Avoid creating un-representable DateTime for transition point (mono#16753)

Fixes mono#16742

Author: MaximLipnin

mcs/class/corlib/System/TimeZoneInfo.cs

@@ -1287,8 +1287,11 @@ private bool TryGetTransitionOffset (DateTime dateTime, out TimeSpan offset, out
 
 		private static DateTime TransitionPoint (TransitionTime transition, int year)
 		{
-			if (transition.IsFixedDateRule)
-				return new DateTime (year, transition.Month, transition.Day) + transition.TimeOfDay.TimeOfDay;
+			if (transition.IsFixedDateRule) {
+				var daysInMonth = DateTime.DaysInMonth (year, transition.Month);
+				var transitionDay = transition.Day <= daysInMonth ? transition.Day : daysInMonth;
+				return new DateTime (year, transition.Month, transitionDay) + transition.TimeOfDay.TimeOfDay;
+			}
 
 			DayOfWeek first = (new DateTime (year, transition.Month, 1)).DayOfWeek;
 			int day = 1 + (transition.Week - 1) * 7 + (transition.DayOfWeek - first + 7) % 7;

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

@@ -854,6 +854,15 @@ public void TestIsDST_DateTimeOffset ()
 				Assert.IsTrue (tzi.IsDaylightSavingTime (dateOffset));
 			}
 
+			// https://github.com/mono/mono/issues/16742
+			[Test]
+			public void Bug_16472 ()
+			{
+				var parsedTime = DateTime.Parse ("1948-02-19T23:00:00Z", CultureInfo.InvariantCulture);
+				var newTime = TimeZoneInfo.ConvertTime (parsedTime, TimeZoneInfo.FindSystemTimeZoneById (MapTimeZoneId ("Europe/Rome")));
+				Assert.AreEqual (1948, newTime.Year);
+			}
+
 			// https://github.com/mono/mono/issues/9664
 			[Test]
 			public void Bug_9664 ()