a key."""
+    _validate_tzfile_path(key)
+    for search_path in TZPATH:
+        filepath = os.path.join(search_path, key)
+        if os.path.isfile(filepath):
+            return filepath
+
+    return None
+
+
+_TEST_PATH = os.path.normpath(os.path.join("_", "_"))[:-1]
+
+
+def _validate_tzfile_path(path, _base=_TEST_PATH):
+    if os.path.isabs(path):
+        raise ValueError(
+            f"ZoneInfo keys may not be absolute paths, got: {path}"
+        )
+
+    # We only care about the kinds of path normalizations that would change the
+    # length of the key - e.g. a/../b -> a/b, or a/b/ -> a/b. On Windows,
+    # normpath will also change from a/b to a\b, but that would still preserve
+    # the length.
+    new_path = os.path.normpath(path)
+    if len(new_path) != len(path):
+        raise ValueError(
+            f"ZoneInfo keys must be normalized relative paths, got: {path}"
+        )
+
+    resolved = os.path.normpath(os.path.join(_base, new_path))
+    if not resolved.startswith(_base):
+        raise ValueError(
+            f"ZoneInfo keys must refer to subdirectories of TZPATH, got: {path}"
+        )
+
+
+del _TEST_PATH
+
+
+class InvalidTZPathWarning(RuntimeWarning):
+    """Warning raised if an invalid path is specified in PYTHONTZPATH."""
+
+
+TZPATH = ()
+reset_tzpath()
diff --git a/Lib/zoneinfo/_zoneinfo.py b/Lib/zoneinfo/_zoneinfo.py
new file mode 100644
index 0000000..69133ae
--- /dev/null
+++ b/Lib/zoneinfo/_zoneinfo.py
@@ -0,0 +1,755 @@
+import bisect
+import calendar
+import collections
+import functools
+import os
+import re
+import struct
+import sys
+import weakref
+from datetime import datetime, timedelta, timezone, tzinfo
+
+from . import _common, _tzpath
+
+EPOCH = datetime(1970, 1, 1)
+EPOCHORDINAL = datetime(1970, 1, 1).toordinal()
+
+# It is relatively expensive to construct new timedelta objects, and in most
+# cases we're looking at the same deltas, like integer numbers of hours, etc.
+# To improve speed and memory use, we'll keep a dictionary with references
+# to the ones we've already used so far.
+#
+# Loading every time zone in the 2020a version of the time zone database
+# requires 447 timedeltas, which requires approximately the amount of space
+# that ZoneInfo("America/New_York") with 236 transitions takes up, so we will
+# set the cache size to 512 so that in the common case we always get cache
+# hits, but specifically crafted ZoneInfo objects don't leak arbitrary amounts
+# of memory.
+@functools.lru_cache(maxsize=512)
+def _load_timedelta(seconds):
+    return timedelta(seconds=seconds)
+
+
+class ZoneInfo(tzinfo):
+    _strong_cache_size = 8
+    _strong_cache = collections.OrderedDict()
+    _weak_cache = weakref.WeakValueDictionary()
+    __module__ = "zoneinfo"
+
+    def __init_subclass__(cls):
+        cls._strong_cache = collections.OrderedDict()
+        cls._weak_cache = weakref.WeakValueDictionary()
+
+    def __new__(cls, key):
+        instance = cls._weak_cache.get(key, None)
+        if instance is None:
+            instance = cls._weak_cache.setdefault(key, cls._new_instance(key))
+            instance._from_cache = True
+
+        # Update the "strong" cache
+        cls._strong_cache[key] = cls._strong_cache.pop(key, instance)
+
+        if len(cls._strong_cache) > cls._strong_cache_size:
+            cls._strong_cache.popitem(last=False)
+
+        return instance
+
+    @classmethod
+    def no_cache(cls, key):
+        obj = cls._new_instance(key)
+        obj._from_cache = False
+
+        return obj
+
+    @classmethod
+    def _new_instance(cls, key):
+        obj = super().__new__(cls)
+        obj._key = key
+        obj._file_path = obj._find_tzfile(key)
+
+        if obj._file_path is not None:
+            file_obj = open(obj._file_path, "rb")
+        else:
+            file_obj = _common.load_tzdata(key)
+
+        with file_obj as f:
+            obj._load_file(f)
+
+        return obj
+
+    @classmethod
+    def from_file(cls, fobj, /, key=None):
+        obj = super().__new__(cls)
+        obj._key = key
+        obj._file_path = None
+        obj._load_file(fobj)
+        obj._file_repr = repr(fobj)
+
+        # Disable pickling for objects created from files
+        obj.__reduce__ = obj._file_reduce
+
+        return obj
+
+    @classmethod
+    def clear_cache(cls, *, only_keys=None):
+        if only_keys is not None:
+            for key in only_keys:
+                cls._weak_cache.pop(key, None)
+                cls._strong_cache.pop(key, None)
+
+        else:
+            cls._weak_cache.clear()
+            cls._strong_cache.clear()
+
+    @property
+    def key(self):
+        return self._key
+
+    def utcoffset(self, dt):
+        return self._find_trans(dt).utcoff
+
+    def dst(self, dt):
+        return self._find_trans(dt).dstoff
+
+    def tzname(self, dt):
+        return self._find_trans(dt).tzname
+
+    def fromutc(self, dt):
+        """Convert from datetime in UTC to datetime in local time"""
+
+        if not isinstance(dt, datetime):
+            raise TypeError("fromutc() requires a datetime argument")
+        if dt.tzinfo is not self:
+            raise ValueError("dt.tzinfo is not self")
+
+        timestamp = self._get_local_timestamp(dt)
+        num_trans = len(self._trans_utc)
+
+        if num_trans >= 1 and timestamp < self._trans_utc[0]:
+            tti = self._tti_before
+            fold = 0
+        elif (
+            num_trans == 0 or timestamp > self._trans_utc[-1]
+        ) and not isinstance(self._tz_after, _ttinfo):
+            tti, fold = self._tz_after.get_trans_info_fromutc(
+                timestamp, dt.year
+            )
+        elif num_trans == 0:
+            tti = self._tz_after
+            fold = 0
+        else:
+            idx = bisect.bisect_right(self._trans_utc, timestamp)
+
+            if num_trans > 1 and timestamp >= self._trans_utc[1]:
+                tti_prev, tti = self._ttinfos[idx - 2 : idx]
+            elif timestamp > self._trans_utc[-1]:
+                tti_prev = self._ttinfos[-1]
+                tti = self._tz_after
+            else:
+                tti_prev = self._tti_before
+                tti = self._ttinfos[0]
+
+            # Detect fold
+            shift = tti_prev.utcoff - tti.utcoff
+            fold = shift.total_seconds() > timestamp - self._trans_utc[idx - 1]
+        dt += tti.utcoff
+        if fold:
+            return dt.replace(fold=1)
+        else:
+            return dt
+
+    def _find_trans(self, dt):
+        if dt is None:
+            if self._fixed_offset:
+                return self._tz_after
+            else:
+                return _NO_TTINFO
+
+        ts = self._get_local_timestamp(dt)
+
+        lt = self._trans_local[dt.fold]
+
+        num_trans = len(lt)
+
+        if num_trans and ts < lt[0]:
+            return self._tti_before
+        elif not num_trans or ts > lt[-1]:
+            if isinstance(self._tz_after, _TZStr):
+                return self._tz_after.get_trans_info(ts, dt.year, dt.fold)
+            else:
+                return self._tz_after
+        else:
+            # idx is the transition that occurs after this timestamp, so we
+            # subtract off 1 to get the current ttinfo
+            idx = bisect.bisect_right(lt, ts) - 1
+            assert idx >= 0
+            return self._ttinfos[idx]
+
+    def _get_local_timestamp(self, dt):
+        return (
+            (dt.toordinal() - EPOCHORDINAL) * 86400
+            + dt.hour * 3600
+            + dt.minute * 60
+            + dt.second
+        )
+
+    def __str__(self):
+        if self._key is not None:
+            return f"{self._key}"
+        else:
+            return repr(self)
+
+    def __repr__(self):
+        if self._key is not None:
+            return f"{self.__class__.__name__}(key={self._key!r})"
+        else:
+            return f"{self.__class__.__name__}.from_file({self._file_repr})"
+
+    def __reduce__(self):
+        return (self.__class__._unpickle, (self._key, self._from_cache))
+
+    def _file_reduce(self):
+        import pickle
+
+        raise pickle.PicklingError(
+            "Cannot pickle a ZoneInfo file created from a file stream."
+        )
+
+    @classmethod
+    def _unpickle(cls, key, from_cache, /):
+        if from_cache:
+            return cls(key)
+        else:
+            return cls.no_cache(key)
+
+    def _find_tzfile(self, key):
+        return _tzpath.find_tzfile(key)
+
+    def _load_file(self, fobj):
+        # Retrieve all the data as it exists in the zoneinfo file
+        trans_idx, trans_utc, utcoff, isdst, abbr, tz_str = _common.load_data(
+            fobj
+        )
+
+        # Infer the DST offsets (needed for .dst()) from the data
+        dstoff = self._utcoff_to_dstoff(trans_idx, utcoff, isdst)
+
+        # Convert all the transition times (UTC) into "seconds since 1970-01-01 local time"
+        trans_local = self._ts_to_local(trans_idx, trans_utc, utcoff)
+
+        # Construct `_ttinfo` objects for each transition in the file
+        _ttinfo_list = [
+            _ttinfo(
+                _load_timedelta(utcoffset), _load_timedelta(dstoffset), tzname
+            )
+            for utcoffset, dstoffset, tzname in zip(utcoff, dstoff, abbr)
+        ]
+
+        self._trans_utc = trans_utc
+        self._trans_local = trans_local
+        self._ttinfos = [_ttinfo_list[idx] for idx in trans_idx]
+
+        # Find the first non-DST transition
+        for i in range(len(isdst)):
+            if not isdst[i]:
+                self._tti_before = _ttinfo_list[i]
+                break
+        else:
+            if self._ttinfos:
+                self._tti_before = self._ttinfos[0]
+            else:
+                self._tti_before = None
+
+        # Set the "fallback" time zone
+        if tz_str is not None and tz_str != b"":
+            self._tz_after = _parse_tz_str(tz_str.decode())
+        else:
+            if not self._ttinfos and not _ttinfo_list:
+                raise ValueError("No time zone information found.")
+
+            if self._ttinfos:
+                self._tz_after = self._ttinfos[-1]
+            else:
+                self._tz_after = _ttinfo_list[-1]
+
+        # Determine if this is a "fixed offset" zone, meaning that the output
+        # of the utcoffset, dst and tzname functions does not depend on the
+        # specific datetime passed.
+        #
+        # We make three simplifying assumptions here:
+        #
+        # 1. If _tz_after is not a _ttinfo, it has transitions that might
+        #    actually occur (it is possible to construct TZ strings that
+        #    specify STD and DST but no transitions ever occur, such as
+        #    AAA0BBB,0/0,J365/25).
+        # 2. If _ttinfo_list contains more than one _ttinfo object, the objects
+        #    represent different offsets.
+        # 3. _ttinfo_list contains no unused _ttinfos (in which case an
+        #    otherwise fixed-offset zone with extra _ttinfos defined may
+        #    appear to *not* be a fixed offset zone).
+        #
+        # Violations to these assumptions would be fairly exotic, and exotic
+        # zones should almost certainly not be used with datetime.time (the
+        # only thing that would be affected by this).
+        if len(_ttinfo_list) > 1 or not isinstance(self._tz_after, _ttinfo):
+            self._fixed_offset = False
+        elif not _ttinfo_list:
+            self._fixed_offset = True
+        else:
+            self._fixed_offset = _ttinfo_list[0] == self._tz_after
+
+    @staticmethod
+    def _utcoff_to_dstoff(trans_idx, utcoffsets, isdsts):
+        # Now we must transform our ttis and abbrs into `_ttinfo` objects,
+        # but there is an issue: .dst() must return a timedelta with the
+        # difference between utcoffset() and the "standard" offset, but
+        # the "base offset" and "DST offset" are not encoded in the file;
+        # we can infer what they are from the isdst flag, but it is not
+        # sufficient to to just look at the last standard offset, because
+        # occasionally countries will shift both DST offset and base offset.
+
+        typecnt = len(isdsts)
+        dstoffs = [0] * typecnt  # Provisionally assign all to 0.
+        dst_cnt = sum(isdsts)
+        dst_found = 0
+
+        for i in range(1, len(trans_idx)):
+            if dst_cnt == dst_found:
+                break
+
+            idx = trans_idx[i]
+
+            dst = isdsts[idx]
+
+            # We're only going to look at daylight saving time
+            if not dst:
+                continue
+
+            # Skip any offsets that have already been assigned
+            if dstoffs[idx] != 0:
+                continue
+
+            dstoff = 0
+            utcoff = utcoffsets[idx]
+
+            comp_idx = trans_idx[i - 1]
+
+            if not isdsts[comp_idx]:
+                dstoff = utcoff - utcoffsets[comp_idx]
+
+            if not dstoff and idx < (typecnt - 1):
+                comp_idx = trans_idx[i + 1]
+
+                # If the following transition is also DST and we couldn't
+                # find the DST offset by this point, we're going ot have to
+                # skip it and hope this transition gets assigned later
+                if isdsts[comp_idx]:
+                    continue
+
+                dstoff = utcoff - utcoffsets[comp_idx]
+
+            if dstoff:
+                dst_found += 1
+                dstoffs[idx] = dstoff
+        else:
+            # If we didn't find a valid value for a given index, we'll end up
+            # with dstoff = 0 for something where `isdst=1`. This is obviously
+            # wrong - one hour will be a much better guess than 0
+            for idx in range(typecnt):
+                if not dstoffs[idx] and isdsts[idx]:
+                    dstoffs[idx] = 3600
+
+        return dstoffs
+
+    @staticmethod
+    def _ts_to_local(trans_idx, trans_list_utc, utcoffsets):
+        """Generate number of seconds since 1970 *in the local time*.
+
+        This is necessary to easily find the transition times in local time"""
+        if not trans_list_utc:
+            return [[], []]
+
+        # Start with the timestamps and modify in-place
+        trans_list_wall = [list(trans_list_utc), list(trans_list_utc)]
+
+        if len(utcoffsets) > 1:
+            offset_0 = utcoffsets[0]
+            offset_1 = utcoffsets[trans_idx[0]]
+            if offset_1 > offset_0:
+                offset_1, offset_0 = offset_0, offset_1
+        else:
+            offset_0 = offset_1 = utcoffsets[0]
+
+        trans_list_wall[0][0] += offset_0
+        trans_list_wall[1][0] += offset_1
+
+        for i in range(1, len(trans_idx)):
+            offset_0 = utcoffsets[trans_idx[i - 1]]
+            offset_1 = utcoffsets[trans_idx[i]]
+
+            if offset_1 > offset_0:
+                offset_1, offset_0 = offset_0, offset_1
+
+            trans_list_wall[0][i] += offset_0
+            trans_list_wall[1][i] += offset_1
+
+        return trans_list_wall
+
+
+class _ttinfo:
+    __slots__ = ["utcoff", "dstoff", "tzname"]
+
+    def __init__(self, utcoff, dstoff, tzname):
+        self.utcoff = utcoff
+        self.dstoff = dstoff
+        self.tzname = tzname
+
+    def __eq__(self, other):
+        return (
+            self.utcoff == other.utcoff
+            and self.dstoff == other.dstoff
+            and self.tzname == other.tzname
+        )
+
+    def __repr__(self):  # pragma: nocover
+        return (
+            f"{self.__class__.__name__}"
+            + f"({self.utcoff}, {self.dstoff}, {self.tzname})"
+        )
+
+
+_NO_TTINFO = _ttinfo(None, None, None)
+
+
+class _TZStr:
+    __slots__ = (
+        "std",
+        "dst",
+        "start",
+        "end",
+        "get_trans_info",
+        "get_trans_info_fromutc",
+        "dst_diff",
+    )
+
+    def __init__(
+        self, std_abbr, std_offset, dst_abbr, dst_offset, start=None, end=None
+    ):
+        self.dst_diff = dst_offset - std_offset
+        std_offset = _load_timedelta(std_offset)
+        self.std = _ttinfo(
+            utcoff=std_offset, dstoff=_load_timedelta(0), tzname=std_abbr
+        )
+
+        self.start = start
+        self.end = end
+
+        dst_offset = _load_timedelta(dst_offset)
+        delta = _load_timedelta(self.dst_diff)
+        self.dst = _ttinfo(utcoff=dst_offset, dstoff=delta, tzname=dst_abbr)
+
+        # These are assertions because the constructor should only be called
+        # by functions that would fail before passing start or end
+        assert start is not None, "No transition start specified"
+        assert end is not None, "No transition end specified"
+
+        self.get_trans_info = self._get_trans_info
+        self.get_trans_info_fromutc = self._get_trans_info_fromutc
+
+    def transitions(self, year):
+        start = self.start.year_to_epoch(year)
+        end = self.end.year_to_epoch(year)
+        return start, end
+
+    def _get_trans_info(self, ts, year, fold):
+        """Get the information about the current transition - tti"""
+        start, end = self.transitions(year)
+
+        # With fold = 0, the period (denominated in local time) with the
+        # smaller offset starts at the end of the gap and ends at the end of
+        # the fold; with fold = 1, it runs from the start of the gap to the
+        # beginning of the fold.
