diff -r 5b2212e0c350 Lib/_strptime.py
--- a/Lib/_strptime.py	Sun Jan 11 12:48:17 2015 +0200
+++ b/Lib/_strptime.py	Sun Jan 11 13:12:13 2015 +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")
diff -r 5b2212e0c350 Lib/test/test_strptime.py
--- a/Lib/test/test_strptime.py	Sun Jan 11 12:48:17 2015 +0200
+++ b/Lib/test/test_strptime.py	Sun Jan 11 13:12:13 2015 +0200
@@ -494,6 +494,24 @@ class CalculationTests(unittest.TestCase
         test_helper((2006, 12, 31), "Last Sunday of 2006")
         test_helper((2006, 12, 24), "Second to last Sunday of 2006")
 
+    def test_week_0(self):
+        def check(value, format, *expected):
+            self.assertEqual(_strptime._strptime_time(value, format)[:-1], expected)
+        check('2015 0 0', '%Y %U %w', 2014, 12, 28, 0, 0, 0, 6, -3)
+        check('2015 0 0', '%Y %W %w', 2015, 1, 4, 0, 0, 0, 6, 4)
+        check('2015 0 1', '%Y %U %w', 2014, 12, 29, 0, 0, 0, 0, -2)
+        check('2015 0 1', '%Y %W %w', 2014, 12, 29, 0, 0, 0, 0, -2)
+        check('2015 0 2', '%Y %U %w', 2014, 12, 30, 0, 0, 0, 1, -1)
+        check('2015 0 2', '%Y %W %w', 2014, 12, 30, 0, 0, 0, 1, -1)
+        check('2015 0 3', '%Y %U %w', 2014, 12, 31, 0, 0, 0, 2, 0)
+        check('2015 0 3', '%Y %W %w', 2014, 12, 31, 0, 0, 0, 2, 0)
+        check('2015 0 4', '%Y %U %w', 2015, 1, 1, 0, 0, 0, 3, 1)
+        check('2015 0 4', '%Y %W %w', 2015, 1, 1, 0, 0, 0, 3, 1)
+        check('2015 0 5', '%Y %U %w', 2015, 1, 2, 0, 0, 0, 4, 2)
+        check('2015 0 5', '%Y %W %w', 2015, 1, 2, 0, 0, 0, 4, 2)
+        check('2015 0 6', '%Y %U %w', 2015, 1, 3, 0, 0, 0, 5, 3)
+        check('2015 0 6', '%Y %W %w', 2015, 1, 3, 0, 0, 0, 5, 3)
+
 
 class CacheTests(unittest.TestCase):
     """Test that caching works properly."""