Fix TimeZoneInfo to handle Yukon zone (#43550)

Author: tarekgh

src/libraries/System.Private.CoreLib/src/System/TimeZoneInfo.AdjustmentRule.cs

@@ -119,8 +119,7 @@ internal static AdjustmentRule CreateAdjustmentRule(
             internal bool IsStartDateMarkerForBeginningOfYear() =>
                 !NoDaylightTransitions &&
                 DaylightTransitionStart.Month == 1 && DaylightTransitionStart.Day == 1 &&
-                DaylightTransitionStart.TimeOfDay.TimeOfDay.Ticks < TimeSpan.TicksPerSecond && // < 12:00:01 AM
-                _dateStart.Year == _dateEnd.Year;
+                DaylightTransitionStart.TimeOfDay.TimeOfDay.Ticks < TimeSpan.TicksPerSecond; // < 12:00:01 AM
 
             //
             // When Windows sets the daylight transition end Jan 1st at 12:00 AM, it means the year ends with the daylight saving on.
@@ -129,8 +128,7 @@ internal bool IsStartDateMarkerForBeginningOfYear() =>
             internal bool IsEndDateMarkerForEndOfYear() =>
                 !NoDaylightTransitions &&
                 DaylightTransitionEnd.Month == 1 && DaylightTransitionEnd.Day == 1 &&
-                DaylightTransitionEnd.TimeOfDay.TimeOfDay.Ticks < TimeSpan.TicksPerSecond && // < 12:00:01 AM
-                _dateStart.Year == _dateEnd.Year;
+                DaylightTransitionEnd.TimeOfDay.TimeOfDay.Ticks < TimeSpan.TicksPerSecond; // < 12:00:01 AM
 
             /// <summary>
             /// Helper function that performs all of the validation checks for the factory methods and deserialization callback.

src/libraries/System.Runtime/tests/System/TimeZoneInfoTests.cs

@@ -119,6 +119,40 @@ public static void LibyaTimeZone()
             Assert.True(libyaLocalTime.Equals(expectResult), string.Format("Expected {0} and got {1}", expectResult, libyaLocalTime));
         }
 
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsWindows))]
+        public static void TestYukunTZ()
+        {
+            try
+            {
+                TimeZoneInfo yukon = TimeZoneInfo.FindSystemTimeZoneById("Yukon Standard Time");
+
+                // First, ensure we have the updated data
+                TimeZoneInfo.AdjustmentRule [] rules = yukon.GetAdjustmentRules();
+                if (rules.Length <= 0 || rules[rules.Length - 1].DateStart.Year != 2021 || rules[rules.Length - 1].DateEnd.Year != 9999)
+                {
+                    return;
+                }
+
+                TimeSpan minus7HoursSpan = new TimeSpan(-7, 0, 0);
+
+                DateTimeOffset midnight = new DateTimeOffset(2021, 1, 1, 0, 0, 0, 0, minus7HoursSpan);
+                DateTimeOffset beforeMidnight = new DateTimeOffset(2020, 12, 31, 23, 59, 59, 999, minus7HoursSpan);
+                DateTimeOffset before1AM = new DateTimeOffset(2021, 1, 1, 0, 59, 59, 999, minus7HoursSpan);
+                DateTimeOffset at1AM = new DateTimeOffset(2021, 1, 1, 1, 0, 0, 0, minus7HoursSpan);
+                DateTimeOffset midnight2022 = new DateTimeOffset(2022, 1, 1, 0, 0, 0, 0, minus7HoursSpan);
+
+                Assert.Equal(minus7HoursSpan, yukon.GetUtcOffset(midnight));
+                Assert.Equal(minus7HoursSpan, yukon.GetUtcOffset(beforeMidnight));
+                Assert.Equal(minus7HoursSpan, yukon.GetUtcOffset(before1AM));
+                Assert.Equal(minus7HoursSpan, yukon.GetUtcOffset(at1AM));
+                Assert.Equal(minus7HoursSpan, yukon.GetUtcOffset(midnight2022));
+            }
+            catch (TimeZoneNotFoundException)
+            {
+                // Some Windows versions don't carry the complete TZ data. Ignore the tests on such versiosn.
+            }
+       }
+
         [Fact]
         public static void RussianTimeZone()
         {