Fix datetime for daylight-saving transitions.

Change-Id: I0ec35937a76ada32991b79bcdac57912731db489
Reviewed-on: https://dart-review.googlesource.com/14380
Reviewed-by: Florian Loitsch <floitsch@google.com>

runtime/lib/date_patch.dart

@@ -297,16 +297,28 @@ class DateTime {
     }
 
     if (!isUtc) {
-      // Note that we need to remove the local timezone adjustment before
-      // asking for the correct zone offset.
+      // Note that we can't literally follow the ECMAScript spec (which this
+      // code is based on), because it leads to incorrect computations at
+      // the DST transition points.
+      //
+      // See V8's comment here:
+      // https://github.com/v8/v8/blob/089dd7d2447d6eaf57c8ba6d8f37957f3a269777/src/date.h#L118
+
+      // We need to remove the local timezone adjustment before asking for the
+      // correct zone offset.
       int adjustment = _localTimeZoneAdjustmentInSeconds() *
           Duration.MICROSECONDS_PER_SECOND;
       // The adjustment is independent of the actual date and of the daylight
       // saving time. It is positive east of the Prime Meridian and negative
       // west of it, e.g. -28800 sec for America/Los_Angeles timezone.
 
+      // We remove one hour to ensure that we have the correct offset at
+      // DST transitioning points. This is a temporary solution and only
+      // correct in timezones that shift for exactly one hour.
+      adjustment += Duration.MICROSECONDS_PER_HOUR;
       int zoneOffset =
           _timeZoneOffsetInSeconds(microsecondsSinceEpoch - adjustment);
+
       // The zoneOffset depends on the actual date and reflects any daylight
       // saving time and/or historical deviation relative to UTC time.
       // It is positive east of the Prime Meridian and negative west of it,

tests/corelib_2/corelib_2.status

@@ -419,6 +419,7 @@ iterable_to_set_test: RuntimeError # is-checks do not implement strong mode type
 regexp/no-extensions_test: RuntimeError
 regexp/lookahead_test: RuntimeError
 regexp/overflow_test: RuntimeError
+date_time11_test: RuntimeError, OK # Bug in Safari.
 
 [ $runtime == safari || $runtime == safarimobilesim ]
 string_trimlr_test/02: RuntimeError # Uses Unicode 6.2.0 or earlier.

