accessing tzinfo for timezones defined via tzwin which don't observe Daylight Saving Time throws ValueError because _dstmonth is 0

on windows:

from dateutil.tz import gettz
from datetime import datetime

tzinfo = gettz('UTC')
f = datetime.now(tzinfo)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python27\lib\site-packages\python_dateutil-1.5-py2.7.egg\dateutil\tzwin.py", line 32, in utcoffset
    if self._isdst(dt):
  File "C:\Python27\lib\site-packages\python_dateutil-1.5-py2.7.egg\dateutil\tzwin.py", line 67, in _isdst
    self._dstweeknumber)
  File "C:\Python27\lib\site-packages\python_dateutil-1.5-py2.7.egg\dateutil\tzwin.py", line 166, in picknthweekday
    first = datetime.datetime(year, month, 1, hour, minute)
ValueError: month must be in 1..12

Patch:
--- tzwin.py~ 2012-06-05 09:08:36.212393800 -0400
+++ tzwin.py 2012-06-07 10:24:33.878270600 -0400
@@ -62,6 +62,8 @@
         return self._display

     def _isdst(self, dt):
+ if self._dstmonth == 0:
+ return False
         dston = picknthweekday(dt.year, self._dstmonth, self._dstdayofweek,
                                self._dsthour, self._dstminute,
                                self._dstweeknumber)