+        #
+        # So in order to determine the DST boundaries we need to know both
+        # the fold and whether DST is positive or negative (rare), and it
+        # turns out that this boils down to fold XOR is_positive.
+        if fold == (self.dst_diff >= 0):
+            end -= self.dst_diff
+        else:
+            start += self.dst_diff
+
+        if start < end:
+            isdst = start <= ts < end
+        else:
+            isdst = not (end <= ts < start)
+
+        return self.dst if isdst else self.std
+
+    def _get_trans_info_fromutc(self, ts, year):
+        start, end = self.transitions(year)
+        start -= self.std.utcoff.total_seconds()
+        end -= self.dst.utcoff.total_seconds()
+
+        if start < end:
+            isdst = start <= ts < end
+        else:
+            isdst = not (end <= ts < start)
+
+        # For positive DST, the ambiguous period is one dst_diff after the end
+        # of DST; for negative DST, the ambiguous period is one dst_diff before
+        # the start of DST.
+        if self.dst_diff > 0:
+            ambig_start = end
+            ambig_end = end + self.dst_diff
+        else:
+            ambig_start = start
+            ambig_end = start - self.dst_diff
+
+        fold = ambig_start <= ts < ambig_end
+
+        return (self.dst if isdst else self.std, fold)
+
+
+def _post_epoch_days_before_year(year):
+    """Get the number of days between 1970-01-01 and YEAR-01-01"""
+    y = year - 1
+    return y * 365 + y // 4 - y // 100 + y // 400 - EPOCHORDINAL
+
+
+class _DayOffset:
+    __slots__ = ["d", "julian", "hour", "minute", "second"]
+
+    def __init__(self, d, julian, hour=2, minute=0, second=0):
+        if not (0 + julian) <= d <= 365:
+            min_day = 0 + julian
+            raise ValueError(f"d must be in [{min_day}, 365], not: {d}")
+
+        self.d = d
+        self.julian = julian
+        self.hour = hour
+        self.minute = minute
+        self.second = second
+
+    def year_to_epoch(self, year):
+        days_before_year = _post_epoch_days_before_year(year)
+
+        d = self.d
+        if self.julian and d >= 59 and calendar.isleap(year):
+            d += 1
+
+        epoch = (days_before_year + d) * 86400
+        epoch += self.hour * 3600 + self.minute * 60 + self.second
+
+        return epoch
+
+
+class _CalendarOffset:
+    __slots__ = ["m", "w", "d", "hour", "minute", "second"]
+
+    _DAYS_BEFORE_MONTH = (
+        -1,
+        0,
+        31,
+        59,
+        90,
+        120,
+        151,
+        181,
+        212,
+        243,
+        273,
+        304,
+        334,
+    )
+
+    def __init__(self, m, w, d, hour=2, minute=0, second=0):
+        if not 0 < m <= 12:
+            raise ValueError("m must be in (0, 12]")
+
+        if not 0 < w <= 5:
+            raise ValueError("w must be in (0, 5]")
+
+        if not 0 <= d <= 6:
+            raise ValueError("d must be in [0, 6]")
+
+        self.m = m
+        self.w = w
+        self.d = d
+        self.hour = hour
+        self.minute = minute
+        self.second = second
+
+    @classmethod
+    def _ymd2ord(cls, year, month, day):
+        return (
+            _post_epoch_days_before_year(year)
+            + cls._DAYS_BEFORE_MONTH[month]
+            + (month > 2 and calendar.isleap(year))
+            + day
+        )
+
+    # TODO: These are not actually epoch dates as they are expressed in local time
+    def year_to_epoch(self, year):
+        """Calculates the datetime of the occurrence from the year"""
+        # We know year and month, we need to convert w, d into day of month
+        #
+        # Week 1 is the first week in which day `d` (where 0 = Sunday) appears.
+        # Week 5 represents the last occurrence of day `d`, so we need to know
+        # the range of the month.
+        first_day, days_in_month = calendar.monthrange(year, self.m)
+
+        # This equation seems magical, so I'll break it down:
+        # 1. calendar says 0 = Monday, POSIX says 0 = Sunday
+        #    so we need first_day + 1 to get 1 = Monday -> 7 = Sunday,
+        #    which is still equivalent because this math is mod 7
+        # 2. Get first day - desired day mod 7: -1 % 7 = 6, so we don't need
+        #    to do anything to adjust negative numbers.
+        # 3. Add 1 because month days are a 1-based index.
+        month_day = (self.d - (first_day + 1)) % 7 + 1
+
+        # Now use a 0-based index version of `w` to calculate the w-th
+        # occurrence of `d`
+        month_day += (self.w - 1) * 7
+
+        # month_day will only be > days_in_month if w was 5, and `w` means
+        # "last occurrence of `d`", so now we just check if we over-shot the
+        # end of the month and if so knock off 1 week.
+        if month_day > days_in_month:
+            month_day -= 7
+
+        ordinal = self._ymd2ord(year, self.m, month_day)
+        epoch = ordinal * 86400
+        epoch += self.hour * 3600 + self.minute * 60 + self.second
+        return epoch
+
+
+def _parse_tz_str(tz_str):
+    # The tz string has the format:
+    #
+    # std[offset[dst[offset],start[/time],end[/time]]]
+    #
+    # std and dst must be 3 or more characters long and must not contain
+    # a leading colon, embedded digits, commas, nor a plus or minus signs;
+    # The spaces between "std" and "offset" are only for display and are
+    # not actually present in the string.
+    #
+    # The format of the offset is ``[+|-]hh[:mm[:ss]]``
+
+    offset_str, *start_end_str = tz_str.split(",", 1)
+
+    # fmt: off
+    parser_re = re.compile(
+        r"(?P<std>[^<0-9:.+-]+|<[a-zA-Z0-9+\-]+>)" +
+        r"((?P<stdoff>[+-]?\d{1,2}(:\d{2}(:\d{2})?)?)" +
+            r"((?P<dst>[^0-9:.+-]+|<[a-zA-Z0-9+\-]+>)" +
+                r"((?P<dstoff>[+-]?\d{1,2}(:\d{2}(:\d{2})?)?))?" +
+            r")?" + # dst
+        r")?$" # stdoff
+    )
+    # fmt: on
+
+    m = parser_re.match(offset_str)
+
+    if m is None:
+        raise ValueError(f"{tz_str} is not a valid TZ string")
+
+    std_abbr = m.group("std")
+    dst_abbr = m.group("dst")
+    dst_offset = None
+
+    std_abbr = std_abbr.strip("<>")
+
+    if dst_abbr:
+        dst_abbr = dst_abbr.strip("<>")
+
+    if std_offset := m.group("stdoff"):
+        try:
+            std_offset = _parse_tz_delta(std_offset)
+        except ValueError as e:
+            raise ValueError(f"Invalid STD offset in {tz_str}") from e
+    else:
+        std_offset = 0
+
+    if dst_abbr is not None:
+        if dst_offset := m.group("dstoff"):
+            try:
+                dst_offset = _parse_tz_delta(dst_offset)
+            except ValueError as e:
+                raise ValueError(f"Invalid DST offset in {tz_str}") from e
+        else:
+            dst_offset = std_offset + 3600
+
+        if not start_end_str:
+            raise ValueError(f"Missing transition rules: {tz_str}")
+
+        start_end_strs = start_end_str[0].split(",", 1)
+        try:
+            start, end = (_parse_dst_start_end(x) for x in start_end_strs)
+        except ValueError as e:
+            raise ValueError(f"Invalid TZ string: {tz_str}") from e
+
+        return _TZStr(std_abbr, std_offset, dst_abbr, dst_offset, start, end)
+    elif start_end_str:
+        raise ValueError(f"Transition rule present without DST: {tz_str}")
+    else:
+        # This is a static ttinfo, don't return _TZStr
+        return _ttinfo(
+            _load_timedelta(std_offset), _load_timedelta(0), std_abbr
+        )
+
+
+def _parse_dst_start_end(dststr):
+    date, *time = dststr.split("/")
+    if date[0] == "M":
+        n_is_julian = False
+        m = re.match(r"M(\d{1,2})\.(\d).(\d)$", date)
+        if m is None:
+            raise ValueError(f"Invalid dst start/end date: {dststr}")
+        date_offset = tuple(map(int, m.groups()))
+        offset = _CalendarOffset(*date_offset)
+    else:
+        if date[0] == "J":
+            n_is_julian = True
+            date = date[1:]
+        else:
+            n_is_julian = False
+
+        doy = int(date)
+        offset = _DayOffset(doy, n_is_julian)
+
+    if time:
+        time_components = list(map(int, time[0].split(":")))
+        n_components = len(time_components)
+        if n_components < 3:
+            time_components.extend([0] * (3 - n_components))
+        offset.hour, offset.minute, offset.second = time_components
+
+    return offset
+
+
+def _parse_tz_delta(tz_delta):
+    match = re.match(
+        r"(?P<sign>[+-])?(?P<h>\d{1,2})(:(?P<m>\d{2})(:(?P<s>\d{2}))?)?",
+        tz_delta,
+    )
+    # Anything passed to this function should already have hit an equivalent
+    # regular expression to find the section to parse.
+    assert match is not None, tz_delta
+
+    h, m, s = (
+        int(v) if v is not None else 0
+        for v in map(match.group, ("h", "m", "s"))
+    )
+
+    total = h * 3600 + m * 60 + s
+
+    if not -86400 < total < 86400:
+        raise ValueError(
+            "Offset must be strictly between -24h and +24h:" + tz_delta
+        )
+
+    # Yes, +5 maps to an offset of -5h
+    if match.group("sign") != "-":
+        total *= -1
+
+    return total
diff --git a/Makefile.pre.in b/Makefile.pre.in
index d545a9e..dbfd805 100644
--- a/Makefile.pre.in
+++ b/Makefile.pre.in
@@ -197,6 +197,9 @@
 OPENSSL_LIBS=@OPENSSL_LIBS@
 OPENSSL_LDFLAGS=@OPENSSL_LDFLAGS@
 
+# Default zoneinfo.TZPATH. Added here to expose it in sysconfig.get_config_var
+TZPATH=@TZPATH@
+
 # Modes for directories, executables and data files created by the
 # install process.  Default to user-only-writable for all file types.
 DIRMODE=	755
diff --git a/Misc/requirements-test.txt b/Misc/requirements-test.txt
new file mode 100644
index 0000000..6e46c12
--- /dev/null
+++ b/Misc/requirements-test.txt
@@ -0,0 +1 @@
+tzdata==2020.1rc0
diff --git a/Modules/Setup b/Modules/Setup
index 87e73ba..02cfb67 100644
--- a/Modules/Setup
+++ b/Modules/Setup
@@ -181,6 +181,7 @@
 #_elementtree -I$(srcdir)/Modules/expat -DHAVE_EXPAT_CONFIG_H -DUSE_PYEXPAT_CAPI _elementtree.c	# elementtree accelerator
 #_pickle _pickle.c	# pickle accelerator
 #_datetime _datetimemodule.c	# datetime accelerator
+#_zoneinfo _zoneinfo.c	# zoneinfo accelerator
 #_bisect _bisectmodule.c	# Bisection algorithms
 #_heapq _heapqmodule.c	# Heap queue algorithm
 #_asyncio _asynciomodule.c  # Fast asyncio Future
diff --git a/Modules/_zoneinfo.c b/Modules/_zoneinfo.c
new file mode 100644
index 0000000..9f5e64d
--- /dev/null
+++ b/Modules/_zoneinfo.c
@@ -0,0 +1,2695 @@
+#include "Python.h"
+#include "structmember.h"
+
+#include <ctype.h>
+#include <stddef.h>
+#include <stdint.h>
+
+#include "datetime.h"
+
+// Imports
+static PyObject *io_open = NULL;
+static PyObject *_tzpath_find_tzfile = NULL;
+static PyObject *_common_mod = NULL;
+
+typedef struct TransitionRuleType TransitionRuleType;
+typedef struct StrongCacheNode StrongCacheNode;
+
+typedef struct {
+    PyObject *utcoff;
+    PyObject *dstoff;
+    PyObject *tzname;
+    long utcoff_seconds;
+} _ttinfo;
+
+typedef struct {
+    _ttinfo std;
+    _ttinfo dst;
+    int dst_diff;
+    TransitionRuleType *start;
+    TransitionRuleType *end;
+    unsigned char std_only;
+} _tzrule;
+
+typedef struct {
+    PyDateTime_TZInfo base;
+    PyObject *key;
+    PyObject *file_repr;
+    PyObject *weakreflist;
+    unsigned int num_transitions;
+    unsigned int num_ttinfos;
+    int64_t *trans_list_utc;
+    int64_t *trans_list_wall[2];
+    _ttinfo **trans_ttinfos;  // References to the ttinfo for each transition
+    _ttinfo *ttinfo_before;
+    _tzrule tzrule_after;
+    _ttinfo *_ttinfos;  // Unique array of ttinfos for ease of deallocation
+    unsigned char fixed_offset;
+    unsigned char source;
+} PyZoneInfo_ZoneInfo;
+
+struct TransitionRuleType {
+    int64_t (*year_to_timestamp)(TransitionRuleType *, int);
+};
+
+typedef struct {
+    TransitionRuleType base;
+    uint8_t month;
+    uint8_t week;
+    uint8_t day;
+    int8_t hour;
+    int8_t minute;
+    int8_t second;
+} CalendarRule;
+
+typedef struct {
+    TransitionRuleType base;
+    uint8_t julian;
+    unsigned int day;
+    int8_t hour;
+    int8_t minute;
+    int8_t second;
+} DayRule;
+
+struct StrongCacheNode {
+    StrongCacheNode *next;
+    StrongCacheNode *prev;
+    PyObject *key;
+    PyObject *zone;
+};
+
+static PyTypeObject PyZoneInfo_ZoneInfoType;
+
+// Globals
+static PyObject *TIMEDELTA_CACHE = NULL;
+static PyObject *ZONEINFO_WEAK_CACHE = NULL;
+static StrongCacheNode *ZONEINFO_STRONG_CACHE = NULL;
+static size_t ZONEINFO_STRONG_CACHE_MAX_SIZE = 8;
+
+static _ttinfo NO_TTINFO = {NULL, NULL, NULL, 0};
+
+// Constants
+static const int EPOCHORDINAL = 719163;
+static int DAYS_IN_MONTH[] = {
+    -1, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31,
+};
+
+static int DAYS_BEFORE_MONTH[] = {
+    -1, 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334,
+};
+
+static const int SOURCE_NOCACHE = 0;
+static const int SOURCE_CACHE = 1;
+static const int SOURCE_FILE = 2;
+
+// Forward declarations
+static int
+load_data(PyZoneInfo_ZoneInfo *self, PyObject *file_obj);
+static void
+utcoff_to_dstoff(size_t *trans_idx, long *utcoffs, long *dstoffs,
+                 unsigned char *isdsts, size_t num_transitions,
+                 size_t num_ttinfos);
+static int
+ts_to_local(size_t *trans_idx, int64_t *trans_utc, long *utcoff,
+            int64_t *trans_local[2], size_t num_ttinfos,
+            size_t num_transitions);
+
+static int
+parse_tz_str(PyObject *tz_str_obj, _tzrule *out);
+
+static ssize_t
+parse_abbr(const char *const p, PyObject **abbr);
+static ssize_t
+parse_tz_delta(const char *const p, long *total_seconds);
+static ssize_t
+parse_transition_time(const char *const p, int8_t *hour, int8_t *minute,
+                      int8_t *second);
+static ssize_t
+parse_transition_rule(const char *const p, TransitionRuleType **out);
+
+static _ttinfo *
+find_tzrule_ttinfo(_tzrule *rule, int64_t ts, unsigned char fold, int year);
+static _ttinfo *
+find_tzrule_ttinfo_fromutc(_tzrule *rule, int64_t ts, int year,
+                           unsigned char *fold);
+
+static int
+build_ttinfo(long utcoffset, long dstoffset, PyObject *tzname, _ttinfo *out);
+static void
+xdecref_ttinfo(_ttinfo *ttinfo);
+static int
+ttinfo_eq(const _ttinfo *const tti0, const _ttinfo *const tti1);
+
+static int
+build_tzrule(PyObject *std_abbr, PyObject *dst_abbr, long std_offset,
+             long dst_offset, TransitionRuleType *start,
+             TransitionRuleType *end, _tzrule *out);
+static void
+free_tzrule(_tzrule *tzrule);
+
+static PyObject *
+load_timedelta(long seconds);
+
+static int
+get_local_timestamp(PyObject *dt, int64_t *local_ts);
+static _ttinfo *
+find_ttinfo(PyZoneInfo_ZoneInfo *self, PyObject *dt);
+
+static int
+ymd_to_ord(int y, int m, int d);
+static int
+is_leap_year(int year);
+
+static size_t
+_bisect(const int64_t value, const int64_t *arr, size_t size);
+
+static void
+eject_from_strong_cache(const PyTypeObject *const type, PyObject *key);
+static void
+clear_strong_cache(const PyTypeObject *const type);
+static void
+update_strong_cache(const PyTypeObject *const type, PyObject *key,
+                    PyObject *zone);
+static PyObject *
+zone_from_strong_cache(const PyTypeObject *const type, PyObject *key);
+
+static PyObject *
+zoneinfo_new_instance(PyTypeObject *type, PyObject *key)
+{
+    PyObject *file_obj = NULL;
+    PyObject *file_path = NULL;
+
+    file_path = PyObject_CallFunctionObjArgs(_tzpath_find_tzfile, key, NULL);
+    if (file_path == NULL) {
+        return NULL;
+    }
+    else if (file_path == Py_None) {
+        file_obj = PyObject_CallMethod(_common_mod, "load_tzdata", "O", key);
+        if (file_obj == NULL) {
+            Py_DECREF(file_path);
+            return NULL;
+        }
+    }
+
+    PyObject *self = (PyObject *)(type->tp_alloc(type, 0));
+    if (self == NULL) {
+        goto error;
+    }
+
+    if (file_obj == NULL) {
+        file_obj = PyObject_CallFunction(io_open, "Os", file_path, "rb");
+        if (file_obj == NULL) {
+            goto error;
+        }
+    }
+
+    if (load_data((PyZoneInfo_ZoneInfo *)self, file_obj)) {
+        goto error;
+    }
+
+    PyObject *rv = PyObject_CallMethod(file_obj, "close", NULL);
+    Py_DECREF(file_obj);
+    file_obj = NULL;
+    if (rv == NULL) {
+        goto error;
+    }
+    Py_DECREF(rv);
+
+    ((PyZoneInfo_ZoneInfo *)self)->key = key;
+    Py_INCREF(key);
+
+    goto cleanup;
+error:
+    Py_XDECREF(self);
+    self = NULL;
+cleanup:
+    if (file_obj != NULL) {
+        PyObject *tmp = PyObject_CallMethod(file_obj, "close", NULL);
+        Py_DECREF(tmp);
+        Py_DECREF(file_obj);
+    }
+    Py_DECREF(file_path);
+    return self;
+}
+
+static PyObject *
+get_weak_cache(PyTypeObject *type)
+{
+    if (type == &PyZoneInfo_ZoneInfoType) {
+        return ZONEINFO_WEAK_CACHE;
+    }
+    else {
+        PyObject *cache =
+            PyObject_GetAttrString((PyObject *)type, "_weak_cache");
+        // We are assuming that the type lives at least as long as the function
+        // that calls get_weak_cache, and that it holds a reference to the
+        // cache, so we'll return a "borrowed reference".
