improve comment
diff --git a/Lib/datetime.py b/Lib/datetime.py
index b64637f..1789714 100644
--- a/Lib/datetime.py
+++ b/Lib/datetime.py
@@ -23,10 +23,10 @@
# for all computations. See the book for algorithms for converting between
# proleptic Gregorian ordinals and many other calendar systems.
-# -1 is a placeholder for 0.
+# -1 is a placeholder for indexing purposes.
_DAYS_IN_MONTH = [-1, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
-_DAYS_BEFORE_MONTH = [-1] # -1 is a placeholder for 0.
+_DAYS_BEFORE_MONTH = [-1] # -1 is a placeholder for indexing purposes.
dbm = 0
for dim in _DAYS_IN_MONTH[1:]:
_DAYS_BEFORE_MONTH.append(dbm)