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

src/date.h

 // Copyright 2012 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 104 matching lines @@
     }
     return OS::LocalTimezone(static_cast<double>(time_ms));
   }
 
   // ECMA 262 - 15.9.5.26
   int TimezoneOffset(int64_t time_ms) {
     int64_t local_ms = ToLocal(time_ms);
     return static_cast<int>((time_ms - local_ms) / kMsPerMin);
   }
 
+  void CheckTimezone() {
+    if (local_offset_ms_ != kInvalidLocalOffsetInMs &&
+        local_offset_ms_ != GetLocalOffsetFromOS()) {
+      ResetDateCache();
+    }
+  }
+
   // ECMA 262 - 15.9.1.9
   int64_t ToLocal(int64_t time_ms) {
     return time_ms + LocalOffsetInMs() + DaylightSavingsOffsetInMs(time_ms);
   }
 
   // ECMA 262 - 15.9.1.9
   int64_t ToUTC(int64_t time_ms) {
     time_ms -= LocalOffsetInMs();
     return time_ms - DaylightSavingsOffsetInMs(time_ms);
   }
@@ skipping 116 matching lines @@
   bool ymd_valid_;
   int ymd_days_;
   int ymd_year_;
   int ymd_month_;
   int ymd_day_;
 };
 
 } }   // namespace v8::internal
 
 #endif