LibJS+LibTimeZone: Begin handling DST when computing time zone offsets

This also updates some expectations in a Temporal time zone offset test
that is using a time stamp which is in DST for a few time zones.

Meta/Lagom/Tools/CodeGenerators/LibTimeZone/GenerateTimeZoneData.cpp

@@ -450,9 +450,55 @@ static constexpr Array<@type@, @size@> @name@ { {
     append_string_conversions("DaylightSavingsRule"sv, "daylight_savings_rule"sv, time_zone_data.dst_offset_names);
 
     generator.append(R"~~~(
+static i64 get_dst_offset(TimeZoneOffset const& time_zone_offset, AK::Time time)
+{
+    auto const& dst_rules = s_dst_offsets[time_zone_offset.dst_rule];
+
+    DaylightSavingsOffset const* standard_offset = nullptr;
+    DaylightSavingsOffset const* daylight_offset = nullptr;
+
+    auto time_in_effect_for_rule = [&](auto const& dst_rule) {
+        auto in_effect = dst_rule.in_effect;
+        in_effect.year = seconds_since_epoch_to_year(time.to_seconds());
+
+        return in_effect.time_since_epoch();
+    };
+
+    auto preferred_rule = [&](auto* current_offset, auto& new_offset) {
+        if (!current_offset)
+            return &new_offset;
+
+        auto new_time_in_effect = time_in_effect_for_rule(new_offset);
+        return (time >= new_time_in_effect) ? &new_offset : current_offset;
+    };
+
+    for (size_t index = 0; (index < dst_rules.size()) && (!standard_offset || !daylight_offset); ++index) {
+        auto const& dst_rule = dst_rules[index];
+
+        auto year_from = AK::Time::from_timestamp(dst_rule.year_from, 1, 1, 0, 0, 0, 0);
+        auto year_to = AK::Time::from_timestamp(dst_rule.year_to + 1, 1, 1, 0, 0, 0, 0);
+        if ((time < year_from) || (time >= year_to))
+            continue;
+
+        if (dst_rule.offset == 0)
+            standard_offset = preferred_rule(standard_offset, dst_rule);
+        else
+            daylight_offset = preferred_rule(daylight_offset, dst_rule);
+    }
+
+    if (!standard_offset || !daylight_offset)
+        return 0;
+
+    auto standard_time_in_effect = time_in_effect_for_rule(*standard_offset);
+    auto daylight_time_in_effect = time_in_effect_for_rule(*daylight_offset);
+
+    if ((time < daylight_time_in_effect) || (time >= standard_time_in_effect))
+        return standard_offset->offset;
+    return daylight_offset->offset;
+}
+
 Optional<i64> get_time_zone_offset(TimeZone time_zone, AK::Time time)
 {
-    // FIXME: This implementation completely ignores DST.
     auto const& time_zone_offsets = s_time_zone_offsets[to_underlying(time_zone)];
 
     size_t index = 0;
@@ -464,7 +510,15 @@ Optional<i64> get_time_zone_offset(TimeZone time_zone, AK::Time time)
     }
 
     VERIFY(index < time_zone_offsets.size());
-    return time_zone_offsets[index].offset;
+    auto const& time_zone_offset = time_zone_offsets[index];
+
+    i64 dst_offset = 0;
+    if (time_zone_offset.dst_rule != -1)
+        dst_offset = get_dst_offset(time_zone_offset, time);
+    else
+        dst_offset = time_zone_offset.dst_offset;
+
+    return time_zone_offset.offset + dst_offset;
 }
 
 }

Tests/LibTimeZone/TestTimeZone.cpp

@@ -85,18 +85,20 @@ TEST_CASE(canonicalize_time_zone)
     EXPECT(!TimeZone::canonicalize_time_zone("I don't exist"sv).has_value());
 }
 
