Update the time zone offset and dst offset when time zone name changes.

src/date.js

Index: src/date.js
===================================================================
--- src/date.js	(revision 4331)
+++ src/date.js	(working copy)
@@ -588,6 +588,20 @@
 
 
 function LocalTimezoneString(time) {
+  var old_timezone = timezone_cache_timezone;
+  var timezone = LocalTimezone(time);
+  if (old_timezone && timezone != old_timezone) {
+    // If the timezone string has changed from the one that we cached,
+    // the local time offset may now be wrong. So we need to update it
+    // and try again.
+    local_time_offset = %DateLocalTimeOffset();
+    // We also need to invalidate the DST cache as the new timezone may have
+    // different DST times.
+    var dst_cache = DST_offset_cache;
+    dst_cache.start = 0;
+    dst_cache.end = -1;
+  }
+
   var timezoneOffset =
       (DaylightSavingsOffset(time) + local_time_offset) / msPerMinute;
   var sign = (timezoneOffset >= 0) ? 1 : -1;
@@ -595,7 +609,7 @@
   var min   = FLOOR((sign * timezoneOffset)%60);
   var gmt = ' GMT' + ((sign == 1) ? '+' : '-') +
       TwoDigitString(hours) + TwoDigitString(min);
-  return gmt + ' (' +  LocalTimezone(time) + ')';
+  return gmt + ' (' +  timezone + ')';
 }