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

src/date.js

 // Copyright 2006-2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 24 matching lines @@
 
 // This file contains date support implemented in JavaScript.
 
 // Helper function to throw error.
 function ThrowDateTypeError() {
   throw new $TypeError('this is not a Date object.');
 }
 
 
 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;
 }
 
 
 function UTC(time) {
   if (NUMBER_IS_NAN(time)) return time;
   // local_time_offset is needed before the call to DaylightSavingsOffset,
   // so it may be uninitialized.
   return %DateToUTC(time);
 }
@@ skipping 754 matching lines @@
     "toGMTString", DateToGMTString,
     "toUTCString", DateToUTCString,
     "getYear", DateGetYear,
     "setYear", DateSetYear,
     "toISOString", DateToISOString,
     "toJSON", DateToJSON
   ));
 }
 
 SetUpDate();