+        Py_XDECREF(cache);
+        return cache;
+    }
+}
+
+static PyObject *
+zoneinfo_new(PyTypeObject *type, PyObject *args, PyObject *kw)
+{
+    PyObject *key = NULL;
+    static char *kwlist[] = {"key", NULL};
+    if (PyArg_ParseTupleAndKeywords(args, kw, "O", kwlist, &key) == 0) {
+        return NULL;
+    }
+
+    PyObject *instance = zone_from_strong_cache(type, key);
+    if (instance != NULL) {
+        return instance;
+    }
+
+    PyObject *weak_cache = get_weak_cache(type);
+    instance = PyObject_CallMethod(weak_cache, "get", "O", key, Py_None);
+    if (instance == NULL) {
+        return NULL;
+    }
+
+    if (instance == Py_None) {
+        Py_DECREF(instance);
+        PyObject *tmp = zoneinfo_new_instance(type, key);
+        if (tmp == NULL) {
+            return NULL;
+        }
+
+        instance =
+            PyObject_CallMethod(weak_cache, "setdefault", "OO", key, tmp);
+        ((PyZoneInfo_ZoneInfo *)instance)->source = SOURCE_CACHE;
+
+        Py_DECREF(tmp);
+
+        if (instance == NULL) {
+            return NULL;
+        }
+    }
+
+    update_strong_cache(type, key, instance);
+    return instance;
+}
+
+static void
+zoneinfo_dealloc(PyObject *obj_self)
+{
+    PyZoneInfo_ZoneInfo *self = (PyZoneInfo_ZoneInfo *)obj_self;
+
+    if (self->weakreflist != NULL) {
+        PyObject_ClearWeakRefs(obj_self);
+    }
+
+    if (self->trans_list_utc != NULL) {
+        PyMem_Free(self->trans_list_utc);
+    }
+
+    for (size_t i = 0; i < 2; i++) {
+        if (self->trans_list_wall[i] != NULL) {
+            PyMem_Free(self->trans_list_wall[i]);
+        }
+    }
+
+    if (self->_ttinfos != NULL) {
+        for (size_t i = 0; i < self->num_ttinfos; ++i) {
+            xdecref_ttinfo(&(self->_ttinfos[i]));
+        }
+        PyMem_Free(self->_ttinfos);
+    }
+
+    if (self->trans_ttinfos != NULL) {
+        PyMem_Free(self->trans_ttinfos);
+    }
+
+    free_tzrule(&(self->tzrule_after));
+
+    Py_XDECREF(self->key);
+    Py_XDECREF(self->file_repr);
+
+    Py_TYPE(self)->tp_free((PyObject *)self);
+}
+
+static PyObject *
+zoneinfo_from_file(PyTypeObject *type, PyObject *args, PyObject *kwargs)
+{
+    PyObject *file_obj = NULL;
+    PyObject *file_repr = NULL;
+    PyObject *key = Py_None;
+    PyZoneInfo_ZoneInfo *self = NULL;
+
+    static char *kwlist[] = {"", "key", NULL};
+    if (!PyArg_ParseTupleAndKeywords(args, kwargs, "O|O", kwlist, &file_obj,
+                                     &key)) {
+        return NULL;
+    }
+
+    PyObject *obj_self = (PyObject *)(type->tp_alloc(type, 0));
+    self = (PyZoneInfo_ZoneInfo *)obj_self;
+    if (self == NULL) {
+        return NULL;
+    }
+
+    file_repr = PyUnicode_FromFormat("%R", file_obj);
+    if (file_repr == NULL) {
+        goto error;
+    }
+
+    if (load_data(self, file_obj)) {
+        goto error;
+    }
+
+    self->source = SOURCE_FILE;
+    self->file_repr = file_repr;
+    self->key = key;
+    Py_INCREF(key);
+
+    return obj_self;
+error:
+    Py_XDECREF(file_repr);
+    Py_XDECREF(self);
+    return NULL;
+}
+
+static PyObject *
+zoneinfo_no_cache(PyTypeObject *cls, PyObject *args, PyObject *kwargs)
+{
+    static char *kwlist[] = {"key", NULL};
+    PyObject *key = NULL;
+    if (!PyArg_ParseTupleAndKeywords(args, kwargs, "O", kwlist, &key)) {
+        return NULL;
+    }
+
+    PyObject *out = zoneinfo_new_instance(cls, key);
+    if (out != NULL) {
+        ((PyZoneInfo_ZoneInfo *)out)->source = SOURCE_NOCACHE;
+    }
+
+    return out;
+}
+
+static PyObject *
+zoneinfo_clear_cache(PyObject *cls, PyObject *args, PyObject *kwargs)
+{
+    PyObject *only_keys = NULL;
+    static char *kwlist[] = {"only_keys", NULL};
+
+    if (!(PyArg_ParseTupleAndKeywords(args, kwargs, "|$O", kwlist,
+                                      &only_keys))) {
+        return NULL;
+    }
+
+    PyTypeObject *type = (PyTypeObject *)cls;
+    PyObject *weak_cache = get_weak_cache(type);
+
+    if (only_keys == NULL || only_keys == Py_None) {
+        PyObject *rv = PyObject_CallMethod(weak_cache, "clear", NULL);
+        if (rv != NULL) {
+            Py_DECREF(rv);
+        }
+
+        clear_strong_cache(type);
+        ZONEINFO_STRONG_CACHE = NULL;
+    }
+    else {
+        PyObject *item = NULL;
+        PyObject *pop = PyUnicode_FromString("pop");
+        if (pop == NULL) {
+            return NULL;
+        }
+
+        PyObject *iter = PyObject_GetIter(only_keys);
+        if (iter == NULL) {
+            Py_DECREF(pop);
+            return NULL;
+        }
+
+        while ((item = PyIter_Next(iter))) {
+            // Remove from strong cache
+            eject_from_strong_cache(type, item);
+
+            // Remove from weak cache
+            PyObject *tmp = PyObject_CallMethodObjArgs(weak_cache, pop, item,
+                                                       Py_None, NULL);
+
+            Py_DECREF(item);
+            if (tmp == NULL) {
+                break;
+            }
+            Py_DECREF(tmp);
+        }
+        Py_DECREF(iter);
+        Py_DECREF(pop);
+    }
+
+    if (PyErr_Occurred()) {
+        return NULL;
+    }
+
+    Py_RETURN_NONE;
+}
+
+static PyObject *
+zoneinfo_utcoffset(PyObject *self, PyObject *dt)
+{
+    _ttinfo *tti = find_ttinfo((PyZoneInfo_ZoneInfo *)self, dt);
+    if (tti == NULL) {
+        return NULL;
+    }
+    Py_INCREF(tti->utcoff);
+    return tti->utcoff;
+}
+
+static PyObject *
+zoneinfo_dst(PyObject *self, PyObject *dt)
+{
+    _ttinfo *tti = find_ttinfo((PyZoneInfo_ZoneInfo *)self, dt);
+    if (tti == NULL) {
+        return NULL;
+    }
+    Py_INCREF(tti->dstoff);
+    return tti->dstoff;
+}
+
+static PyObject *
+zoneinfo_tzname(PyObject *self, PyObject *dt)
+{
+    _ttinfo *tti = find_ttinfo((PyZoneInfo_ZoneInfo *)self, dt);
+    if (tti == NULL) {
+        return NULL;
+    }
+    Py_INCREF(tti->tzname);
+    return tti->tzname;
+}
+
+#define HASTZINFO(p) (((_PyDateTime_BaseTZInfo *)(p))->hastzinfo)
+#define GET_DT_TZINFO(p) \
+    (HASTZINFO(p) ? ((PyDateTime_DateTime *)(p))->tzinfo : Py_None)
+
+static PyObject *
+zoneinfo_fromutc(PyObject *obj_self, PyObject *dt)
+{
+    if (!PyDateTime_Check(dt)) {
+        PyErr_SetString(PyExc_TypeError,
+                        "fromutc: argument must be a datetime");
+        return NULL;
+    }
+    if (GET_DT_TZINFO(dt) != obj_self) {
+        PyErr_SetString(PyExc_ValueError,
+                        "fromutc: dt.tzinfo "
+                        "is not self");
+        return NULL;
+    }
+
+    PyZoneInfo_ZoneInfo *self = (PyZoneInfo_ZoneInfo *)obj_self;
+
+    int64_t timestamp;
+    if (get_local_timestamp(dt, &timestamp)) {
+        return NULL;
+    }
+    size_t num_trans = self->num_transitions;
+
+    _ttinfo *tti = NULL;
+    unsigned char fold = 0;
+
+    if (num_trans >= 1 && timestamp < self->trans_list_utc[0]) {
+        tti = self->ttinfo_before;
+    }
+    else if (num_trans == 0 ||
+             timestamp > self->trans_list_utc[num_trans - 1]) {
+        tti = find_tzrule_ttinfo_fromutc(&(self->tzrule_after), timestamp,
+                                         PyDateTime_GET_YEAR(dt), &fold);
+
+        // Immediately after the last manual transition, the fold/gap is
+        // between self->trans_ttinfos[num_transitions - 1] and whatever
+        // ttinfo applies immediately after the last transition, not between
+        // the STD and DST rules in the tzrule_after, so we may need to
+        // adjust the fold value.
+        if (num_trans) {
+            _ttinfo *tti_prev = NULL;
+            if (num_trans == 1) {
+                tti_prev = self->ttinfo_before;
+            }
+            else {
+                tti_prev = self->trans_ttinfos[num_trans - 2];
+            }
+            int64_t diff = tti_prev->utcoff_seconds - tti->utcoff_seconds;
+            if (diff > 0 &&
+                timestamp < (self->trans_list_utc[num_trans - 1] + diff)) {
+                fold = 1;
+            }
+        }
+    }
+    else {
+        size_t idx = _bisect(timestamp, self->trans_list_utc, num_trans);
+        _ttinfo *tti_prev = NULL;
+
+        if (idx >= 2) {
+            tti_prev = self->trans_ttinfos[idx - 2];
+            tti = self->trans_ttinfos[idx - 1];
+        }
+        else {
+            tti_prev = self->ttinfo_before;
+            tti = self->trans_ttinfos[0];
+        }
+
+        // Detect fold
+        int64_t shift =
+            (int64_t)(tti_prev->utcoff_seconds - tti->utcoff_seconds);
+        if (shift > (timestamp - self->trans_list_utc[idx - 1])) {
+            fold = 1;
+        }
+    }
+
+    PyObject *tmp = PyNumber_Add(dt, tti->utcoff);
+    if (tmp == NULL) {
+        return NULL;
+    }
+
+    if (fold) {
+        if (PyDateTime_CheckExact(tmp)) {
+            ((PyDateTime_DateTime *)tmp)->fold = 1;
+            dt = tmp;
+        }
+        else {
+            PyObject *replace = PyObject_GetAttrString(tmp, "replace");
+            PyObject *args = PyTuple_New(0);
+            PyObject *kwargs = PyDict_New();
+
+            Py_DECREF(tmp);
+            if (args == NULL || kwargs == NULL || replace == NULL) {
+                Py_XDECREF(args);
+                Py_XDECREF(kwargs);
+                Py_XDECREF(replace);
+                return NULL;
+            }
+
+            dt = NULL;
+            if (!PyDict_SetItemString(kwargs, "fold", _PyLong_One)) {
+                dt = PyObject_Call(replace, args, kwargs);
+            }
+
+            Py_DECREF(args);
+            Py_DECREF(kwargs);
+            Py_DECREF(replace);
+
+            if (dt == NULL) {
+                return NULL;
+            }
+        }
+    }
+    else {
+        dt = tmp;
+    }
+    return dt;
+}
+
+static PyObject *
+zoneinfo_repr(PyZoneInfo_ZoneInfo *self)
+{
+    PyObject *rv = NULL;
+    const char *type_name = Py_TYPE((PyObject *)self)->tp_name;
+    if (!(self->key == Py_None)) {
+        rv = PyUnicode_FromFormat("%s(key=%R)", type_name, self->key);
+    }
+    else {
+        assert(PyUnicode_Check(self->file_repr));
+        rv = PyUnicode_FromFormat("%s.from_file(%U)", type_name,
+                                  self->file_repr);
+    }
+
+    return rv;
+}
+
+static PyObject *
+zoneinfo_str(PyZoneInfo_ZoneInfo *self)
+{
+    if (!(self->key == Py_None)) {
+        Py_INCREF(self->key);
+        return self->key;
+    }
+    else {
+        return zoneinfo_repr(self);
+    }
+}
+
+/* Pickles the ZoneInfo object by key and source.
+ *
+ * ZoneInfo objects are pickled by reference to the TZif file that they came
+ * from, which means that the exact transitions may be different or the file
+ * may not un-pickle if the data has changed on disk in the interim.
+ *
+ * It is necessary to include a bit indicating whether or not the object
+ * was constructed from the cache, because from-cache objects will hit the
+ * unpickling process's cache, whereas no-cache objects will bypass it.
