Fix computation of UTC time from local time at DST change points.

src/date.h

Index: src/date.h
diff --git a/src/date.h b/src/date.h
index 2e5ce39a042c791de83a4446c960225a4784cf61..813d3126ede972f628e325336e94e9e5661f132c 100644
--- a/src/date.h
+++ b/src/date.h
@@ -104,21 +104,51 @@ class DateCache {
 
   // 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) {
-    time_ms += LocalOffsetInMs();
-    return time_ms + DaylightSavingsOffsetInMs(time_ms);
+    return time_ms + LocalOffsetInMs() + 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) {
-    return time_ms - LocalOffsetInMs() - DaylightSavingsOffsetInMs(time_ms);
+    // We need to compute UTC time that corresponds to the given local time.
+    // Literally following spec here leads to incorrect time computation at
+    // the points were we transition to and from DST.
+    //
+    // The following shows that using DST for (t - LocalTZA - hour) produces
+    // correct conversion.
+    //
+    // Consider transition to DST at local time L1.
+    // Let L0 = L1 - hour, L2 = L1 + hour,
+    //     U1 = UTC time that corresponds to L1,
+    //     U0 = U1 - hour.
+    // Transitioning to DST moves local clock one hour forward L1 => L2, so
+    // U0 = UTC time that corresponds to L0 = L0 - LocalTZA,
+    // U1 = UTC time that corresponds to L1 = L1 - LocalTZA,
+    // U1 = UTC time that corresponds to L2 = L2 - LocalTZA - hour.
+    // Note that DST(U0 - hour) = 0, DST(U0) = 0, DST(U1) = 1.
+    // U0 = L0 - LocalTZA - DST(L0 - LocalTZA - hour),
+    // U1 = L1 - LocalTZA - DST(L1 - LocalTZA - hour),
+    // U1 = L2 - LocalTZA - DST(L2 - LocalTZA - hour).
+    //
+    // Consider transition from DST at local time L1.
+    // Let L0 = L1 - hour,
+    //     U1 = UTC time that corresponds to L1,
+    //     U0 = U1 - hour, U2 = U1 + hour.
+    // Transitioning from DST moves local clock one hour back L1 => L0, so
+    // U0 = UTC time that corresponds to L0 (before transition)
+    //    = L0 - LocalTZA - hour.
+    // U1 = UTC time that corresponds to L0 (after transition)
+    //    = L0 - LocalTZA = L1 - LocalTZA - hour
+    // U2 = UTC time that corresponds to L1 = L1 - LocalTZA.
+    // Note that DST(U0) = 1, DST(U1) = 0, DST(U2) = 0.
+    // U0 = L0 - LocalTZA - DST(L0 - LocalTZA - hour) = L0 - LocalTZA - DST(U0).
+    // U2 = L1 - LocalTZA - DST(L1 - LocalTZA - hour) = L1 - LocalTZA - DST(U1).
+    // It is impossible to get U1 from local time.
+
+    const int kMsPerHour = 3600 * 1000;
+    time_ms -= LocalOffsetInMs();
+    return time_ms - DaylightSavingsOffsetInMs(time_ms - kMsPerHour);
   }