Reapply "Add support for timezone offset and timezone name."

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 96a00359df610e396aefefd1da4776a427c61a2d..9c26dd8ef5898450ba0758b52101366009e21a01 100644
--- a/runtime/vm/os_win.cc
+++ b/runtime/vm/os_win.cc
@@ -18,9 +18,11 @@ bool OS::GmTime(int64_t seconds_since_epoch, tm* tm_result) {
 }
 
 
+// As a side-effect sets the globals _timezone, _daylight and _tzname.
 bool OS::LocalTime(int64_t seconds_since_epoch, tm* tm_result) {
   time_t seconds = static_cast<time_t>(seconds_since_epoch);
   if (seconds != seconds_since_epoch) return false;
+  // localtime_s implicitly sets _timezone, _daylight and _tzname.
   errno_t error_code = localtime_s(tm_result, &seconds);
   return error_code == 0;
 }
@@ -54,6 +56,53 @@ bool OS::MkTime(tm* tm, int64_t* seconds_result) {
 }
 
 
+static int GetDaylightSavingBiasInSeconds() {
+  TIME_ZONE_INFORMATION zone_information;
+  memset(&zone_information, 0, sizeof(zone_information));
+  if (GetTimeZoneInformation(&zone_information) == TIME_ZONE_ID_INVALID) {
+    // By default the daylight saving offset is an hour.
+    return -60 * 60;
+  } else {
+    return static_cast<int>(zone_information.DaylightBias * 60);
+  }
+}
+
+bool OS::GetTimeZoneName(int64_t seconds_since_epoch,
+                         const char** name_result) {
+  tm decomposed;
+  // LocalTime will set _tzname.
+  bool succeeded = LocalTime(seconds_since_epoch, &decomposed);
+  if (!succeeded) return false;
+  int inDaylightSavingsTime = decomposed.tm_isdst;
+  if (inDaylightSavingsTime != 0 && inDaylightSavingsTime != 1) {
+    return false;
+  }
+  *name_result = _tzname[inDaylightSavingsTime];
+  return true;
+}
+
+
+bool OS::GetTimeZoneOffsetInSeconds(int64_t seconds_since_epoch,
+                                    int* offset_result) {
+  tm decomposed;
+  // LocalTime will set _timezone.
+  bool succeeded = LocalTime(seconds_since_epoch, &decomposed);
+  if (!succeeded) return false;
+  int inDaylightSavingsTime = decomposed.tm_isdst;
+  if (inDaylightSavingsTime != 0 && inDaylightSavingsTime != 1) {
+    return false;
+  }
+  // Dart and Windows disagree on the sign of the bias.
+  *offset_result = static_cast<int>(-_timezone);
+  if (inDaylightSavingsTime == 1) {
+    static int daylight_bias = GetDaylightSavingBiasInSeconds()
+    // Subtract because windows and Dart disagree on the sign.
+    *offset_result = *offset_result - daylight_bias;
+  }
+  return true;
+}
+
+
 int64_t OS::GetCurrentTimeMillis() {
   return GetCurrentTimeMicros() / 1000;
 }