diff --git a/Lib/_strptime.py b/Lib/_strptime.py
index 1bd570d..63dca02 100644
--- a/Lib/_strptime.py
+++ b/Lib/_strptime.py
@@ -75,6 +75,8 @@
         self.__calc_date_time()
         if _getlang() != self.lang:
             raise ValueError("locale changed during initialization")
+        if time.tzname != self.tzname or time.daylight != self.daylight:
+            raise ValueError("timezone changed during initialization")
 
     def __pad(self, seq, front):
         # Add '' to seq to either the front (is True), else the back.
@@ -159,15 +161,17 @@
 
     def __calc_timezone(self):
         # Set self.timezone by using time.tzname.
-        # Do not worry about possibility of time.tzname[0] == timetzname[1]
-        # and time.daylight; handle that in strptime .
+        # Do not worry about possibility of time.tzname[0] == time.tzname[1]
+        # and time.daylight; handle that in strptime.
         try:
             time.tzset()
         except AttributeError:
             pass
-        no_saving = frozenset(["utc", "gmt", time.tzname[0].lower()])
-        if time.daylight:
-            has_saving = frozenset([time.tzname[1].lower()])
+        self.tzname = time.tzname
+        self.daylight = time.daylight
+        no_saving = frozenset(["utc", "gmt", self.tzname[0].lower()])
+        if self.daylight:
+            has_saving = frozenset([self.tzname[1].lower()])
         else:
             has_saving = frozenset()
         self.timezone = (no_saving, has_saving)
@@ -296,12 +300,15 @@
     """Return a time struct based on the input string and the format string."""
     global _TimeRE_cache, _regex_cache
     with _cache_lock:
-        if _getlang() != _TimeRE_cache.locale_time.lang:
+        locale_time = _TimeRE_cache.locale_time
+        if (_getlang() != locale_time.lang or
+            time.tzname != locale_time.tzname or
+            time.daylight != locale_time.daylight):
             _TimeRE_cache = TimeRE()
             _regex_cache.clear()
+            locale_time = _TimeRE_cache.locale_time
         if len(_regex_cache) > _CACHE_MAX_SIZE:
             _regex_cache.clear()
-        locale_time = _TimeRE_cache.locale_time
         format_regex = _regex_cache.get(format)
         if not format_regex:
             try:
diff --git a/Lib/test/test_strptime.py b/Lib/test/test_strptime.py
index 7a47f9e..e309d06 100644
--- a/Lib/test/test_strptime.py
+++ b/Lib/test/test_strptime.py
@@ -4,8 +4,9 @@
 import time
 import locale
 import re
+import os
 import sys
-from test import test_support
+from test import test_support as support
 from datetime import date as datetime_date
 
 import _strptime
@@ -314,9 +315,10 @@
         tz_name = time.tzname[0]
         if tz_name.upper() in ("UTC", "GMT"):
             self.skipTest('need non-UTC/GMT timezone')
-        try:
-            original_tzname = time.tzname
-            original_daylight = time.daylight
+
+        with support.swap_attr(time, 'tzname', (tz_name, tz_name)), \
+             support.swap_attr(time, 'daylight', 1), \
+             support.swap_attr(time, 'tzset', lambda: None):
             time.tzname = (tz_name, tz_name)
             time.daylight = 1
             tz_value = _strptime._strptime_time(tz_name, "%Z")[8]
@@ -324,9 +326,6 @@
                     "%s lead to a timezone value of %s instead of -1 when "
                     "time.daylight set to %s and passing in %s" %
                     (time.tzname, tz_value, time.daylight, tz_name))
-        finally:
-            time.tzname = original_tzname
-            time.daylight = original_daylight
 
     def test_date_time(self):
         # Test %c directive
@@ -538,7 +537,7 @@
         _strptime._strptime_time("10", "%d")
         self.assertIsNot(locale_time_id, _strptime._TimeRE_cache.locale_time)
 