+ *
+ * Objects constructed from ZoneInfo.from_file cannot be pickled.
+ */
+static PyObject *
+zoneinfo_reduce(PyObject *obj_self, PyObject *unused)
+{
+    PyZoneInfo_ZoneInfo *self = (PyZoneInfo_ZoneInfo *)obj_self;
+    if (self->source == SOURCE_FILE) {
+        // Objects constructed from files cannot be pickled.
+        PyObject *pickle = PyImport_ImportModule("pickle");
+        if (pickle == NULL) {
+            return NULL;
+        }
+
+        PyObject *pickle_error =
+            PyObject_GetAttrString(pickle, "PicklingError");
+        Py_DECREF(pickle);
+        if (pickle_error == NULL) {
+            return NULL;
+        }
+
+        PyErr_Format(pickle_error,
+                     "Cannot pickle a ZoneInfo file from a file stream.");
+        Py_DECREF(pickle_error);
+        return NULL;
+    }
+
+    unsigned char from_cache = self->source == SOURCE_CACHE ? 1 : 0;
+    PyObject *constructor = PyObject_GetAttrString(obj_self, "_unpickle");
+
+    if (constructor == NULL) {
+        return NULL;
+    }
+
+    PyObject *rv = Py_BuildValue("O(OB)", constructor, self->key, from_cache);
+    Py_DECREF(constructor);
+    return rv;
+}
+
+static PyObject *
+zoneinfo__unpickle(PyTypeObject *cls, PyObject *args)
+{
+    PyObject *key;
+    unsigned char from_cache;
+    if (!PyArg_ParseTuple(args, "OB", &key, &from_cache)) {
+        return NULL;
+    }
+
+    if (from_cache) {
+        PyObject *val_args = Py_BuildValue("(O)", key);
+        if (val_args == NULL) {
+            return NULL;
+        }
+
+        PyObject *rv = zoneinfo_new(cls, val_args, NULL);
+
+        Py_DECREF(val_args);
+        return rv;
+    }
+    else {
+        return zoneinfo_new_instance(cls, key);
+    }
+}
+
+/* It is relatively expensive to construct new timedelta objects, and in most
+ * cases we're looking at a relatively small number of timedeltas, such as
+ * integer number of hours, etc. We will keep a cache so that we construct
+ * a minimal number of these.
+ *
+ * Possibly this should be replaced with an LRU cache so that it's not possible
+ * for the memory usage to explode from this, but in order for this to be a
+ * serious problem, one would need to deliberately craft a malicious time zone
+ * file with many distinct offsets. As of tzdb 2019c, loading every single zone
+ * fills the cache with ~450 timedeltas for a total size of ~12kB.
+ *
+ * This returns a new reference to the timedelta.
+ */
+static PyObject *
+load_timedelta(long seconds)
+{
+    PyObject *rv = NULL;
+    PyObject *pyoffset = PyLong_FromLong(seconds);
+    if (pyoffset == NULL) {
+        return NULL;
+    }
+    int contains = PyDict_Contains(TIMEDELTA_CACHE, pyoffset);
+    if (contains == -1) {
+        goto error;
+    }
+
+    if (!contains) {
+        PyObject *tmp = PyDateTimeAPI->Delta_FromDelta(
+            0, seconds, 0, 1, PyDateTimeAPI->DeltaType);
+
+        if (tmp == NULL) {
+            goto error;
+        }
+
+        rv = PyDict_SetDefault(TIMEDELTA_CACHE, pyoffset, tmp);
+        Py_DECREF(tmp);
+    }
+    else {
+        rv = PyDict_GetItem(TIMEDELTA_CACHE, pyoffset);
+    }
+
+    Py_DECREF(pyoffset);
+    Py_INCREF(rv);
+    return rv;
+error:
+    Py_DECREF(pyoffset);
+    return NULL;
+}
+
+/* Constructor for _ttinfo object - this starts by initializing the _ttinfo
+ * to { NULL, NULL, NULL }, so that Py_XDECREF will work on partially
+ * initialized _ttinfo objects.
+ */
+static int
+build_ttinfo(long utcoffset, long dstoffset, PyObject *tzname, _ttinfo *out)
+{
+    out->utcoff = NULL;
+    out->dstoff = NULL;
+    out->tzname = NULL;
+
+    out->utcoff_seconds = utcoffset;
+    out->utcoff = load_timedelta(utcoffset);
+    if (out->utcoff == NULL) {
+        return -1;
+    }
+
+    out->dstoff = load_timedelta(dstoffset);
+    if (out->dstoff == NULL) {
+        return -1;
+    }
+
+    out->tzname = tzname;
+    Py_INCREF(tzname);
+
+    return 0;
+}
+
+/* Decrease reference count on any non-NULL members of a _ttinfo  */
+static void
+xdecref_ttinfo(_ttinfo *ttinfo)
+{
+    if (ttinfo != NULL) {
+        Py_XDECREF(ttinfo->utcoff);
+        Py_XDECREF(ttinfo->dstoff);
+        Py_XDECREF(ttinfo->tzname);
+    }
+}
+
+/* Equality function for _ttinfo. */
+static int
+ttinfo_eq(const _ttinfo *const tti0, const _ttinfo *const tti1)
+{
+    int rv;
+    if ((rv = PyObject_RichCompareBool(tti0->utcoff, tti1->utcoff, Py_EQ)) <
+        1) {
+        goto end;
+    }
+
+    if ((rv = PyObject_RichCompareBool(tti0->dstoff, tti1->dstoff, Py_EQ)) <
+        1) {
+        goto end;
+    }
+
+    if ((rv = PyObject_RichCompareBool(tti0->tzname, tti1->tzname, Py_EQ)) <
+        1) {
+        goto end;
+    }
+end:
+    return rv;
+}
+
+/* Given a file-like object, this populates a ZoneInfo object
+ *
+ * The current version calls into a Python function to read the data from
+ * file into Python objects, and this translates those Python objects into
+ * C values and calculates derived values (e.g. dstoff) in C.
+ *
+ * This returns 0 on success and -1 on failure.
+ *
+ * The function will never return while `self` is partially initialized —
+ * the object only needs to be freed / deallocated if this succeeds.
+ */
+static int
+load_data(PyZoneInfo_ZoneInfo *self, PyObject *file_obj)
+{
+    PyObject *data_tuple = NULL;
+
+    long *utcoff = NULL;
+    long *dstoff = NULL;
+    size_t *trans_idx = NULL;
+    unsigned char *isdst = NULL;
+
+    self->trans_list_utc = NULL;
+    self->trans_list_wall[0] = NULL;
+    self->trans_list_wall[1] = NULL;
+    self->trans_ttinfos = NULL;
+    self->_ttinfos = NULL;
+    self->file_repr = NULL;
+
+    size_t ttinfos_allocated = 0;
+
+    data_tuple = PyObject_CallMethod(_common_mod, "load_data", "O", file_obj);
+
+    if (data_tuple == NULL) {
+        goto error;
+    }
+
+    if (!PyTuple_CheckExact(data_tuple)) {
+        PyErr_Format(PyExc_TypeError, "Invalid data result type: %r",
+                     data_tuple);
+        goto error;
+    }
+
+    // Unpack the data tuple
+    PyObject *trans_idx_list = PyTuple_GetItem(data_tuple, 0);
+    if (trans_idx_list == NULL) {
+        goto error;
+    }
+
+    PyObject *trans_utc = PyTuple_GetItem(data_tuple, 1);
+    if (trans_utc == NULL) {
+        goto error;
+    }
+
+    PyObject *utcoff_list = PyTuple_GetItem(data_tuple, 2);
+    if (utcoff_list == NULL) {
+        goto error;
+    }
+
+    PyObject *isdst_list = PyTuple_GetItem(data_tuple, 3);
+    if (isdst_list == NULL) {
+        goto error;
+    }
+
+    PyObject *abbr = PyTuple_GetItem(data_tuple, 4);
+    if (abbr == NULL) {
+        goto error;
+    }
+
+    PyObject *tz_str = PyTuple_GetItem(data_tuple, 5);
+    if (tz_str == NULL) {
+        goto error;
+    }
+
+    // Load the relevant sizes
+    Py_ssize_t num_transitions = PyTuple_Size(trans_utc);
+    if (num_transitions == -1) {
+        goto error;
+    }
+
+    Py_ssize_t num_ttinfos = PyTuple_Size(utcoff_list);
+    if (num_ttinfos == -1) {
+        goto error;
+    }
+
+    self->num_transitions = (size_t)num_transitions;
+    self->num_ttinfos = (size_t)num_ttinfos;
+
+    // Load the transition indices and list
+    self->trans_list_utc =
+        PyMem_Malloc(self->num_transitions * sizeof(int64_t));
+    trans_idx = PyMem_Malloc(self->num_transitions * sizeof(Py_ssize_t));
+
+    for (Py_ssize_t i = 0; i < self->num_transitions; ++i) {
+        PyObject *num = PyTuple_GetItem(trans_utc, i);
+        if (num == NULL) {
+            goto error;
+        }
+        self->trans_list_utc[i] = PyLong_AsLongLong(num);
+        if (self->trans_list_utc[i] == -1 && PyErr_Occurred()) {
+            goto error;
+        }
+
+        num = PyTuple_GetItem(trans_idx_list, i);
+        if (num == NULL) {
+            goto error;
+        }
+
+        Py_ssize_t cur_trans_idx = PyLong_AsSsize_t(num);
+        if (cur_trans_idx == -1) {
+            goto error;
+        }
+
+        trans_idx[i] = (size_t)cur_trans_idx;
+        if (trans_idx[i] > self->num_ttinfos) {
+            PyErr_Format(
+                PyExc_ValueError,
+                "Invalid transition index found while reading TZif: %zd",
+                cur_trans_idx);
+
+            goto error;
+        }
+    }
+
+    // Load UTC offsets and isdst (size num_ttinfos)
+    utcoff = PyMem_Malloc(self->num_ttinfos * sizeof(long));
+    isdst = PyMem_Malloc(self->num_ttinfos * sizeof(unsigned char));
+
+    if (utcoff == NULL || isdst == NULL) {
+        goto error;
+    }
+    for (Py_ssize_t i = 0; i < self->num_ttinfos; ++i) {
+        PyObject *num = PyTuple_GetItem(utcoff_list, i);
+        if (num == NULL) {
+            goto error;
+        }
+
+        utcoff[i] = PyLong_AsLong(num);
+        if (utcoff[i] == -1 && PyErr_Occurred()) {
+            goto error;
+        }
+
+        num = PyTuple_GetItem(isdst_list, i);
+        if (num == NULL) {
+            goto error;
+        }
+
+        int isdst_with_error = PyObject_IsTrue(num);
+        if (isdst_with_error == -1) {
+            goto error;
+        }
+        else {
+            isdst[i] = (unsigned char)isdst_with_error;
+        }
+    }
+
+    dstoff = PyMem_Calloc(self->num_ttinfos, sizeof(long));
+    if (dstoff == NULL) {
+        goto error;
+    }
+
+    // Derive dstoff and trans_list_wall from the information we've loaded
+    utcoff_to_dstoff(trans_idx, utcoff, dstoff, isdst, self->num_transitions,
+                     self->num_ttinfos);
+
+    if (ts_to_local(trans_idx, self->trans_list_utc, utcoff,
+                    self->trans_list_wall, self->num_ttinfos,
+                    self->num_transitions)) {
+        goto error;
+    }
+
+    // Build _ttinfo objects from utcoff, dstoff and abbr
+    self->_ttinfos = PyMem_Malloc(self->num_ttinfos * sizeof(_ttinfo));
+    for (size_t i = 0; i < self->num_ttinfos; ++i) {
+        PyObject *tzname = PyTuple_GetItem(abbr, i);
+        if (tzname == NULL) {
+            goto error;
+        }
+
+        ttinfos_allocated++;
+        if (build_ttinfo(utcoff[i], dstoff[i], tzname, &(self->_ttinfos[i]))) {
+            goto error;
+        }
+    }
+
+    // Build our mapping from transition to the ttinfo that applies
+    self->trans_ttinfos =
+        PyMem_Calloc(self->num_transitions, sizeof(_ttinfo *));
+    for (size_t i = 0; i < self->num_transitions; ++i) {
+        size_t ttinfo_idx = trans_idx[i];
+        assert(ttinfo_idx < self->num_ttinfos);
+        self->trans_ttinfos[i] = &(self->_ttinfos[ttinfo_idx]);
+    }
+
+    // Set ttinfo_before to the first non-DST transition
+    for (size_t i = 0; i < self->num_ttinfos; ++i) {
+        if (!isdst[i]) {
+            self->ttinfo_before = &(self->_ttinfos[i]);
+            break;
+        }
+    }
+
+    // If there are only DST ttinfos, pick the first one, if there are no
+    // ttinfos at all, set ttinfo_before to NULL
+    if (self->ttinfo_before == NULL && self->num_ttinfos > 0) {
+        self->ttinfo_before = &(self->_ttinfos[0]);
+    }
+
+    if (tz_str != Py_None && PyObject_IsTrue(tz_str)) {
+        if (parse_tz_str(tz_str, &(self->tzrule_after))) {
+            goto error;
+        }
+    }
+    else {
+        if (!self->num_ttinfos) {
+            PyErr_Format(PyExc_ValueError, "No time zone information found.");
+            goto error;
+        }
+
+        size_t idx;
+        if (!self->num_transitions) {
+            idx = self->num_ttinfos - 1;
+        }
+        else {
+            idx = trans_idx[self->num_transitions - 1];
+        }
+
+        _ttinfo *tti = &(self->_ttinfos[idx]);
+        build_tzrule(tti->tzname, NULL, tti->utcoff_seconds, 0, NULL, NULL,
+                     &(self->tzrule_after));
+
+        // We've abused the build_tzrule constructor to construct an STD-only
+        // rule mimicking whatever ttinfo we've picked up, but it's possible
+        // that the one we've picked up is a DST zone, so we need to make sure
+        // that the dstoff is set correctly in that case.
+        if (PyObject_IsTrue(tti->dstoff)) {
+            _ttinfo *tti_after = &(self->tzrule_after.std);
+            Py_DECREF(tti_after->dstoff);
+            tti_after->dstoff = tti->dstoff;
+            Py_INCREF(tti_after->dstoff);
+        }
+    }
+
+    // Determine if this is a "fixed offset" zone, meaning that the output of
+    // the utcoffset, dst and tzname functions does not depend on the specific
+    // datetime passed.
+    //
+    // We make three simplifying assumptions here:
+    //
+    // 1. If tzrule_after is not std_only, it has transitions that might occur
+    //    (it is possible to construct TZ strings that specify STD and DST but
+    //    no transitions ever occur, such as AAA0BBB,0/0,J365/25).
+    // 2. If self->_ttinfos contains more than one _ttinfo object, the objects
+    //    represent different offsets.
+    // 3. self->ttinfos contains no unused _ttinfos (in which case an otherwise
+    //    fixed-offset zone with extra _ttinfos defined may appear to *not* be
+    //    a fixed offset zone).
+    //
+    // Violations to these assumptions would be fairly exotic, and exotic
+    // zones should almost certainly not be used with datetime.time (the
+    // only thing that would be affected by this).
+    if (self->num_ttinfos > 1 || !self->tzrule_after.std_only) {
+        self->fixed_offset = 0;
+    }
+    else if (self->num_ttinfos == 0) {
+        self->fixed_offset = 1;
+    }
+    else {
+        int constant_offset =
+            ttinfo_eq(&(self->_ttinfos[0]), &self->tzrule_after.std);
+        if (constant_offset < 0) {
+            goto error;
+        }
+        else {
+            self->fixed_offset = constant_offset;
+        }
+    }
+
+    int rv = 0;
+    goto cleanup;
+error:
+    // These resources only need to be freed if we have failed, if we succeed
+    // in initializing a PyZoneInfo_ZoneInfo object, we can rely on its dealloc
+    // method to free the relevant resources.
