Fix time zone transition out of DST (#15401)

When a datetime appears in range of [end_of_DST_period - Daylight_delta; end_of_DST_period] it's not DST and should return base offset.

Fixes #9664.

mcs/class/corlib/System/TimeZoneInfo.cs

@@ -1272,6 +1272,12 @@ namespace System
 						isDst = true;
 					}
 
+					if (date >= new DateTime (tEnd.Ticks - current.DaylightDelta.Ticks, DateTimeKind.Utc))
+					{
+						offset = baseUtcOffset;
+						isDst = false;
+					}
+
 					return true;
 				}
 			}

mcs/class/corlib/Test/System/TimeZoneInfoTest.cs

@@ -71,6 +71,10 @@ namespace MonoTests.System
 					return "W. Europe Standard Time";
 				case "Canada/Eastern":
 					return "Eastern Standard Time";
+				case "Asia/Tehran":
+					return "Iran Standard Time";
+				case "Europe/Guernsey":
+					return "GMT Standard Time";
 				default:
 					Assert.Fail ($"No mapping defined for zone id '{id}'");
 					return null;
@@ -873,6 +877,24 @@ namespace MonoTests.System
 				date = new DateTime (2019, 3, 10, 3, 0, 0);
 				Assert.IsTrue (tzi.IsDaylightSavingTime (date));
 				Assert.AreEqual (new TimeSpan (-5, 0, 0), tzi.GetUtcOffset (date));
+
+#if !WINAOT // https://github.com/mono/mono/issues/15439
+				tzi = TimeZoneInfo.FindSystemTimeZoneById (MapTimeZoneId ("Europe/Vatican"));
+				date = new DateTime (2018, 10, 28, 2, 15, 0);
+				Assert.IsFalse (tzi.IsDaylightSavingTime (date));
+				Assert.AreEqual (new TimeSpan (1, 0, 0), tzi.GetUtcOffset (date));
+
+				tzi = TimeZoneInfo.FindSystemTimeZoneById (MapTimeZoneId ("Asia/Tehran"));
+				date = new DateTime (2018, 9, 21, 23, 15, 0);
+				Assert.IsFalse (tzi.IsDaylightSavingTime (date));
+				Assert.AreEqual (new TimeSpan (3, 30, 0), tzi.GetUtcOffset (date));
+
+				// for Greenwitch Mean Time (Guernsey)
+				tzi = TimeZoneInfo.FindSystemTimeZoneById (MapTimeZoneId ("Europe/Guernsey"));
+				date = new DateTime (2019, 10, 27, 1, 15, 0);
+				Assert.IsFalse (tzi.IsDaylightSavingTime (date));
+				Assert.AreEqual (new TimeSpan (0, 0, 0), tzi.GetUtcOffset (date));
+#endif
 			}
 		}