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

runtime/vm/os_macos.cc

Index: runtime/vm/os_macos.cc
diff --git a/runtime/vm/os_macos.cc b/runtime/vm/os_macos.cc
index e6107650d00cec279b45ca5bdf231d49aa18e632..1cc1c5f04fab3b56c1a9e15a8b5d26afb0cc703e 100644
--- a/runtime/vm/os_macos.cc
+++ b/runtime/vm/os_macos.cc
@@ -26,9 +26,6 @@ bool OS::BreakDownSecondsSinceEpoch(time_t seconds_since_epoch,
   if (in_utc) {
     error_code = gmtime_r(&seconds_since_epoch, &tm_result);
   } 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]

I am confused by this call being removed for Win32 and MacOS but not for Linux.

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

removed.

     error_code = localtime_r(&seconds_since_epoch, &tm_result);
   }
   result->year = tm_result.tm_year;
@@ -41,6 +38,14 @@ bool OS::BreakDownSecondsSinceEpoch(time_t seconds_since_epoch,
 }
 
 
+// TODO(floitsch): cache utc offset.

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

This seems unnecessary.  See below.

+static int UtcOffsetInSeconds(time_t seconds_since_epoch) {
+  struct tm t;
+  struct tm* error_code = localtime_r(&seconds_since_epoch, &t);
+  ASSERT(error_code != NULL);
+  return t.tm_gmtoff;
+}
+
 bool OS::BrokenDownToSecondsSinceEpoch(
     const BrokenDownDate& broken_down, bool in_utc, time_t* result) {
   struct tm tm_broken_down;
@@ -54,27 +59,18 @@ 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();
-  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;
-  } else {
-    // Let libc figure out if daylight saving is active.
-    tm_broken_down.tm_isdst = -1;
-    *result = mktime(&tm_broken_down);
-  }
+  // 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)) {
     return false;
   }
+  if (in_utc) {
+    *result += UtcOffsetInSeconds(*result);

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

use timegm instead of mktime and correcting with UtcOffsetInSeconds. This
function is available on Linux and MacOS X.

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

The manpage of timegm explicitly states that it is deprecated and should never
be used.

[cshapiro, 2012/04/20 20:47:49]

I see no mention of timegm being deprecated or that it should never be used in
any of the documentation.  Relevant references

https://developer.apple.com/library/mac/#documentation/darwin/reference/manpa...

http://man7.org/linux/man-pages/man3/timegm.3.html

http://www.gnu.org/software/libc/manual/html_node/Broken_002ddown-Time.html#i...

[floitsch, 2012/04/23 08:37:09]

Ok. "deprecated" was too strong. But they explicitly state that usage should be
avoided. They furthermore don't give any guarantees on multi-threading. That
said, I just went through the sources of the libc and they don't change the env
and also use a lock.
Since this is not guaranteed I would still go with v8's approach, but it's your
call.

+  }
   return true;
 }