Set the TZ env to get the UTC-ms since epoch.

runtime/vm/os_win.cc

Index: runtime/vm/os_win.cc
diff --git a/runtime/vm/os_win.cc b/runtime/vm/os_win.cc
index 420a06876e5fd79a5da6857c930a67d59feaf706..6e1c1d9b027d4e265ab398e4ac80225f79f95f19 100644
--- a/runtime/vm/os_win.cc
+++ b/runtime/vm/os_win.cc
@@ -18,9 +18,6 @@ bool OS::BreakDownSecondsSinceEpoch(time_t seconds_since_epoch,
   if (in_utc) {
     error_code = gmtime_s(&tm_result, &seconds_since_epoch);
   } else {
-    // TODO(floitsch): we should be able to call tzset only once during
-    // initialization.
-    tzset();  // Make sure the libc knows about the local zone.

[cshapiro, 2012/04/19 23:52:12]

Why was this removed?  The documentation states that localtime consults the TZ
environment variable

http://msdn.microsoft.com/en-us/library/a442x3ye(v=vs.80).aspx

Either this call is needed for all conversions using the C library routines, or
it is not.

[floitsch, 2012/04/20 16:09:57]

localtime_s only looks at the env variable. tzset *uses* the env variable but
doesn't set it. Actually I think that localtime_s invokes tzset itself (quoting
the doc):
When TZ is set, three other environment variables (_timezone, _daylight, and
_tzname) are automatically set as well.

     error_code = localtime_s(&tm_result, &seconds_since_epoch);
   }
   result->year = tm_result.tm_year;
@@ -46,25 +43,19 @@ bool OS::BrokenDownToSecondsSinceEpoch(
   // Set wday to an impossible day, so that we can catch bad input.
   tm_broken_down.tm_wday = -1;
   // Make sure the libc knows about the local zone.
-  // In case of 'in_utc' this call is mainly for multi-threading issues. If
-  // another thread uses a time-function it will set the timezone. The timezone
-  // adjustement below would then not work anymore.
   // TODO(floitsch): we should be able to call tzset only once during
   // initialization.
   tzset();

[cshapiro, 2012/04/19 23:52:12]

Why is this being called before a conversion for a timezone independent value? 
This looks like it should precede the mktime call, only.

[floitsch, 2012/04/20 16:09:57]

Looking through the docs it looks as calling tzset should never be necessary.
Removing.

   if (in_utc) {
     // Disable daylight saving in utc mode.
     tm_broken_down.tm_isdst = 0;
-    // mktime assumes that the given date is local time zone.
-    *result = mktime(&tm_broken_down);
-    // Remove the timezone.
-    *result -= timezone;
+    *result = _mkgmtime(&tm_broken_down);
   } else {
     // Let libc figure out if daylight saving is active.
     tm_broken_down.tm_isdst = -1;
     *result = mktime(&tm_broken_down);
   }
-  if ((*result == -1) && (tm_broken_down.tm_wday == -1)) {
+  if ((*result == -1 || *result == 1) && (tm_broken_down.tm_wday == -1)) {

[cshapiro, 2012/04/19 23:52:12]

How is returning 1 an error?

[floitsch, 2012/04/20 16:09:57]

Couldn't find any reference to it either. Probably added before I switched to
_mkgmtime. I will remove it.

     return false;
   }
   return true;