+static i64 offset(i64 sign, i64 hours, i64 minutes, i64 seconds)
+{
+    return sign * ((hours * 3600) + (minutes * 60) + seconds);
+}
+
+static void test_offset(StringView time_zone, i64 time, i64 expected_offset)
+{
+    auto actual_offset = TimeZone::get_time_zone_offset(time_zone, AK::Time::from_seconds(time));
+    VERIFY(actual_offset.has_value());
+    EXPECT_EQ(*actual_offset, expected_offset);
+}
+
 TEST_CASE(get_time_zone_offset)
 {
-    auto offset = [](i64 sign, i64 hours, i64 minutes, i64 seconds) {
-        return sign * ((hours * 3600) + (minutes * 60) + seconds);
-    };
-
-    auto test_offset = [](auto time_zone, i64 time, i64 expected_offset) {
-        auto actual_offset = TimeZone::get_time_zone_offset(time_zone, AK::Time::from_seconds(time));
-        VERIFY(actual_offset.has_value());
-        EXPECT_EQ(*actual_offset, expected_offset);
-    };
-
     test_offset("America/Chicago"sv, -2717668237, offset(-1, 5, 50, 36)); // Sunday, November 18, 1883 12:09:23 PM
     test_offset("America/Chicago"sv, -2717668236, offset(-1, 6, 00, 00)); // Sunday, November 18, 1883 12:09:24 PM
     test_offset("America/Chicago"sv, -1067810460, offset(-1, 6, 00, 00)); // Sunday, March 1, 1936 1:59:00 AM
@@ -126,6 +128,24 @@ TEST_CASE(get_time_zone_offset)
     EXPECT(!TimeZone::get_time_zone_offset("I don't exist"sv, {}).has_value());
 }
 
+TEST_CASE(get_time_zone_offset_with_dst)
+{
+    test_offset("America/New_York"sv, 1642558528, offset(-1, 5, 00, 00)); // Wednesday, January 19, 2022 2:15:28 AM
+    test_offset("America/New_York"sv, 1663553728, offset(-1, 4, 00, 00)); // Monday, September 19, 2022 2:15:28 AM
+    test_offset("America/New_York"sv, 1671453238, offset(-1, 5, 00, 00)); // Monday, December 19, 2022 12:33:58 PM
+
+    // Phoenix does not observe DST.
+    test_offset("America/Phoenix"sv, 1642558528, offset(-1, 7, 00, 00)); // Wednesday, January 19, 2022 2:15:28 AM
+    test_offset("America/Phoenix"sv, 1663553728, offset(-1, 7, 00, 00)); // Monday, September 19, 2022 2:15:28 AM
+    test_offset("America/Phoenix"sv, 1671453238, offset(-1, 7, 00, 00)); // Monday, December 19, 2022 12:33:58 PM
+
+    // Moscow's observed DST changed several times in 1919.
+    test_offset("Europe/Moscow"sv, -1609459200, offset(+1, 2, 31, 19)); // Wednesday, January 1, 1919 12:00:00 AM
+    test_offset("Europe/Moscow"sv, -1596412800, offset(+1, 4, 31, 19)); // Sunday, June 1, 1919 12:00:00 AM
+    test_offset("Europe/Moscow"sv, -1592611200, offset(+1, 4, 00, 00)); // Tuesday, July 15, 1919 12:00:00 AM
+    test_offset("Europe/Moscow"sv, -1589068800, offset(+1, 3, 00, 00)); // Monday, August 25, 1919 12:00:00 AM
+}
+
 #else
 
 TEST_CASE(time_zone_from_string)

Userland/Libraries/LibJS/Tests/builtins/Temporal/TimeZone/TimeZone.prototype.getOffsetStringFor.js

@@ -9,15 +9,15 @@ describe("correct behavior", () => {
             ["GMT", "+00:00"],
             ["Etc/GMT+12", "-12:00"],
             ["Etc/GMT-12", "+12:00"],
-            ["Europe/London", "+00:00"],
+            ["Europe/London", "+01:00"],
-            ["Europe/Berlin", "+01:00"],
+            ["Europe/Berlin", "+02:00"],
-            ["America/New_York", "-05:00"],
+            ["America/New_York", "-04:00"],
-            ["America/Los_Angeles", "-08:00"],
+            ["America/Los_Angeles", "-07:00"],
             ["+00:00", "+00:00"],
             ["+01:30", "+01:30"],
         ];
         for (const [arg, expected] of values) {
-            const instant = new Temporal.Instant(1600000000000000000n);
+            const instant = new Temporal.Instant(1600000000000000000n); // Sunday, September 13, 2020 12:26:40 PM
             expect(new Temporal.TimeZone(arg).getOffsetStringFor(instant)).toBe(expected);
         }
     });