+    if (self->trans_list_utc != NULL) {
+        PyMem_Free(self->trans_list_utc);
+        self->trans_list_utc = NULL;
+    }
+
+    for (size_t i = 0; i < 2; ++i) {
+        if (self->trans_list_wall[i] != NULL) {
+            PyMem_Free(self->trans_list_wall[i]);
+            self->trans_list_wall[i] = NULL;
+        }
+    }
+
+    if (self->_ttinfos != NULL) {
+        for (size_t i = 0; i < ttinfos_allocated; ++i) {
+            xdecref_ttinfo(&(self->_ttinfos[i]));
+        }
+        PyMem_Free(self->_ttinfos);
+        self->_ttinfos = NULL;
+    }
+
+    if (self->trans_ttinfos != NULL) {
+        PyMem_Free(self->trans_ttinfos);
+        self->trans_ttinfos = NULL;
+    }
+
+    rv = -1;
+cleanup:
+    Py_XDECREF(data_tuple);
+
+    if (utcoff != NULL) {
+        PyMem_Free(utcoff);
+    }
+
+    if (dstoff != NULL) {
+        PyMem_Free(dstoff);
+    }
+
+    if (isdst != NULL) {
+        PyMem_Free(isdst);
+    }
+
+    if (trans_idx != NULL) {
+        PyMem_Free(trans_idx);
+    }
+
+    return rv;
+}
+
+/* Function to calculate the local timestamp of a transition from the year. */
+int64_t
+calendarrule_year_to_timestamp(TransitionRuleType *base_self, int year)
+{
+    CalendarRule *self = (CalendarRule *)base_self;
+
+    // We want (year, month, day of month); we have year and month, but we
+    // need to turn (week, day-of-week) into day-of-month
+    //
+    // Week 1 is the first week in which day `day` (where 0 = Sunday) appears.
+    // Week 5 represents the last occurrence of day `day`, so we need to know
+    // the first weekday of the month and the number of days in the month.
+    int8_t first_day = (ymd_to_ord(year, self->month, 1) + 6) % 7;
+    uint8_t days_in_month = DAYS_IN_MONTH[self->month];
+    if (self->month == 2 && is_leap_year(year)) {
+        days_in_month += 1;
+    }
+
+    // This equation seems magical, so I'll break it down:
+    // 1. calendar says 0 = Monday, POSIX says 0 = Sunday so we need first_day
+    //    + 1 to get 1 = Monday -> 7 = Sunday, which is still equivalent
+    //    because this math is mod 7
+    // 2. Get first day - desired day mod 7 (adjusting by 7 for negative
+    //    numbers so that -1 % 7 = 6).
+    // 3. Add 1 because month days are a 1-based index.
+    int8_t month_day = ((int8_t)(self->day) - (first_day + 1)) % 7;
+    if (month_day < 0) {
+        month_day += 7;
+    }
+    month_day += 1;
+
+    // Now use a 0-based index version of `week` to calculate the w-th
+    // occurrence of `day`
+    month_day += ((int8_t)(self->week) - 1) * 7;
+
+    // month_day will only be > days_in_month if w was 5, and `w` means "last
+    // occurrence of `d`", so now we just check if we over-shot the end of the
+    // month and if so knock off 1 week.
+    if (month_day > days_in_month) {
+        month_day -= 7;
+    }
+
+    int64_t ordinal = ymd_to_ord(year, self->month, month_day) - EPOCHORDINAL;
+    return ((ordinal * 86400) + (int64_t)(self->hour * 3600) +
+            (int64_t)(self->minute * 60) + (int64_t)(self->second));
+}
+
+/* Constructor for CalendarRule. */
+int
+calendarrule_new(uint8_t month, uint8_t week, uint8_t day, int8_t hour,
+                 int8_t minute, int8_t second, CalendarRule *out)
+{
+    // These bounds come from the POSIX standard, which describes an Mm.n.d
+    // rule as:
+    //
+    //   The d'th day (0 <= d <= 6) of week n of month m of the year (1 <= n <=
+    //   5, 1 <= m <= 12, where week 5 means "the last d day in month m" which
+    //   may occur in either the fourth or the fifth week). Week 1 is the first
+    //   week in which the d'th day occurs. Day zero is Sunday.
+    if (month <= 0 || month > 12) {
+        PyErr_Format(PyExc_ValueError, "Month must be in (0, 12]");
+        return -1;
+    }
+
+    if (week <= 0 || week > 5) {
+        PyErr_Format(PyExc_ValueError, "Week must be in (0, 5]");
+        return -1;
+    }
+
+    // day is an unsigned integer, so day < 0 should always return false, but
+    // if day's type changes to a signed integer *without* changing this value,
+    // it may create a bug. Considering that the compiler should be able to
+    // optimize out the first comparison if day is an unsigned integer anyway,
+    // we will leave this comparison in place and disable the compiler warning.
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wtype-limits"
+    if (day < 0 || day > 6) {
+#pragma GCC diagnostic pop
+        PyErr_Format(PyExc_ValueError, "Day must be in [0, 6]");
+        return -1;
+    }
+
+    TransitionRuleType base = {&calendarrule_year_to_timestamp};
+
+    CalendarRule new_offset = {
+        .base = base,
+        .month = month,
+        .week = week,
+        .day = day,
+        .hour = hour,
+        .minute = minute,
+        .second = second,
+    };
+
+    *out = new_offset;
+    return 0;
+}
+
+/* Function to calculate the local timestamp of a transition from the year.
+ *
+ * This translates the day of the year into a local timestamp — either a
+ * 1-based Julian day, not including leap days, or the 0-based year-day,
+ * including leap days.
+ * */
+int64_t
+dayrule_year_to_timestamp(TransitionRuleType *base_self, int year)
+{
+    // The function signature requires a TransitionRuleType pointer, but this
+    // function is only applicable to DayRule* objects.
+    DayRule *self = (DayRule *)base_self;
+
+    // ymd_to_ord calculates the number of days since 0001-01-01, but we want
+    // to know the number of days since 1970-01-01, so we must subtract off
+    // the equivalent of ymd_to_ord(1970, 1, 1).
+    //
+    // We subtract off an additional 1 day to account for January 1st (we want
+    // the number of full days *before* the date of the transition - partial
+    // days are accounted for in the hour, minute and second portions.
+    int64_t days_before_year = ymd_to_ord(year, 1, 1) - EPOCHORDINAL - 1;
+
+    // The Julian day specification skips over February 29th in leap years,
+    // from the POSIX standard:
+    //
+    //   Leap days shall not be counted. That is, in all years-including leap
+    //   years-February 28 is day 59 and March 1 is day 60. It is impossible to
+    //   refer explicitly to the occasional February 29.
+    //
+    // This is actually more useful than you'd think — if you want a rule that
+    // always transitions on a given calendar day (other than February 29th),
+    // you would use a Julian day, e.g. J91 always refers to April 1st and J365
+    // always refers to December 31st.
+    unsigned int day = self->day;
+    if (self->julian && day >= 59 && is_leap_year(year)) {
+        day += 1;
+    }
+
+    return ((days_before_year + day) * 86400) + (self->hour * 3600) +
+           (self->minute * 60) + self->second;
+}
+
+/* Constructor for DayRule. */
+static int
+dayrule_new(uint8_t julian, unsigned int day, int8_t hour, int8_t minute,
+            int8_t second, DayRule *out)
+{
+    // The POSIX standard specifies that Julian days must be in the range (1 <=
+    // n <= 365) and that non-Julian (they call it "0-based Julian") days must
+    // be in the range (0 <= n <= 365).
+    if (day < julian || day > 365) {
+        PyErr_Format(PyExc_ValueError, "day must be in [%u, 365], not: %u",
+                     julian, day);
+        return -1;
+    }
+
+    TransitionRuleType base = {
+        &dayrule_year_to_timestamp,
+    };
+
+    DayRule tmp = {
+        .base = base,
+        .julian = julian,
+        .day = day,
+        .hour = hour,
+        .minute = minute,
+        .second = second,
+    };
+
+    *out = tmp;
+
+    return 0;
+}
+
+/* Calculate the start and end rules for a _tzrule in the given year. */
+static void
+tzrule_transitions(_tzrule *rule, int year, int64_t *start, int64_t *end)
+{
+    assert(rule->start != NULL);
+    assert(rule->end != NULL);
+    *start = rule->start->year_to_timestamp(rule->start, year);
+    *end = rule->end->year_to_timestamp(rule->end, year);
+}
+
+/* Calculate the _ttinfo that applies at a given local time from a _tzrule.
+ *
+ * This takes a local timestamp and fold for disambiguation purposes; the year
+ * could technically be calculated from the timestamp, but given that the
+ * callers of this function already have the year information accessible from
+ * the datetime struct, it is taken as an additional parameter to reduce
+ * unncessary calculation.
+ * */
+static _ttinfo *
+find_tzrule_ttinfo(_tzrule *rule, int64_t ts, unsigned char fold, int year)
+{
+    if (rule->std_only) {
+        return &(rule->std);
+    }
+
+    int64_t start, end;
+    uint8_t isdst;
+
+    tzrule_transitions(rule, year, &start, &end);
+
+    // With fold = 0, the period (denominated in local time) with the smaller
+    // offset starts at the end of the gap and ends at the end of the fold;
+    // with fold = 1, it runs from the start of the gap to the beginning of the
+    // fold.
+    //
+    // So in order to determine the DST boundaries we need to know both the
+    // fold and whether DST is positive or negative (rare), and it turns out
+    // that this boils down to fold XOR is_positive.
+    if (fold == (rule->dst_diff >= 0)) {
+        end -= rule->dst_diff;
+    }
+    else {
+        start += rule->dst_diff;
+    }
+
+    if (start < end) {
+        isdst = (ts >= start) && (ts < end);
+    }
+    else {
+        isdst = (ts < end) || (ts >= start);
+    }
+
+    if (isdst) {
+        return &(rule->dst);
+    }
+    else {
+        return &(rule->std);
+    }
+}
+
+/* Calculate the ttinfo and fold that applies for a _tzrule at an epoch time.
+ *
+ * This function can determine the _ttinfo that applies at a given epoch time,
+ * (analogous to trans_list_utc), and whether or not the datetime is in a fold.
+ * This is to be used in the .fromutc() function.
+ *
+ * The year is technically a redundant parameter, because it can be calculated
+ * from the timestamp, but all callers of this function should have the year
+ * in the datetime struct anyway, so taking it as a parameter saves unnecessary
+ * calculation.
+ **/
+static _ttinfo *
+find_tzrule_ttinfo_fromutc(_tzrule *rule, int64_t ts, int year,
+                           unsigned char *fold)
+{
+    if (rule->std_only) {
+        *fold = 0;
+        return &(rule->std);
+    }
+
+    int64_t start, end;
+    uint8_t isdst;
+    tzrule_transitions(rule, year, &start, &end);
+    start -= rule->std.utcoff_seconds;
+    end -= rule->dst.utcoff_seconds;
+
+    if (start < end) {
+        isdst = (ts >= start) && (ts < end);
+    }
+    else {
+        isdst = (ts < end) || (ts >= start);
+    }
+
+    // For positive DST, the ambiguous period is one dst_diff after the end of
+    // DST; for negative DST, the ambiguous period is one dst_diff before the
+    // start of DST.
+    int64_t ambig_start, ambig_end;
+    if (rule->dst_diff > 0) {
+        ambig_start = end;
+        ambig_end = end + rule->dst_diff;
+    }
+    else {
+        ambig_start = start;
+        ambig_end = start - rule->dst_diff;
+    }
+
+    *fold = (ts >= ambig_start) && (ts < ambig_end);
+
+    if (isdst) {
+        return &(rule->dst);
+    }
+    else {
+        return &(rule->std);
+    }
+}
+
+/* Parse a TZ string in the format specified by the POSIX standard:
+ *
+ *  std offset[dst[offset],start[/time],end[/time]]
+ *
+ *  std and dst must be 3 or more characters long and must not contain a
+ *  leading colon, embedded digits, commas, nor a plus or minus signs; The
+ *  spaces between "std" and "offset" are only for display and are not actually
+ *  present in the string.
+ *
+ *  The format of the offset is ``[+|-]hh[:mm[:ss]]``
+ *
+ * See the POSIX.1 spec: IEE Std 1003.1-2018 §8.3:
+ *
+ * https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html
+ */
+static int
+parse_tz_str(PyObject *tz_str_obj, _tzrule *out)
+{
+    PyObject *std_abbr = NULL;
+    PyObject *dst_abbr = NULL;
+    TransitionRuleType *start = NULL;
+    TransitionRuleType *end = NULL;
+    long std_offset, dst_offset;
+
+    char *tz_str = PyBytes_AsString(tz_str_obj);
+    if (tz_str == NULL) {
+        return -1;
+    }
+    char *p = tz_str;
+
+    // Read the `std` abbreviation, which must be at least 3 characters long.
+    ssize_t num_chars = parse_abbr(p, &std_abbr);
+    if (num_chars < 1) {
+        PyErr_Format(PyExc_ValueError, "Invalid STD format in %R", tz_str_obj);
+        goto error;
+    }
+
+    p += num_chars;
+
+    // Now read the STD offset, which is required
+    num_chars = parse_tz_delta(p, &std_offset);
+    if (num_chars < 0) {
+        PyErr_Format(PyExc_ValueError, "Invalid STD offset in %R", tz_str_obj);
+        goto error;
+    }
+    p += num_chars;
+
+    // If the string ends here, there is no DST, otherwise we must parse the
+    // DST abbreviation and start and end dates and times.
+    if (*p == '\0') {
+        goto complete;
+    }
+
+    num_chars = parse_abbr(p, &dst_abbr);
+    if (num_chars < 1) {
+        PyErr_Format(PyExc_ValueError, "Invalid DST format in %R", tz_str_obj);
+        goto error;
+    }
+    p += num_chars;
+
+    if (*p == ',') {
+        // From the POSIX standard:
+        //
+        // If no offset follows dst, the alternative time is assumed to be one
+        // hour ahead of standard time.
+        dst_offset = std_offset + 3600;
+    }
+    else {
+        num_chars = parse_tz_delta(p, &dst_offset);
+        if (num_chars < 0) {
+            PyErr_Format(PyExc_ValueError, "Invalid DST offset in %R",
+                         tz_str_obj);
+            goto error;
+        }
+
+        p += num_chars;
+    }
+
+    TransitionRuleType **transitions[2] = {&start, &end};
+    for (size_t i = 0; i < 2; ++i) {
+        if (*p != ',') {
+            PyErr_Format(PyExc_ValueError,
+                         "Missing transition rules in TZ string: %R",
+                         tz_str_obj);
+            goto error;
+        }
+        p++;
+
+        num_chars = parse_transition_rule(p, transitions[i]);
+        if (num_chars < 0) {
+            PyErr_Format(PyExc_ValueError,
+                         "Malformed transition rule in TZ string: %R",
+                         tz_str_obj);
+            goto error;
+        }
+        p += num_chars;
+    }
+
+    if (*p != '\0') {
+        PyErr_Format(PyExc_ValueError,
+                     "Extraneous characters at end of TZ string: %R",
+                     tz_str_obj);
+        goto error;
+    }
+
+complete:
+    build_tzrule(std_abbr, dst_abbr, std_offset, dst_offset, start, end, out);
+    Py_DECREF(std_abbr);
+    Py_XDECREF(dst_abbr);
+
+    return 0;
+error:
+    Py_XDECREF(std_abbr);
+    if (dst_abbr != NULL && dst_abbr != Py_None) {
+        Py_DECREF(dst_abbr);
+    }
+
+    if (start != NULL) {
+        PyMem_Free(start);
+    }
+
+    if (end != NULL) {
+        PyMem_Free(end);
+    }
+
+    return -1;
+}
+
+static ssize_t
+parse_uint(const char *const p)
+{
+    if (!isdigit(*p)) {
+        return -1;
+    }
+
+    return (*p) - '0';
+}
+
+/* Parse the STD and DST abbreviations from a TZ string. */
+static ssize_t
+parse_abbr(const char *const p, PyObject **abbr)
+{
+    const char *ptr = p;
+    char buff = *ptr;
+    const char *str_start;
+    const char *str_end;
+
+    if (*ptr == '<') {
+        ptr++;
+        str_start = ptr;
+        while ((buff = *ptr) != '>') {
+            // From the POSIX standard:
+            //
+            //   In the quoted form, the first character shall be the less-than
+            //   ( '<' ) character and the last character shall be the
+            //   greater-than ( '>' ) character. All characters between these
+            //   quoting characters shall be alphanumeric characters from the
+            //   portable character set in the current locale, the plus-sign (
+            //   '+' ) character, or the minus-sign ( '-' ) character. The std
+            //   and dst fields in this case shall not include the quoting
+            //   characters.
+            if (!isalpha(buff) && !isdigit(buff) && buff != '+' &&
+                buff != '-') {
+                return -1;
+            }
+            ptr++;
+        }
+        str_end = ptr;
+        ptr++;
+    }
+    else {
+        str_start = p;
+        // From the POSIX standard:
+        //
+        //   In the unquoted form, all characters in these fields shall be
+        //   alphabetic characters from the portable character set in the
+        //   current locale.
