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 2acef13fe304b92e735082d851fb144e2524bd51..91dacd723e7beaed5911ffbd778344568d46a16b 100644
--- a/runtime/vm/os_win.cc
+++ b/runtime/vm/os_win.cc
@@ -7,9 +7,12 @@
 #include <time.h>
 
 #include "platform/assert.h"
+#include "vm/thread.h"
 
 namespace dart {
 
+static Mutex timezone_mutex;
+
 bool OS::BreakDownSecondsSinceEpoch(time_t seconds_since_epoch,
                                     bool in_utc,
                                     BrokenDownDate* result) {
@@ -18,7 +21,10 @@ 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
+    MutexLocker ml(&timezone_mutex);
+    // TODO(floitsch): we should not need the tzset. According to the man-page
+    // the function already acts as if it invoked tzset. In any case we
+    // shouldn't need to invoke it every time, but only once during
     // initialization.
     tzset();  // Make sure the libc knows about the local zone.
     error_code = localtime_s(&tm_result, &seconds_since_epoch);
@@ -35,6 +41,8 @@ bool OS::BreakDownSecondsSinceEpoch(time_t seconds_since_epoch,
 
 bool OS::BrokenDownToSecondsSinceEpoch(
     const BrokenDownDate& broken_down, bool in_utc, time_t* result) {
+  // Changing the timezone environment is not thread-safe.
+  MutexLocker ml(&timezone_mutex);
   struct tm tm_broken_down;
   // mktime takes the years since 1900.
   tm_broken_down.tm_year = broken_down.year;
@@ -45,21 +53,29 @@ bool OS::BrokenDownToSecondsSinceEpoch(
   tm_broken_down.tm_sec = broken_down.seconds;
   // 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();
   if (in_utc) {
+    char* saved_timezone = getenv("TZ");
+    // Set to UTC.
+    setenv("TZ", "", 1);
+    tzset();
     // 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;
+    // Restore old value of the timezone.
+    if (saved_timezone) {
+      setenv("TZ", saved_timezone, 1);
+    } else {
+      unsetenv("TZ");
+    }
+    tzset();
   } else {
+    // 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();
     // Let libc figure out if daylight saving is active.
     tm_broken_down.tm_isdst = -1;
     *result = mktime(&tm_broken_down);