Fix Date DST computation.

src/date.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_DATE_H_
 #define V8_DATE_H_
 
 #include "src/allocation.h"
 #include "src/base/platform/platform.h"
 #include "src/globals.h"
@@ skipping 85 matching lines @@
     return base::OS::LocalTimezone(static_cast<double>(time_ms), tz_cache_);
   }
 
   // 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);
   }
 
   // ECMA 262 - 15.9.1.9
+  // LocalTime(t) = t + LocalTZA + DaylightSavingTA(t)
+  // ECMA 262 assumes that DaylightSavingTA is computed using UTC time,
+  // but we fetch DST from OS using local time, therefore we need:
+  // LocalTime(t) = t + LocalTZA + DaylightSavingTA(t + LocalTZA).
   int64_t ToLocal(int64_t time_ms) {
-    return time_ms + LocalOffsetInMs() + DaylightSavingsOffsetInMs(time_ms);
+    time_ms += LocalOffsetInMs();
+    return time_ms + DaylightSavingsOffsetInMs(time_ms);
   }
 
   // ECMA 262 - 15.9.1.9
+  // UTC(t) = t - LocalTZA - DaylightSavingTA(t - LocalTZA)
+  // ECMA 262 assumes that DaylightSavingTA is computed using UTC time,
+  // but we fetch DST from OS using local time, therefore we need:
+  // UTC(t) = t - LocalTZA - DaylightSavingTA(t).
   int64_t ToUTC(int64_t time_ms) {
-    time_ms -= LocalOffsetInMs();
-    return time_ms - DaylightSavingsOffsetInMs(time_ms);
+    return time_ms - LocalOffsetInMs() - DaylightSavingsOffsetInMs(time_ms);
   }
 
 
   // Computes a time equivalent to the given time according
   // to ECMA 262 - 15.9.1.9.
   // The issue here is that some library calls don't work right for dates
   // that cannot be represented using a non-negative signed 32 bit integer
   // (measured in whole seconds based on the 1970 epoch).
   // We solve this by mapping the time to a year with same leap-year-ness
   // and same starting day for the year. The ECMAscript specification says
@@ skipping 110 matching lines @@
   int ymd_year_;
   int ymd_month_;
   int ymd_day_;
 
   base::TimezoneCache* tz_cache_;
 };
 
 } }   // namespace v8::internal
 
 #endif