+        while (isalpha(*ptr)) {
+            ptr++;
+        }
+        str_end = ptr;
+    }
+
+    *abbr = PyUnicode_FromStringAndSize(str_start, str_end - str_start);
+    if (abbr == NULL) {
+        return -1;
+    }
+
+    return ptr - p;
+}
+
+/* Parse a UTC offset from a TZ str. */
+static ssize_t
+parse_tz_delta(const char *const p, long *total_seconds)
+{
+    // From the POSIX spec:
+    //
+    //   Indicates the value added to the local time to arrive at Coordinated
+    //   Universal Time. The offset has the form:
+    //
+    //   hh[:mm[:ss]]
+    //
+    //   One or more digits may be used; the value is always interpreted as a
+    //   decimal number.
+    //
+    // The POSIX spec says that the values for `hour` must be between 0 and 24
+    // hours, but RFC 8536 §3.3.1 specifies that the hours part of the
+    // transition times may be signed and range from -167 to 167.
+    long sign = -1;
+    long hours = 0;
+    long minutes = 0;
+    long seconds = 0;
+
+    const char *ptr = p;
+    char buff = *ptr;
+    if (buff == '-' || buff == '+') {
+        // Negative numbers correspond to *positive* offsets, from the spec:
+        //
+        //   If preceded by a '-', the timezone shall be east of the Prime
+        //   Meridian; otherwise, it shall be west (which may be indicated by
+        //   an optional preceding '+' ).
+        if (buff == '-') {
+            sign = 1;
+        }
+
+        ptr++;
+    }
+
+    // The hour can be 1 or 2 numeric characters
+    for (size_t i = 0; i < 2; ++i) {
+        buff = *ptr;
+        if (!isdigit(buff)) {
+            if (i == 0) {
+                return -1;
+            }
+            else {
+                break;
+            }
+        }
+
+        hours *= 10;
+        hours += buff - '0';
+        ptr++;
+    }
+
+    if (hours > 24 || hours < 0) {
+        return -1;
+    }
+
+    // Minutes and seconds always of the format ":dd"
+    long *outputs[2] = {&minutes, &seconds};
+    for (size_t i = 0; i < 2; ++i) {
+        if (*ptr != ':') {
+            goto complete;
+        }
+        ptr++;
+
+        for (size_t j = 0; j < 2; ++j) {
+            buff = *ptr;
+            if (!isdigit(buff)) {
+                return -1;
+            }
+            *(outputs[i]) *= 10;
+            *(outputs[i]) += buff - '0';
+            ptr++;
+        }
+    }
+
+complete:
+    *total_seconds = sign * ((hours * 3600) + (minutes * 60) + seconds);
+
+    return ptr - p;
+}
+
+/* Parse the date portion of a transition rule. */
+static ssize_t
+parse_transition_rule(const char *const p, TransitionRuleType **out)
+{
+    // The full transition rule indicates when to change back and forth between
+    // STD and DST, and has the form:
+    //
+    //   date[/time],date[/time]
+    //
+    // This function parses an individual date[/time] section, and returns
+    // the number of characters that contributed to the transition rule. This
+    // does not include the ',' at the end of the first rule.
+    //
+    // The POSIX spec states that if *time* is not given, the default is 02:00.
+    const char *ptr = p;
+    int8_t hour = 2;
+    int8_t minute = 0;
+    int8_t second = 0;
+
+    // Rules come in one of three flavors:
+    //
+    //   1. Jn: Julian day n, with no leap days.
+    //   2. n: Day of year (0-based, with leap days)
+    //   3. Mm.n.d: Specifying by month, week and day-of-week.
+
+    if (*ptr == 'M') {
+        uint8_t month, week, day;
+        ptr++;
+        ssize_t tmp = parse_uint(ptr);
+        if (tmp < 0) {
+            return -1;
+        }
+        month = (uint8_t)tmp;
+        ptr++;
+        if (*ptr != '.') {
+            tmp = parse_uint(ptr);
+            if (tmp < 0) {
+                return -1;
+            }
+
+            month *= 10;
+            month += (uint8_t)tmp;
+            ptr++;
+        }
+
+        uint8_t *values[2] = {&week, &day};
+        for (size_t i = 0; i < 2; ++i) {
+            if (*ptr != '.') {
+                return -1;
+            }
+            ptr++;
+
+            tmp = parse_uint(ptr);
+            if (tmp < 0) {
+                return -1;
+            }
+            ptr++;
+
+            *(values[i]) = tmp;
+        }
+
+        if (*ptr == '/') {
+            ptr++;
+            ssize_t num_chars =
+                parse_transition_time(ptr, &hour, &minute, &second);
+            if (num_chars < 0) {
+                return -1;
+            }
+            ptr += num_chars;
+        }
+
+        CalendarRule *rv = PyMem_Calloc(1, sizeof(CalendarRule));
+        if (rv == NULL) {
+            return -1;
+        }
+
+        if (calendarrule_new(month, week, day, hour, minute, second, rv)) {
+            PyMem_Free(rv);
+            return -1;
+        }
+
+        *out = (TransitionRuleType *)rv;
+    }
+    else {
+        uint8_t julian = 0;
+        unsigned int day = 0;
+        if (*ptr == 'J') {
+            julian = 1;
+            ptr++;
+        }
+
+        for (size_t i = 0; i < 3; ++i) {
+            if (!isdigit(*ptr)) {
+                if (i == 0) {
+                    return -1;
+                }
+                break;
+            }
+            day *= 10;
+            day += (*ptr) - '0';
+            ptr++;
+        }
+
+        if (*ptr == '/') {
+            ptr++;
+            ssize_t num_chars =
+                parse_transition_time(ptr, &hour, &minute, &second);
+            if (num_chars < 0) {
+                return -1;
+            }
+            ptr += num_chars;
+        }
+
+        DayRule *rv = PyMem_Calloc(1, sizeof(DayRule));
+        if (rv == NULL) {
+            return -1;
+        }
+
+        if (dayrule_new(julian, day, hour, minute, second, rv)) {
+            PyMem_Free(rv);
+            return -1;
+        }
+        *out = (TransitionRuleType *)rv;
+    }
+
+    return ptr - p;
+}
+
+/* Parse the time portion of a transition rule (e.g. following an /) */
+static ssize_t
+parse_transition_time(const char *const p, int8_t *hour, int8_t *minute,
+                      int8_t *second)
+{
+    // From the spec:
+    //
+    //   The time has the same format as offset except that no leading sign
+    //   ( '-' or '+' ) is allowed.
+    //
+    // The format for the offset is:
+    //
+    //   h[h][:mm[:ss]]
+    //
+    // RFC 8536 also allows transition times to be signed and to range from
+    // -167 to +167, but the current version only supports [0, 99].
+    //
+    // TODO: Support the full range of transition hours.
+    int8_t *components[3] = {hour, minute, second};
+    const char *ptr = p;
+    int8_t sign = 1;
+
+    if (*ptr == '-' || *ptr == '+') {
+        if (*ptr == '-') {
+            sign = -1;
+        }
+        ptr++;
+    }
+
+    for (size_t i = 0; i < 3; ++i) {
+        if (i > 0) {
+            if (*ptr != ':') {
+                break;
+            }
+            ptr++;
+        }
+
+        uint8_t buff = 0;
+        for (size_t j = 0; j < 2; j++) {
+            if (!isdigit(*ptr)) {
+                if (i == 0 && j > 0) {
+                    break;
+                }
+                return -1;
+            }
+
+            buff *= 10;
+            buff += (*ptr) - '0';
+            ptr++;
+        }
+
+        *(components[i]) = sign * buff;
+    }
+
+    return ptr - p;
+}
+
+/* Constructor for a _tzrule.
+ *
+ * If `dst_abbr` is NULL, this will construct an "STD-only" _tzrule, in which
+ * case `dst_offset` will be ignored and `start` and `end` are expected to be
+ * NULL as well.
+ *
+ * Returns 0 on success.
+ */
+static int
+build_tzrule(PyObject *std_abbr, PyObject *dst_abbr, long std_offset,
+             long dst_offset, TransitionRuleType *start,
+             TransitionRuleType *end, _tzrule *out)
+{
+    _tzrule rv = {0};
+
+    rv.start = start;
+    rv.end = end;
+
+    if (build_ttinfo(std_offset, 0, std_abbr, &rv.std)) {
+        goto error;
+    }
+
+    if (dst_abbr != NULL) {
+        rv.dst_diff = dst_offset - std_offset;
+        if (build_ttinfo(dst_offset, rv.dst_diff, dst_abbr, &rv.dst)) {
+            goto error;
+        }
+    }
+    else {
+        rv.std_only = 1;
+    }
+
+    *out = rv;
+
+    return 0;
+error:
+    xdecref_ttinfo(&rv.std);
+    xdecref_ttinfo(&rv.dst);
+    return -1;
+}
+
+/* Destructor for _tzrule. */
+static void
+free_tzrule(_tzrule *tzrule)
+{
+    xdecref_ttinfo(&(tzrule->std));
+    if (!tzrule->std_only) {
+        xdecref_ttinfo(&(tzrule->dst));
+    }
+
+    if (tzrule->start != NULL) {
+        PyMem_Free(tzrule->start);
+    }
+
+    if (tzrule->end != NULL) {
+        PyMem_Free(tzrule->end);
+    }
+}
+
+/* Calculate DST offsets from transitions and UTC offsets
+ *
+ * This is necessary because each C `ttinfo` only contains the UTC offset,
+ * time zone abbreviation and an isdst boolean - it does not include the
+ * amount of the DST offset, but we need the amount for the dst() function.
+ *
+ * Thus function uses heuristics to infer what the offset should be, so it
+ * is not guaranteed that this will work for all zones. If we cannot assign
+ * a value for a given DST offset, we'll assume it's 1H rather than 0H, so
+ * bool(dt.dst()) will always match ttinfo.isdst.
+ */
+static void
+utcoff_to_dstoff(size_t *trans_idx, long *utcoffs, long *dstoffs,
+                 unsigned char *isdsts, size_t num_transitions,
+                 size_t num_ttinfos)
+{
+    size_t dst_count = 0;
+    size_t dst_found = 0;
+    for (size_t i = 0; i < num_ttinfos; ++i) {
+        dst_count++;
+    }
+
+    for (size_t i = 1; i < num_transitions; ++i) {
+        if (dst_count == dst_found) {
+            break;
+        }
+
+        size_t idx = trans_idx[i];
+        size_t comp_idx = trans_idx[i - 1];
+
+        // Only look at DST offsets that have nto been assigned already
+        if (!isdsts[idx] || dstoffs[idx] != 0) {
+            continue;
+        }
+
+        long dstoff = 0;
+        long utcoff = utcoffs[idx];
+
+        if (!isdsts[comp_idx]) {
+            dstoff = utcoff - utcoffs[comp_idx];
+        }
+
+        if (!dstoff && idx < (num_ttinfos - 1)) {
+            comp_idx = trans_idx[i + 1];
+
+            // If the following transition is also DST and we couldn't find
+            // the DST offset by this point, we're going to have to skip it
+            // and hope this transition gets assigned later
+            if (isdsts[comp_idx]) {
+                continue;
+            }
+
+            dstoff = utcoff - utcoffs[comp_idx];
+        }
+
+        if (dstoff) {
+            dst_found++;
+            dstoffs[idx] = dstoff;
+        }
+    }
+
+    if (dst_found < dst_count) {
+        // If there are time zones we didn't find a value for, we'll end up
+        // with dstoff = 0 for something where isdst=1. This is obviously
+        // wrong — one hour will be a much better guess than 0.
+        for (size_t idx = 0; idx < num_ttinfos; ++idx) {
+            if (isdsts[idx] && !dstoffs[idx]) {
+                dstoffs[idx] = 3600;
+            }
+        }
+    }
+}
+
+#define _swap(x, y, buffer) \
+    buffer = x;             \
+    x = y;                  \
+    y = buffer;
+
+/* Calculate transitions in local time from UTC time and offsets.
+ *
+ * We want to know when each transition occurs, denominated in the number of
+ * nominal wall-time seconds between 1970-01-01T00:00:00 and the transition in
+ * *local time* (note: this is *not* equivalent to the output of
+ * datetime.timestamp, which is the total number of seconds actual elapsed
+ * since 1970-01-01T00:00:00Z in UTC).
+ *
+ * This is an ambiguous question because "local time" can be ambiguous — but it
+ * is disambiguated by the `fold` parameter, so we allocate two arrays:
+ *
+ *  trans_local[0]: The wall-time transitions for fold=0
+ *  trans_local[1]: The wall-time transitions for fold=1
+ *
+ * This returns 0 on success and a negative number of failure. The trans_local
+ * arrays must be freed if they are not NULL.
+ */
+static int
+ts_to_local(size_t *trans_idx, int64_t *trans_utc, long *utcoff,
+            int64_t *trans_local[2], size_t num_ttinfos,
+            size_t num_transitions)
+{
+    if (num_transitions == 0) {
+        return 0;
+    }
+
+    // Copy the UTC transitions into each array to be modified in place later
+    for (size_t i = 0; i < 2; ++i) {
+        trans_local[i] = PyMem_Malloc(num_transitions * sizeof(int64_t));
+        if (trans_local[i] == NULL) {
+            return -1;
+        }
+
+        memcpy(trans_local[i], trans_utc, num_transitions * sizeof(int64_t));
+    }
+
+    int64_t offset_0, offset_1, buff;
+    if (num_ttinfos > 1) {
+        offset_0 = utcoff[0];
+        offset_1 = utcoff[trans_idx[0]];
+
+        if (offset_1 > offset_0) {
+            _swap(offset_0, offset_1, buff);
+        }
+    }
+    else {
+        offset_0 = utcoff[0];
+        offset_1 = utcoff[0];
+    }
+
+    trans_local[0][0] += offset_0;
+    trans_local[1][0] += offset_1;
+
+    for (size_t i = 1; i < num_transitions; ++i) {
+        offset_0 = utcoff[trans_idx[i - 1]];
+        offset_1 = utcoff[trans_idx[i]];
+
+        if (offset_1 > offset_0) {
+            _swap(offset_1, offset_0, buff);
+        }
+
+        trans_local[0][i] += offset_0;
+        trans_local[1][i] += offset_1;
+    }
+
+    return 0;
+}
+
+/* Simple bisect_right binary search implementation */
+static size_t
+_bisect(const int64_t value, const int64_t *arr, size_t size)
+{
+    size_t lo = 0;
+    size_t hi = size;
+    size_t m;
+
+    while (lo < hi) {
+        m = (lo + hi) / 2;
+        if (arr[m] > value) {
+            hi = m;
+        }
+        else {
+            lo = m + 1;
+        }
+    }
+
+    return hi;
+}
+
+/* Find the ttinfo rules that apply at a given local datetime. */
+static _ttinfo *
+find_ttinfo(PyZoneInfo_ZoneInfo *self, PyObject *dt)
+{
+    // datetime.time has a .tzinfo attribute that passes None as the dt
+    // argument; it only really has meaning for fixed-offset zones.
+    if (dt == Py_None) {
+        if (self->fixed_offset) {
+            return &(self->tzrule_after.std);
+        }
+        else {
+            return &NO_TTINFO;
+        }
+    }
+
+    int64_t ts;
+    if (get_local_timestamp(dt, &ts)) {
+        return NULL;
+    }
+
+    unsigned char fold = PyDateTime_DATE_GET_FOLD(dt);
+    assert(fold < 2);
+    int64_t *local_transitions = self->trans_list_wall[fold];
+    size_t num_trans = self->num_transitions;
+
+    if (num_trans && ts < local_transitions[0]) {
+        return self->ttinfo_before;
+    }
+    else if (!num_trans || ts > local_transitions[self->num_transitions - 1]) {
+        return find_tzrule_ttinfo(&(self->tzrule_after), ts, fold,
+                                  PyDateTime_GET_YEAR(dt));
+    }
+    else {
+        size_t idx = _bisect(ts, local_transitions, self->num_transitions) - 1;
+        assert(idx < self->num_transitions);
+        return self->trans_ttinfos[idx];
+    }
+}
+
+static int
+is_leap_year(int year)
+{
+    const unsigned int ayear = (unsigned int)year;
+    return ayear % 4 == 0 && (ayear % 100 != 0 || ayear % 400 == 0);
+}
+
+/* Calculates ordinal datetime from year, month and day. */
+static int
+ymd_to_ord(int y, int m, int d)
+{
+    y -= 1;
+    int days_before_year = (y * 365) + (y / 4) - (y / 100) + (y / 400);
+    int yearday = DAYS_BEFORE_MONTH[m];
+    if (m > 2 && is_leap_year(y + 1)) {
+        yearday += 1;
+    }
+
+    return days_before_year + yearday + d;
+}
+
+/* Calculate the number of seconds since 1970-01-01 in local time.
