windows: fix tzinfo for timezones without dst

fixes lp:1010050 and closes #2

This patch was taken almost as is from the launchpad merge request by
@ccanepa :
https://code.launchpad.net/~ccanepacc/dateutil/fix-isdst-in-tzwin/+merge/224424

Applyed to the git repo by @jarondl

2 files changed, 17 insertions, 0 deletions

diff --git a/dateutil/test/test.py b/dateutil/test/test.py
index c00c3a8..af2e6d2 100755
--- a/dateutil/test/test.py
+++ b/dateutil/test/test.py
@@ -4,6 +4,8 @@ from __future__ import unicode_literals
 import unittest
 import calendar
 import base64
+import sys
+
 
 from datetime import *
 
@@ -16,6 +18,11 @@ from dateutil.rrule import *
 from dateutil.tz import *
 from dateutil import zoneinfo
 
+try:
+    from dateutil import tzwin
+except ImportError:
+    pass
+
 
 class RelativeDeltaTest(unittest.TestCase):
     now = datetime(2003, 9, 17, 20, 54, 47, 282310)
@@ -4003,5 +4010,10 @@ END:VTIMEZONE
         self.assertEqual(dt.astimezone(tz=gettz("UTC-2")),
                           datetime(2007, 8, 6, 2, 10, tzinfo=tzstr("UTC-2")))
 
+    @unittest.skipUnless(sys.platform.startswith("win"), "requires Windows")
+    def testIsdstZoneWithNoDaylightSaving(self):
+        tz = tzwin.tzwin("UTC")
+        dt = parse("2013-03-06 19:08:15")
+        self.assertFalse(tz._isdst(dt))
 
 # vim:ts=4:sw=4
diff --git a/dateutil/tzwin.py b/dateutil/tzwin.py
index 4276930..5e6864b 100644
--- a/dateutil/tzwin.py
+++ b/dateutil/tzwin.py
@@ -63,6 +63,9 @@ class tzwinbase(datetime.tzinfo):
         return self._display
 
     def _isdst(self, dt):
+        if not self._dstmonth:
+            # dstmonth == 0 signals the zone has no daylight saving time
+            return False
         dston = picknthweekday(dt.year, self._dstmonth, self._dstdayofweek,
                                self._dsthour, self._dstminute,
                                self._dstweeknumber)
@@ -96,6 +99,8 @@ class tzwin(tzwinbase):
         self._stdoffset = -tup[0]-tup[1]          # Bias + StandardBias * -1
         self._dstoffset = self._stdoffset-tup[2]  # + DaylightBias * -1
 
+        # for the meaning see the win32 TIME_ZONE_INFORMATION structure docs
+        # http://msdn.microsoft.com/en-us/library/windows/desktop/ms725481(v=vs.85).aspx
         (self._stdmonth,
          self._stddayofweek,   # Sunday = 0
          self._stdweeknumber,  # Last = 5