-    def test_TimeRE_recreation(self):
+    def test_TimeRE_recreation_locale(self):
         # The TimeRE instance should be recreated upon changing the locale.
         locale_info = locale.getlocale(locale.LC_TIME)
         try:
@@ -567,9 +566,36 @@
         finally:
             locale.setlocale(locale.LC_TIME, locale_info)
 
+    @support.run_with_tz('STD-1DST')
+    def test_TimeRE_recreation_timezone(self):
+        # The TimeRE instance should be recreated upon changing the timezone.
+        oldtzname = time.tzname
+        tm = _strptime._strptime_time(time.tzname[0], '%Z')
+        self.assertEqual(tm.tm_isdst, 0)
+        tm = _strptime._strptime_time(time.tzname[1], '%Z')
+        self.assertEqual(tm.tm_isdst, 1)
+        # Get id of current cache object.
+        first_time_re = _strptime._TimeRE_cache
+        # Change the timezone and force a recreation of the cache.
+        os.environ['TZ'] = 'EST+05EDT,M3.2.0,M11.1.0'
+        time.tzset()
+        tm = _strptime._strptime_time(time.tzname[0], '%Z')
+        self.assertEqual(tm.tm_isdst, 0)
+        tm = _strptime._strptime_time(time.tzname[1], '%Z')
+        self.assertEqual(tm.tm_isdst, 1)
+        # Get the new cache object's id.
+        second_time_re = _strptime._TimeRE_cache
+        # They should not be equal.
+        self.assertIsNot(first_time_re, second_time_re)
+        # Make sure old names no longer accepted.
+        with self.assertRaises(ValueError):
+            _strptime._strptime_time(oldtzname[0], '%Z')
+        with self.assertRaises(ValueError):
+            _strptime._strptime_time(oldtzname[1], '%Z')
+
 
 def test_main():
-    test_support.run_unittest(
+    support.run_unittest(
         getlang_Tests,
         LocaleTime_Tests,
         TimeRETests,
diff --git a/Lib/test/test_support.py b/Lib/test/test_support.py
index 7ce1c73..956936f 100644
--- a/Lib/test/test_support.py
+++ b/Lib/test/test_support.py
@@ -40,7 +40,7 @@
            "threading_cleanup", "reap_threads", "start_threads", "cpython_only",
            "check_impl_detail", "get_attribute", "py3k_bytes",
            "import_fresh_module", "threading_cleanup", "reap_children",
-           "strip_python_stderr", "IPV6_ENABLED"]
+           "strip_python_stderr", "IPV6_ENABLED", "run_with_tz"]
 
 class Error(Exception):
     """Base class for regression test exceptions."""
@@ -1226,6 +1226,39 @@
     return decorator
 
 #=======================================================================
+# Decorator for running a function in a specific timezone, correctly
+# resetting it afterwards.
+
+def run_with_tz(tz):
+    def decorator(func):
+        def inner(*args, **kwds):
+            try:
+                tzset = time.tzset
+            except AttributeError:
+                raise unittest.SkipTest("tzset required")
+            if 'TZ' in os.environ:
+                orig_tz = os.environ['TZ']
+            else:
+                orig_tz = None
+            os.environ['TZ'] = tz
+            tzset()
+
+            # now run the function, resetting the tz on exceptions
+            try:
+                return func(*args, **kwds)
+            finally:
+                if orig_tz is None:
+                    del os.environ['TZ']
+                else:
+                    os.environ['TZ'] = orig_tz
+                time.tzset()
+
+        inner.__name__ = func.__name__
+        inner.__doc__ = func.__doc__
+        return inner
+    return decorator
+
+#=======================================================================
 # Big-memory-test support. Separate from 'resources' because memory use should be configurable.
 
 # Some handy shorthands. Note that these are used for byte-limits as well
diff --git a/Misc/NEWS b/Misc/NEWS
index ee3fcca..b073ec6 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -22,6 +22,9 @@
 Library
 -------
 
+- Issue #6478: _strptime's regexp cache now is reset after changing timezone
+  with time.tzset().
+
 - Issue #25718: Fixed copying object with state with boolean value is false.
 
 - Issue #25742: :func:`locale.setlocale` now accepts a Unicode string for