+ *
+ * This gets a datetime in the same "units" as self->trans_list_wall so that we
+ * can easily determine which transitions a datetime falls between. See the
+ * comment above ts_to_local for more information.
+ * */
+static int
+get_local_timestamp(PyObject *dt, int64_t *local_ts)
+{
+    assert(local_ts != NULL);
+
+    int hour, minute, second;
+    int ord;
+    if (PyDateTime_CheckExact(dt)) {
+        int y = PyDateTime_GET_YEAR(dt);
+        int m = PyDateTime_GET_MONTH(dt);
+        int d = PyDateTime_GET_DAY(dt);
+        hour = PyDateTime_DATE_GET_HOUR(dt);
+        minute = PyDateTime_DATE_GET_MINUTE(dt);
+        second = PyDateTime_DATE_GET_SECOND(dt);
+
+        ord = ymd_to_ord(y, m, d);
+    }
+    else {
+        PyObject *num = PyObject_CallMethod(dt, "toordinal", NULL);
+        if (num == NULL) {
+            return -1;
+        }
+
+        ord = PyLong_AsLong(num);
+        Py_DECREF(num);
+        if (ord == -1 && PyErr_Occurred()) {
+            return -1;
+        }
+
+        num = PyObject_GetAttrString(dt, "hour");
+        if (num == NULL) {
+            return -1;
+        }
+        hour = PyLong_AsLong(num);
+        Py_DECREF(num);
+        if (hour == -1) {
+            return -1;
+        }
+
+        num = PyObject_GetAttrString(dt, "minute");
+        if (num == NULL) {
+            return -1;
+        }
+        minute = PyLong_AsLong(num);
+        Py_DECREF(num);
+        if (minute == -1) {
+            return -1;
+        }
+
+        num = PyObject_GetAttrString(dt, "second");
+        if (num == NULL) {
+            return -1;
+        }
+        second = PyLong_AsLong(num);
+        Py_DECREF(num);
+        if (second == -1) {
+            return -1;
+        }
+    }
+
+    *local_ts = (int64_t)(ord - EPOCHORDINAL) * 86400 +
+                (int64_t)(hour * 3600 + minute * 60 + second);
+
+    return 0;
+}
+
+/////
+// Functions for cache handling
+
+/* Constructor for StrongCacheNode */
+static StrongCacheNode *
+strong_cache_node_new(PyObject *key, PyObject *zone)
+{
+    StrongCacheNode *node = PyMem_Malloc(sizeof(StrongCacheNode));
+    if (node == NULL) {
+        return NULL;
+    }
+
+    Py_INCREF(key);
+    Py_INCREF(zone);
+
+    node->next = NULL;
+    node->prev = NULL;
+    node->key = key;
+    node->zone = zone;
+
+    return node;
+}
+
+/* Destructor for StrongCacheNode */
+void
+strong_cache_node_free(StrongCacheNode *node)
+{
+    Py_XDECREF(node->key);
+    Py_XDECREF(node->zone);
+
+    PyMem_Free(node);
+}
+
+/* Frees all nodes at or after a specified root in the strong cache.
+ *
+ * This can be used on the root node to free the entire cache or it can be used
+ * to clear all nodes that have been expired (which, if everything is going
+ * right, will actually only be 1 node at a time).
+ */
+void
+strong_cache_free(StrongCacheNode *root)
+{
+    StrongCacheNode *node = root;
+    StrongCacheNode *next_node;
+    while (node != NULL) {
+        next_node = node->next;
+        strong_cache_node_free(node);
+
+        node = next_node;
+    }
+}
+
+/* Removes a node from the cache and update its neighbors.
+ *
+ * This is used both when ejecting a node from the cache and when moving it to
+ * the front of the cache.
+ */
+static void
+remove_from_strong_cache(StrongCacheNode *node)
+{
+    if (ZONEINFO_STRONG_CACHE == node) {
+        ZONEINFO_STRONG_CACHE = node->next;
+    }
+
+    if (node->prev != NULL) {
+        node->prev->next = node->next;
+    }
+
+    if (node->next != NULL) {
+        node->next->prev = node->prev;
+    }
+
+    node->next = NULL;
+    node->prev = NULL;
+}
+
+/* Retrieves the node associated with a key, if it exists.
+ *
+ * This traverses the strong cache until it finds a matching key and returns a
+ * pointer to the relevant node if found. Returns NULL if no node is found.
+ *
+ * root may be NULL, indicating an empty cache.
+ */
+static StrongCacheNode *
+find_in_strong_cache(const StrongCacheNode *const root, PyObject *const key)
+{
+    const StrongCacheNode *node = root;
+    while (node != NULL) {
+        if (PyObject_RichCompareBool(key, node->key, Py_EQ)) {
+            return (StrongCacheNode *)node;
+        }
+
+        node = node->next;
+    }
+
+    return NULL;
+}
+
+/* Ejects a given key from the class's strong cache, if applicable.
+ *
+ * This function is used to enable the per-key functionality in clear_cache.
+ */
+static void
+eject_from_strong_cache(const PyTypeObject *const type, PyObject *key)
+{
+    if (type != &PyZoneInfo_ZoneInfoType) {
+        return;
+    }
+
+    StrongCacheNode *node = find_in_strong_cache(ZONEINFO_STRONG_CACHE, key);
+    if (node != NULL) {
+        remove_from_strong_cache(node);
+
+        strong_cache_node_free(node);
+    }
+}
+
+/* Moves a node to the front of the LRU cache.
+ *
+ * The strong cache is an LRU cache, so whenever a given node is accessed, if
+ * it is not at the front of the cache, it needs to be moved there.
+ */
+static void
+move_strong_cache_node_to_front(StrongCacheNode **root, StrongCacheNode *node)
+{
+    StrongCacheNode *root_p = *root;
+    if (root_p == node) {
+        return;
+    }
+
+    remove_from_strong_cache(node);
+
+    node->prev = NULL;
+    node->next = root_p;
+
+    if (root_p != NULL) {
+        root_p->prev = node;
+    }
+
+    *root = node;
+}
+
+/* Retrieves a ZoneInfo from the strong cache if it's present.
+ *
+ * This function finds the ZoneInfo by key and if found will move the node to
+ * the front of the LRU cache and return a new reference to it. It returns NULL
+ * if the key is not in the cache.
+ *
+ * The strong cache is currently only implemented for the base class, so this
+ * always returns a cache miss for subclasses.
+ */
+static PyObject *
+zone_from_strong_cache(const PyTypeObject *const type, PyObject *const key)
+{
+    if (type != &PyZoneInfo_ZoneInfoType) {
+        return NULL;  // Strong cache currently only implemented for base class
+    }
+
+    StrongCacheNode *node = find_in_strong_cache(ZONEINFO_STRONG_CACHE, key);
+
+    if (node != NULL) {
+        move_strong_cache_node_to_front(&ZONEINFO_STRONG_CACHE, node);
+        Py_INCREF(node->zone);
+        return node->zone;
+    }
+
+    return NULL;  // Cache miss
+}
+
+/* Inserts a new key into the strong LRU cache.
+ *
+ * This function is only to be used after a cache miss — it creates a new node
+ * at the front of the cache and ejects any stale entries (keeping the size of
+ * the cache to at most ZONEINFO_STRONG_CACHE_MAX_SIZE).
+ */
+static void
+update_strong_cache(const PyTypeObject *const type, PyObject *key,
+                    PyObject *zone)
+{
+    if (type != &PyZoneInfo_ZoneInfoType) {
+        return;
+    }
+
+    StrongCacheNode *new_node = strong_cache_node_new(key, zone);
+
+    move_strong_cache_node_to_front(&ZONEINFO_STRONG_CACHE, new_node);
+
+    StrongCacheNode *node = new_node->next;
+    for (size_t i = 1; i < ZONEINFO_STRONG_CACHE_MAX_SIZE; ++i) {
+        if (node == NULL) {
+            return;
+        }
+        node = node->next;
+    }
+
+    // Everything beyond this point needs to be freed
+    if (node != NULL) {
+        if (node->prev != NULL) {
+            node->prev->next = NULL;
+        }
+        strong_cache_free(node);
+    }
+}
+
+/* Clears all entries into a type's strong cache.
+ *
+ * Because the strong cache is not implemented for subclasses, this is a no-op
+ * for everything except the base class.
+ */
+void
+clear_strong_cache(const PyTypeObject *const type)
+{
+    if (type != &PyZoneInfo_ZoneInfoType) {
+        return;
+    }
+
+    strong_cache_free(ZONEINFO_STRONG_CACHE);
+}
+
+static PyObject *
+new_weak_cache()
+{
+    PyObject *weakref_module = PyImport_ImportModule("weakref");
+    if (weakref_module == NULL) {
+        return NULL;
+    }
+
+    PyObject *weak_cache =
+        PyObject_CallMethod(weakref_module, "WeakValueDictionary", "");
+    Py_DECREF(weakref_module);
+    return weak_cache;
+}
+
+static int
+initialize_caches()
+{
+    if (TIMEDELTA_CACHE == NULL) {
+        TIMEDELTA_CACHE = PyDict_New();
+    }
+    else {
+        Py_INCREF(TIMEDELTA_CACHE);
+    }
+
+    if (TIMEDELTA_CACHE == NULL) {
+        return -1;
+    }
+
+    if (ZONEINFO_WEAK_CACHE == NULL) {
+        ZONEINFO_WEAK_CACHE = new_weak_cache();
+    }
+    else {
+        Py_INCREF(ZONEINFO_WEAK_CACHE);
+    }
+
+    if (ZONEINFO_WEAK_CACHE == NULL) {
+        return -1;
+    }
+
+    return 0;
+}
+
+static PyObject *
+zoneinfo_init_subclass(PyTypeObject *cls, PyObject *args, PyObject **kwargs)
+{
+    PyObject *weak_cache = new_weak_cache();
+    if (weak_cache == NULL) {
+        return NULL;
+    }
+
+    PyObject_SetAttrString((PyObject *)cls, "_weak_cache", weak_cache);
+    Py_RETURN_NONE;
+}
+
+/////
+// Specify the ZoneInfo type
+static PyMethodDef zoneinfo_methods[] = {
+    {"clear_cache", (PyCFunction)(void (*)(void))zoneinfo_clear_cache,
+     METH_VARARGS | METH_KEYWORDS | METH_CLASS,
+     PyDoc_STR("Clear the ZoneInfo cache.")},
+    {"no_cache", (PyCFunction)(void (*)(void))zoneinfo_no_cache,
+     METH_VARARGS | METH_KEYWORDS | METH_CLASS,
+     PyDoc_STR("Get a new instance of ZoneInfo, bypassing the cache.")},
+    {"from_file", (PyCFunction)(void (*)(void))zoneinfo_from_file,
+     METH_VARARGS | METH_KEYWORDS | METH_CLASS,
+     PyDoc_STR("Create a ZoneInfo file from a file object.")},
+    {"utcoffset", (PyCFunction)zoneinfo_utcoffset, METH_O,
+     PyDoc_STR("Retrieve a timedelta representing the UTC offset in a zone at "
+               "the given datetime.")},
+    {"dst", (PyCFunction)zoneinfo_dst, METH_O,
+     PyDoc_STR("Retrieve a timedelta representing the amount of DST applied "
+               "in a zone at the given datetime.")},
+    {"tzname", (PyCFunction)zoneinfo_tzname, METH_O,
+     PyDoc_STR("Retrieve a string containing the abbreviation for the time "
+               "zone that applies in a zone at a given datetime.")},
+    {"fromutc", (PyCFunction)zoneinfo_fromutc, METH_O,
+     PyDoc_STR("Given a datetime with local time in UTC, retrieve an adjusted "
+               "datetime in local time.")},
+    {"__reduce__", (PyCFunction)zoneinfo_reduce, METH_NOARGS,
+     PyDoc_STR("Function for serialization with the pickle protocol.")},
+    {"_unpickle", (PyCFunction)zoneinfo__unpickle, METH_VARARGS | METH_CLASS,
+     PyDoc_STR("Private method used in unpickling.")},
+    {"__init_subclass__", (PyCFunction)(void (*)(void))zoneinfo_init_subclass,
+     METH_VARARGS | METH_KEYWORDS,
+     PyDoc_STR("Function to initialize subclasses.")},
+    {NULL} /* Sentinel */
+};
+
+static PyMemberDef zoneinfo_members[] = {
+    {.name = "key",
+     .offset = offsetof(PyZoneInfo_ZoneInfo, key),
+     .type = T_OBJECT_EX,
+     .flags = READONLY,
+     .doc = NULL},
+    {NULL}, /* Sentinel */
+};
+
+static PyTypeObject PyZoneInfo_ZoneInfoType = {
+    PyVarObject_HEAD_INIT(NULL, 0)  //
+        .tp_name = "zoneinfo.ZoneInfo",
+    .tp_basicsize = sizeof(PyZoneInfo_ZoneInfo),
+    .tp_weaklistoffset = offsetof(PyZoneInfo_ZoneInfo, weakreflist),
+    .tp_repr = (reprfunc)zoneinfo_repr,
+    .tp_str = (reprfunc)zoneinfo_str,
+    .tp_getattro = PyObject_GenericGetAttr,
+    .tp_flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE),
+    /* .tp_doc = zoneinfo_doc, */
+    .tp_methods = zoneinfo_methods,
+    .tp_members = zoneinfo_members,
+    .tp_new = zoneinfo_new,
+    .tp_dealloc = zoneinfo_dealloc,
+};
+
+/////
+// Specify the _zoneinfo module
+static PyMethodDef module_methods[] = {{NULL, NULL}};
+static void
+module_free()
+{
+    Py_XDECREF(_tzpath_find_tzfile);
+    _tzpath_find_tzfile = NULL;
+
+    Py_XDECREF(_common_mod);
+    _common_mod = NULL;
+
+    Py_XDECREF(io_open);
+    io_open = NULL;
+
+    xdecref_ttinfo(&NO_TTINFO);
+
+    Py_XDECREF(TIMEDELTA_CACHE);
+    if (!Py_REFCNT(TIMEDELTA_CACHE)) {
+        TIMEDELTA_CACHE = NULL;
+    }
+
+    Py_XDECREF(ZONEINFO_WEAK_CACHE);
+    if (!Py_REFCNT(ZONEINFO_WEAK_CACHE)) {
+        ZONEINFO_WEAK_CACHE = NULL;
+    }
+
+    strong_cache_free(ZONEINFO_STRONG_CACHE);
+    ZONEINFO_STRONG_CACHE = NULL;
+}
+
+static int
+zoneinfomodule_exec(PyObject *m)
+{
+    PyDateTime_IMPORT;
+    PyZoneInfo_ZoneInfoType.tp_base = PyDateTimeAPI->TZInfoType;
+    if (PyType_Ready(&PyZoneInfo_ZoneInfoType) < 0) {
+        goto error;
+    }
+
+    Py_INCREF(&PyZoneInfo_ZoneInfoType);
+    PyModule_AddObject(m, "ZoneInfo", (PyObject *)&PyZoneInfo_ZoneInfoType);
+
+    /* Populate imports */
+    PyObject *_tzpath_module = PyImport_ImportModule("zoneinfo._tzpath");
+    if (_tzpath_module == NULL) {
+        goto error;
+    }
+
+    _tzpath_find_tzfile =
+        PyObject_GetAttrString(_tzpath_module, "find_tzfile");
+    Py_DECREF(_tzpath_module);
+    if (_tzpath_find_tzfile == NULL) {
+        goto error;
+    }
+
+    PyObject *io_module = PyImport_ImportModule("io");
+    if (io_module == NULL) {
+        goto error;
+    }
+
+    io_open = PyObject_GetAttrString(io_module, "open");
+    Py_DECREF(io_module);
+    if (io_open == NULL) {
+        goto error;
+    }
+
+    _common_mod = PyImport_ImportModule("zoneinfo._common");
+    if (_common_mod == NULL) {
+        goto error;
+    }
+
+    if (NO_TTINFO.utcoff == NULL) {
+        NO_TTINFO.utcoff = Py_None;
+        NO_TTINFO.dstoff = Py_None;
+        NO_TTINFO.tzname = Py_None;
+
+        for (size_t i = 0; i < 3; ++i) {
+            Py_INCREF(Py_None);
+        }
+    }
+
+    if (initialize_caches()) {
+        goto error;
+    }
+
+    return 0;
+
+error:
+    return -1;
+}
+
+static PyModuleDef_Slot zoneinfomodule_slots[] = {
+    {Py_mod_exec, zoneinfomodule_exec}, {0, NULL}};
+
+static struct PyModuleDef zoneinfomodule = {
+    PyModuleDef_HEAD_INIT,
+    .m_name = "_zoneinfo",
+    .m_doc = "C implementation of the zoneinfo module",
+    .m_size = 0,
+    .m_methods = module_methods,
