Check and clear date cache in DateCurrentTime, DateLocalTimezone and getTimezoneOffset.

src/date.js

Index: src/date.js
diff --git a/src/date.js b/src/date.js
index f3d4af244f9692c223ee5b467ace6c52e177d46f..ff66dd97cfade1148cd562f31f5bcc7d81407bed 100644
--- a/src/date.js
+++ b/src/date.js
@@ -42,16 +42,19 @@ function ThrowDateTypeError() {
 
 
 var timezone_cache_time = NAN;
+var timezone_cache_timezone_offset = NAN;
 var timezone_cache_timezone;
 
 function LocalTimezone(t) {
   if (NUMBER_IS_NAN(t)) return "";
-  if (t == timezone_cache_time) {
+  if (t == timezone_cache_time &&
+      TIMEZONE_OFFSET(t) = timezone_cache_timezone_offset) {
     return timezone_cache_timezone;
   }
   var timezone = %DateLocalTimezone(t);
   timezone_cache_time = t;
   timezone_cache_timezone = timezone;
+  timezone_cache_timezone_offset = TIMEZONE_OFFSET(t);
   return timezone;
 }