tests/corelib_2/date_time11_test.dart

@@ -0,0 +1,138 @@
+// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import "package:expect/expect.dart";
+
+// Make sure that date-times close to daylight savings work correctly.
+// See http://dartbug.com/30550
+
+/// A list of (decomposed) date-times where daylight saving changes
+/// happen.
+///
+/// This list covers multiple timezones to increase test coverage on
+/// different machines.
+final daylightSavingChanges = [
+  // TZ environment, y, m, d, h, change.
+  ["Europe/Paris", 2017, 03, 26, 02, 60],
+  ["Europe/Paris", 2017, 10, 29, 03, -60],
+  ["Antarctica/Troll", 2017, 03, 19, 01, 120],
+  ["Antarctica/Troll", 2017, 10, 29, 03, -120],
+  ["Australia/Canberra", 2017, 04, 02, 03, -60],
+  ["Australia/Canberra", 2017, 10, 01, 02, 60],
+  ["Australia/Lord_Howe", 2017, 04, 02, 02, -30],
+  ["Australia/Lord_Howe", 2017, 10, 01, 02, 30],
+  ["Atlantic/Bermuda", 2017, 03, 12, 02, 60], // US and Canada.
+  ["Atlantic/Bermuda", 2017, 11, 05, 02, -60],
+  ["America/Campo_Grande", 2017, 02, 19, 00, -60], // Brazil
+  ["America/Campo_Grande", 2017, 10, 15, 00, 60],
+  ["America/Santiago", 2017, 05, 14, 00, -60],
+  ["America/Santiago", 2017, 08, 13, 00, 60],
+  ["Chile/EasterIsland", 2017, 05, 13, 22, -60],
+  ["Chile/EasterIsland", 2017, 08, 12, 22, 60],
+  ["Pacific/Fiji", 2017, 01, 15, 03, -60],
+  ["Pacific/Fiji", 2017, 11, 05, 02, 60],
+  ["America/Scoresbysund", 2017, 03, 26, 00, 60], // Ittoqqortoormiit.
+  ["America/Scoresbysund", 2017, 10, 29, 01, -60],
+  ["Asia/Tehran", 2017, 03, 22, 00, 60],
+  ["Asia/Tehran", 2017, 09, 22, 00, -60],
+  ["Israel", 2017, 03, 24, 02, 60],
+  ["Israel", 2017, 10, 29, 02, -60],
+  ["Asia/Amman", 2017, 03, 31, 00, 60],
+  ["Asia/Amman", 2017, 10, 27, 01, -60],
+  ["Mexico/General", 2017, 04, 02, 02, 60],
+  ["Mexico/General", 2017, 10, 29, 02, -60],
+];
+
+void runTests() {
+  // Makes sure we don't go into the wrong direction during a
+  // daylight-savings change (as happened in #30550).
+  for (var test in daylightSavingChanges) {
+    for (int i = 0; i < 2; i++) {
+      int year = test[1];
+      int month = test[2];
+      int day = test[3];
+      int hour = test[4];
+
+      int minute = i == 0 ? 0 : test[5];
+      // Rather adjust the hours than keeping the minutes.
+      hour += minute ~/ 60;
+      minute = minute.remainder(60);
+      if (hour < 0) {
+        hour += 24;
+        day--;
+      }
+
+      {
+        // Check that microseconds are taken into account.
+        var dtMillisecond = new DateTime(year, month, day, hour, minute, 0, 1);
+        var dtSecond = new DateTime(year, month, day, hour, minute, 1);
+        Expect.equals(const Duration(milliseconds: 999),
+            dtSecond.difference(dtMillisecond));
+
+        dtMillisecond = new DateTime(year, month, day, hour, minute, 0, -1);
+        dtSecond = new DateTime(year, month, day, hour, minute, -1);
+        Expect.equals(const Duration(milliseconds: 999),
+            dtMillisecond.difference(dtSecond));
+      }
+
+      var dt1 = new DateTime(year, month, day, hour);
+      var dt2 = new DateTime(year, month, day, hour, 1);
+
+      // Earlier:
+      int earlierDay = day;
+      int earlierHour = hour - 1;
+      if (earlierHour < 0) {
+        earlierHour = 23;
+        earlierDay--;
+      }
+      var dt3 = new DateTime(year, month, earlierDay, earlierHour, 59);
+
+      var diff1 = dt2.difference(dt1).inMinutes;
+      var diff2 = dt1.difference(dt3).inMinutes;
+
+      if (diff1 == 1 && diff2 == 1 && dt1.hour == hour && dt1.minute == 0) {
+        // Regular date-time.
+        continue;
+      }
+
+      // At most one is at a distance of more than a minute.
+      Expect.isTrue(diff1 == 1 || diff2 == 1);
+
+      if (diff2 < 0) {
+        // This happens, when we ask for invalid times.
+        // Suppose daylight-saving is at 2:00 and switches to 3:00. If we
+        // ask for 2:59, we get 3:59 (59 minutes after 2:00).
+        Expect.isFalse(dt3.day == earlierDay && dt3.hour == earlierHour);
+        // If that's the case, then removing one minute from dt1 should
+        // not yield a date-time with the earlier values, and it should
+        // be far away from dt3.
+        var dt4 = dt1.add(const Duration(minutes: -1));
+        Expect.isFalse(dt4.day == earlierDay && dt4.hour == earlierHour);
+        Expect.isTrue(dt4.isBefore(dt1));
+        Expect.isTrue(dt4.day < earlierDay ||
+            (dt4.day == earlierDay && dt4.hour < earlierHour));
+        continue;
+      }
+
+      // They must be in the right order.
+      Expect.isTrue(dt1.isBefore(dt2));
+      Expect.isTrue(dt3.isBefore(dt1));
+    }
+  }
+}
+
+void main(List<String> args) {
+  // The following code constructs a String with all timezones that are
+  // relevant for this test.
+  // This can be helpful for running tests in multiple timezones.
+  // Typically, one would write something like:
+  //    for tz in <contents_of_string>; do TZ=$tz tools/test.py ...; done
+  var result = new StringBuffer();
+  for (int i = 0; i < daylightSavingChanges.length; i += 2) {
+    if (i != 0) result.write(" ");
+    result.write(daylightSavingChanges[i][0]);
+  }
+
+  runTests();
+}