diff -r ac176a69d188 Lib/_strptime.py
--- a/Lib/_strptime.py	Sun Dec 28 22:14:23 2014 -0600
+++ b/Lib/_strptime.py	Wed Dec 31 18:30:51 2014 +0200
@@ -348,9 +348,9 @@ def _strptime(data_string, format="%a %b
     # though
     week_of_year = -1
     week_of_year_start = -1
-    # weekday and julian defaulted to -1 so as to signal need to calculate
+    # weekday and julian defaulted to None so as to signal need to calculate
     # values
-    weekday = julian = -1
+    weekday = julian = None
     found_dict = found.groupdict()
     for group_key in found_dict.keys():
         # Directives not explicitly handled below:
@@ -452,14 +452,14 @@ def _strptime(data_string, format="%a %b
         year = 1900
     # If we know the week of the year and what day of that week, we can figure
     # out the Julian day of the year.
-    if julian == -1 and week_of_year != -1 and weekday != -1:
+    if julian is None and week_of_year != -1 and weekday is not None:
         week_starts_Mon = True if week_of_year_start == 0 else False
         julian = _calc_julian_from_U_or_W(year, week_of_year, weekday,
                                             week_starts_Mon)
     # Cannot pre-calculate datetime_date() since can change in Julian
     # calculation and thus could have different value for the day of the week
     # calculation.
-    if julian == -1:
+    if julian is None:
         # Need to add 1 to result since first day of the year is 1, not 0.
         julian = datetime_date(year, month, day).toordinal() - \
                   datetime_date(year, 1, 1).toordinal() + 1
@@ -469,7 +469,7 @@ def _strptime(data_string, format="%a %b
         year = datetime_result.year
         month = datetime_result.month
         day = datetime_result.day
-    if weekday == -1:
+    if weekday is None:
         weekday = datetime_date(year, month, day).weekday()
     # Add timezone info
     tzname = found_dict.get("Z")