+    .m_slots = zoneinfomodule_slots,
+    .m_free = (freefunc)module_free};
+
+PyMODINIT_FUNC
+PyInit__zoneinfo(void)
+{
+    return PyModuleDef_Init(&zoneinfomodule);
+}
diff --git a/PCbuild/_zoneinfo.vcxproj b/PCbuild/_zoneinfo.vcxproj
new file mode 100644
index 0000000..6e6389c
--- /dev/null
+++ b/PCbuild/_zoneinfo.vcxproj
@@ -0,0 +1,109 @@
+﻿<?xml version="1.0" encoding="utf-8"?>
+<Project DefaultTargets="Build" ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <ItemGroup Label="ProjectConfigurations">
+    <ProjectConfiguration Include="Debug|ARM">
+      <Configuration>Debug</Configuration>
+      <Platform>ARM</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="Debug|ARM64">
+      <Configuration>Debug</Configuration>
+      <Platform>ARM64</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="Debug|Win32">
+      <Configuration>Debug</Configuration>
+      <Platform>Win32</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="Debug|x64">
+      <Configuration>Debug</Configuration>
+      <Platform>x64</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="PGInstrument|ARM">
+      <Configuration>PGInstrument</Configuration>
+      <Platform>ARM</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="PGInstrument|ARM64">
+      <Configuration>PGInstrument</Configuration>
+      <Platform>ARM64</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="PGInstrument|Win32">
+      <Configuration>PGInstrument</Configuration>
+      <Platform>Win32</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="PGInstrument|x64">
+      <Configuration>PGInstrument</Configuration>
+      <Platform>x64</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="PGUpdate|ARM">
+      <Configuration>PGUpdate</Configuration>
+      <Platform>ARM</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="PGUpdate|ARM64">
+      <Configuration>PGUpdate</Configuration>
+      <Platform>ARM64</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="PGUpdate|Win32">
+      <Configuration>PGUpdate</Configuration>
+      <Platform>Win32</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="PGUpdate|x64">
+      <Configuration>PGUpdate</Configuration>
+      <Platform>x64</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="Release|ARM">
+      <Configuration>Release</Configuration>
+      <Platform>ARM</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="Release|ARM64">
+      <Configuration>Release</Configuration>
+      <Platform>ARM64</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="Release|Win32">
+      <Configuration>Release</Configuration>
+      <Platform>Win32</Platform>
+    </ProjectConfiguration>
+    <ProjectConfiguration Include="Release|x64">
+      <Configuration>Release</Configuration>
+      <Platform>x64</Platform>
+    </ProjectConfiguration>
+  </ItemGroup>
+  <PropertyGroup Label="Globals">
+    <ProjectGuid>{FCBE1EF2-E0F0-40B1-88B5-00A35D378742}</ProjectGuid>
+    <RootNamespace>_zoneinfo</RootNamespace>
+    <Keyword>Win32Proj</Keyword>
+  </PropertyGroup>
+  <Import Project="python.props" />
+  <Import Project="$(VCTargetsPath)\Microsoft.Cpp.Default.props" />
+  <PropertyGroup Label="Configuration">
+    <ConfigurationType>DynamicLibrary</ConfigurationType>
+    <CharacterSet>NotSet</CharacterSet>
+  </PropertyGroup>
+  <Import Project="$(VCTargetsPath)\Microsoft.Cpp.props" />
+  <PropertyGroup>
+    <TargetExt>.pyd</TargetExt>
+  </PropertyGroup>
+  <ImportGroup Label="ExtensionSettings">
+  </ImportGroup>
+  <ImportGroup Label="PropertySheets">
+    <Import Project="$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props" Condition="exists('$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props')" Label="LocalAppDataPlatform" />
+    <Import Project="pyproject.props" />
+  </ImportGroup>
+  <PropertyGroup Label="UserMacros" />
+  <PropertyGroup>
+    <_ProjectFileVersion>10.0.30319.1</_ProjectFileVersion>
+  </PropertyGroup>
+  <ItemGroup>
+    <ClCompile Include="..\Modules\_zoneinfo.c" />
+  </ItemGroup>
+  <ItemGroup>
+    <ResourceCompile Include="..\PC\python_nt.rc" />
+  </ItemGroup>
+  <ItemGroup>
+    <ProjectReference Include="pythoncore.vcxproj">
+      <Project>{cf7ac3d1-e2df-41d2-bea6-1e2556cdea26}</Project>
+      <ReferenceOutputAssembly>false</ReferenceOutputAssembly>
+    </ProjectReference>
+  </ItemGroup>
+  <Import Project="$(VCTargetsPath)\Microsoft.Cpp.targets" />
+  <ImportGroup Label="ExtensionTargets">
+  </ImportGroup>
+</Project>
diff --git a/PCbuild/_zoneinfo.vcxproj.filters b/PCbuild/_zoneinfo.vcxproj.filters
new file mode 100644
index 0000000..e3c3ef4
--- /dev/null
+++ b/PCbuild/_zoneinfo.vcxproj.filters
@@ -0,0 +1,16 @@
+﻿<?xml version="1.0" encoding="utf-8"?>
+<Project ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <ItemGroup>
+    <ResourceCompile Include="..\PC\python_nt.rc" />
+  </ItemGroup>
+  <ItemGroup>
+    <Filter Include="Source Files">
+      <UniqueIdentifier>{2422278e-eeeb-4241-8182-433e2bc5a7fc}</UniqueIdentifier>
+    </Filter>
+  </ItemGroup>
+  <ItemGroup>
+    <ClCompile Include="..\Modules\_zoneinfo.c">
+      <Filter>Source Files</Filter>
+    </ClCompile>
+  </ItemGroup>
+</Project>
diff --git a/PCbuild/lib.pyproj b/PCbuild/lib.pyproj
index ee01d10..7ce88e5 100644
--- a/PCbuild/lib.pyproj
+++ b/PCbuild/lib.pyproj
@@ -1396,6 +1396,10 @@
     <Compile Include="test\test_zipimport.py" />
     <Compile Include="test\test_zipimport_support.py" />
     <Compile Include="test\test_zlib.py" />
+    <Compile Include="test\test_zoneinfo\__init__.py" />
+    <Compile Include="test\test_zoneinfo\__main__.py" />
+    <Compile Include="test\test_zoneinfo\_support.py" />
+    <Compile Include="test\test_zoneinfo\test_zoneinfo.py" />
     <Compile Include="test\test__locale.py" />
     <Compile Include="test\test__opcode.py" />
     <Compile Include="test\test__osx_support.py" />
@@ -1563,6 +1567,10 @@
     <Compile Include="xml\__init__.py" />
     <Compile Include="zipapp.py" />
     <Compile Include="zipfile.py" />
+    <Compile Include="zoneinfo\_common.py" />
+    <Compile Include="zoneinfo\__init__.py" />
+    <Compile Include="zoneinfo\_tzpath.py" />
+    <Compile Include="zoneinfo\_zoneinfo.py" />
     <Compile Include="_bootlocale.py" />
     <Compile Include="_collections_abc.py" />
     <Compile Include="_compat_pickle.py" />
diff --git a/PCbuild/pcbuild.proj b/PCbuild/pcbuild.proj
index 9c4d352..4d416c5 100644
--- a/PCbuild/pcbuild.proj
+++ b/PCbuild/pcbuild.proj
@@ -51,7 +51,7 @@
     <!-- pyshellext.dll -->
     <Projects Include="pyshellext.vcxproj" />
     <!-- Extension modules -->
-    <ExtensionModules Include="_asyncio;_decimal;_elementtree;_msi;_multiprocessing;_overlapped;pyexpat;_queue;select;unicodedata;winsound;_uuid" />
+    <ExtensionModules Include="_asyncio;_zoneinfo;_decimal;_elementtree;_msi;_multiprocessing;_overlapped;pyexpat;_queue;select;unicodedata;winsound;_uuid" />
     <ExtensionModules Include="_ctypes" Condition="$(IncludeCTypes)" />
     <!-- Extension modules that require external sources -->
     <ExternalModules Include="_bz2;_lzma;_sqlite3" />
diff --git a/PCbuild/pcbuild.sln b/PCbuild/pcbuild.sln
index 6d4c950..61db4e0 100644
--- a/PCbuild/pcbuild.sln
+++ b/PCbuild/pcbuild.sln
@@ -91,6 +91,8 @@
 EndProject
 Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "_asyncio", "_asyncio.vcxproj", "{384C224A-7474-476E-A01B-750EA7DE918C}"
 EndProject
+Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "_zoneinfo", "_zoneinfo.vcxproj", "{FCBE1EF2-E0F0-40B1-88B5-00A35D378742}"
+EndProject
 Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "_queue", "_queue.vcxproj", "{78D80A15-BD8C-44E2-B49E-1F05B0A0A687}"
 EndProject
 Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "liblzma", "liblzma.vcxproj", "{12728250-16EC-4DC6-94D7-E21DD88947F8}"
diff --git a/PCbuild/readme.txt b/PCbuild/readme.txt
index 5fe3e8c..c44910e 100644
--- a/PCbuild/readme.txt
+++ b/PCbuild/readme.txt
@@ -132,6 +132,7 @@
 _asyncio
 _ctypes
 _ctypes_test
+_zoneinfo
 _decimal
 _elementtree
 _hashlib
diff --git a/Tools/msi/lib/lib_files.wxs b/Tools/msi/lib/lib_files.wxs
index 9554159..037fc38 100644
--- a/Tools/msi/lib/lib_files.wxs
+++ b/Tools/msi/lib/lib_files.wxs
@@ -1,6 +1,6 @@
 ﻿<?xml version="1.0" encoding="UTF-8"?>
 <Wix xmlns="http://schemas.microsoft.com/wix/2006/wi">
-    <?define exts=pyexpat;select;unicodedata;winsound;_bz2;_elementtree;_socket;_ssl;_msi;_ctypes;_hashlib;_multiprocessing;_lzma;_decimal;_overlapped;_sqlite3;_asyncio;_queue;_uuid ?>
+    <?define exts=pyexpat;select;unicodedata;winsound;_bz2;_elementtree;_socket;_ssl;_msi;_ctypes;_hashlib;_multiprocessing;_lzma;_decimal;_overlapped;_sqlite3;_asyncio;_queue;_uuid;_zoneinfo ?>
     <Fragment>
         <DirectoryRef Id="Lib_venv_scripts_nt" />
 
diff --git a/configure b/configure
index 64bcde6..56d66d0 100755
--- a/configure
+++ b/configure
@@ -658,6 +658,7 @@
 PKG_CONFIG_LIBDIR
 PKG_CONFIG_PATH
 PKG_CONFIG
+TZPATH
 SHLIBS
 CFLAGSFORSHARED
 LINKFORSHARED
@@ -819,6 +820,7 @@
 enable_optimizations
 with_lto
 with_hash_algorithm
+with_tzpath
 with_address_sanitizer
 with_memory_sanitizer
 with_undefined_behavior_sanitizer
@@ -1525,6 +1527,9 @@
   --with-hash-algorithm=[fnv|siphash24]
                           select hash algorithm for use in Python/pyhash.c
                           (default is SipHash24)
+  --with-tzpath=<list of absolute paths separated by pathsep>
+                           Select the default time zone search path for zoneinfo.TZPATH
+
   --with-address-sanitizer
                           enable AddressSanitizer memory error detector,
                           'asan' (default is no)
@@ -10154,6 +10159,47 @@
 fi
 
 
+validate_tzpath() {
+    # Checks that each element of hte path is an absolute path
+    if test -z "$1"; then
+        # Empty string is allowed: it indicates no system TZPATH
+        return 0
+    fi
+
+    # Bad paths are those that don't start with /
+        if ( echo $1 | grep -qE '(^|:)([^/]|$)' ); then
+        as_fn_error $? "--with-tzpath must contain only absolute paths, not $1" "$LINENO" 5
+        return 1;
+    fi
+}
+
+TZPATH="/usr/share/zoneinfo:/usr/lib/zoneinfo:/usr/share/lib/zoneinfo:/etc/zoneinfo"
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for --with-tzpath" >&5
+$as_echo_n "checking for --with-tzpath... " >&6; }
+
+# Check whether --with-tzpath was given.
+if test "${with_tzpath+set}" = set; then :
+  withval=$with_tzpath;
+case "$withval" in
+    yes)
+        as_fn_error $? "--with-tzpath requires a value" "$LINENO" 5
+        ;;
+    *)
+        validate_tzpath "$withval"
+        TZPATH="$withval"
+        { $as_echo "$as_me:${as_lineno-$LINENO}: result: \"$withval\"" >&5
+$as_echo "\"$withval\"" >&6; }
+        ;;
+esac
+
+else
+  validate_tzpath "$TZPATH"
+ { $as_echo "$as_me:${as_lineno-$LINENO}: result: \"$TZPATH\"" >&5
+$as_echo "\"$TZPATH\"" >&6; }
+fi
+
+
+
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for --with-address-sanitizer" >&5
 $as_echo_n "checking for --with-address-sanitizer... " >&6; }
 
diff --git a/configure.ac b/configure.ac
index 21c47b5..497d7c1 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2946,6 +2946,42 @@
 ],
 [AC_MSG_RESULT(default)])
 
+validate_tzpath() {
+    # Checks that each element of hte path is an absolute path
+    if test -z "$1"; then
+        # Empty string is allowed: it indicates no system TZPATH
+        return 0
+    fi
+
+    # Bad paths are those that don't start with /
+    dnl quadrigraphs "@<:@" and "@:>@" produce "[" and "]" in the output
+    if ( echo $1 | grep -qE '(^|:)(@<:@^/@:>@|$)' ); then
+        AC_MSG_ERROR([--with-tzpath must contain only absolute paths, not $1])
+        return 1;
+    fi
+}
+
+TZPATH="/usr/share/zoneinfo:/usr/lib/zoneinfo:/usr/share/lib/zoneinfo:/etc/zoneinfo"
+AC_MSG_CHECKING(for --with-tzpath)
+AC_ARG_WITH(tzpath,
+            AS_HELP_STRING([--with-tzpath=<list of absolute paths separated by pathsep>]
+                           [Select the default time zone search path for zoneinfo.TZPATH]),
+[
+case "$withval" in
+    yes)
+        AC_MSG_ERROR([--with-tzpath requires a value])
+        ;;
+    *)
+        validate_tzpath "$withval"
+        TZPATH="$withval"
+        AC_MSG_RESULT("$withval")
+        ;;
+esac
+],
+[validate_tzpath "$TZPATH"
+ AC_MSG_RESULT("$TZPATH")])
+AC_SUBST(TZPATH)
+
 AC_MSG_CHECKING(for --with-address-sanitizer)
 AC_ARG_WITH(address_sanitizer,
             AS_HELP_STRING([--with-address-sanitizer],
diff --git a/setup.py b/setup.py
index 794ba2f..68fc312 100644
--- a/setup.py
+++ b/setup.py
@@ -304,6 +304,17 @@
     else:
         assert False, "Internal error: Path not found in std_dirs or paths"
 
+def validate_tzpath():
+    base_tzpath = sysconfig.get_config_var('TZPATH')
+    if not base_tzpath:
+        return
+
+    tzpaths = base_tzpath.split(os.pathsep)
+    bad_paths = [tzpath for tzpath in tzpaths if not os.path.isabs(tzpath)]
+    if bad_paths:
+        raise ValueError('TZPATH must contain only absolute paths, '
+                         + f'found:\n{tzpaths!r}\nwith invalid paths:\n'
+                         + f'{bad_paths!r}')
 
 def find_module_file(module, dirlist):
     """Find a module in a set of possible folders. If it is not found
@@ -816,6 +827,8 @@
         # uses modf().
         self.add(Extension('_datetime', ['_datetimemodule.c'],
                            libraries=['m']))
+        # zoneinfo module
+        self.add(Extension('_zoneinfo', ['_zoneinfo.c'])),
         # random number generator implemented in C
         self.add(Extension("_random", ["_randommodule.c"],
                            extra_compile_args=['-DPy_BUILD_CORE_MODULE']))
@@ -2495,6 +2508,7 @@
         ProcessPoolExecutor = None
 
     sys.modules['concurrent.futures.process'] = DummyProcess
+    validate_tzpath()
 
     # turn off warnings when deprecated modules are imported
     import warnings