diff --git a/Lib/calendar.py b/Lib/calendar.py
index 365ca263444..fb56826f6fb 100644
--- a/Lib/calendar.py
+++ b/Lib/calendar.py
@@ -213,7 +213,7 @@ def calendar(year, w=0, l=0, c=_spacing):
 def timegm(tuple):
     """Unrelated but handy function to calculate Unix timestamp from GMT."""
     year, month, day, hour, minute, second = tuple[:6]
-    days = datetime.date(year, month, day).toordinal() - _EPOCH_ORD
+    days = datetime.date(year, month, 1).toordinal() - _EPOCH_ORD + day - 1
     hours = days*24 + hour
     minutes = hours*60 + minute
     seconds = minutes*60 + second