diff --git a/Lib/datetime.py b/Lib/datetime.py
index 197ef63..b64637f 100644
--- a/Lib/datetime.py
+++ b/Lib/datetime.py
@@ -23,9 +23,10 @@
 # for all computations.  See the book for algorithms for converting between
 # proleptic Gregorian ordinals and many other calendar systems.
 
-_DAYS_IN_MONTH = [None, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
+# -1 is a placeholder for 0.
+_DAYS_IN_MONTH = [-1, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
 
-_DAYS_BEFORE_MONTH = [None]
+_DAYS_BEFORE_MONTH = [-1] # -1 is a placeholder for 0.
 dbm = 0
 for dim in _DAYS_IN_MONTH[1:]:
     _DAYS_BEFORE_MONTH.append(dbm)