Fix TimeZoneInfo.ConvertTime producing wrong results near DateTime.MinValue/MaxValue

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

@@ -288,22 +288,21 @@ private bool IsDaylightSavingOn(DateTime localDateTime)
         /// Tries to convert a local date and time to Coordinated Universal Time (UTC).
         /// </summary>
         /// <param name="localDateTime">The local date and time to convert.</param>
-        /// <param name="utcDateTime">When this method returns, contains the UTC date and time if the conversion succeeded.</param>
+        /// <param name="utcTicks">When this method returns, contains the UTC ticks if the conversion succeeded. The value may be negative or exceed DateTime range.</param>
         /// <returns>True if the conversion was successful; otherwise, false.</returns>
         /// <remarks>
         /// This method attempts to convert a local time to UTC. It returns false if the local time is invalid,
         /// such as during a daylight saving time transition when the local time does not exist.
         /// </remarks>
-        private bool TryLocalToUtc(DateTime localDateTime, out DateTime utcDateTime)
+        private bool TryLocalToUtc(DateTime localDateTime, out long utcTicks)
         {
             if (TryGetUtcOffset(localDateTime, out TimeSpan offset))
             {
-                long ticks = localDateTime.Ticks - offset.Ticks;
-                utcDateTime = SafeCreateDateTimeFromTicks(ticks, DateTimeKind.Utc);
+                utcTicks = localDateTime.Ticks - offset.Ticks;
                 return true;
             }
 
-            utcDateTime = default;
+            utcTicks = 0;
             return false;
         }
 

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

@@ -649,7 +649,7 @@ private static DateTime ConvertTime(DateTime dateTime, TimeZoneInfo sourceTimeZo
                 throw new ArgumentException(SR.Argument_ConvertMismatch, nameof(sourceTimeZone));
             }
 
-            bool isInvalidTime = !sourceTimeZone.TryLocalToUtc(dateTime, out DateTime utcDateTime);
+            bool isInvalidTime = !sourceTimeZone.TryLocalToUtc(dateTime, out long utcTicks);
 
             if (((flags & TimeZoneInfoOptions.NoThrowOnInvalidTime) == 0) && isInvalidTime)
             {
@@ -660,7 +660,10 @@ private static DateTime ConvertTime(DateTime dateTime, TimeZoneInfo sourceTimeZo
             {
                 // This is not logical to do but we are keeping it for app compatibility reason.
                 // We get here if the dateTime is invalid in the source time zone.
-                utcDateTime = new DateTime(dateTime.Ticks + sourceTimeZone.BaseUtcOffset.Ticks, DateTimeKind.Utc);
+                // Preserve the historical behavior of throwing if the computed UTC time is
+                // outside the DateTime range, rather than silently clamping it later.
+                DateTime invalidTimeUtc = new DateTime(dateTime.Ticks + sourceTimeZone.BaseUtcOffset.Ticks, DateTimeKind.Utc);
+                utcTicks = invalidTimeUtc.Ticks;
             }
 
             DateTimeKind targetKind = cachedData.GetCorrespondingKind(destinationTimeZone);
@@ -671,7 +674,15 @@ private static DateTime ConvertTime(DateTime dateTime, TimeZoneInfo sourceTimeZo
                 return dateTime;
             }
 
-            DateTime targetConverted = destinationTimeZone.UtcToLocal(utcDateTime, out bool isDaylightSaving);
+            // Use a clamped DateTime for destination offset lookup because transition-table lookups require
+            // an in-range DateTime. This preserves the existing offset-selection behavior for the lookup,
+            // while the final local ticks are still computed from the raw utcTicks to avoid double-clamping.
+            DateTime utcForLookup = SafeCreateDateTimeFromTicks(utcTicks, DateTimeKind.Utc);
+            TimeSpan destOffset = destinationTimeZone.GetOffsetForUtcDate(utcForLookup, out bool isDaylightSaving);
+
+            // Compute the final result from raw ticks to avoid precision loss from double-clamping.
+            // The intermediate UTC ticks may be outside DateTime range, but the final local ticks may be valid.
+            DateTime targetConverted = SafeCreateDateTimeFromTicks(utcTicks + destOffset.Ticks);
 
             if (targetKind == DateTimeKind.Local)
             {

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

@@ -290,6 +290,45 @@ public static void ConvertTime_DateTimeOffset_NearMinMaxValue()
             VerifyConvert(DateTime.MinValue.AddHours(0.5) + earlyTimesDifference, s_strSydney, s_strPacific, DateTime.MinValue.AddHours(0.5));
         }
 
+        [Theory]
+        [InlineData(4, 3, 1, 13, 15, 0, 13, 15)]   // UTC+4 to UTC+3: subtract 1 hour
+        [InlineData(5, 3, 2, 30, 0, 0, 30, 0)]      // UTC+5 to UTC+3: subtract 2 hours
+        [InlineData(3, 4, 0, 30, 0, 1, 30, 0)]      // UTC+3 to UTC+4: add 1 hour
+        [InlineData(8, 3, 4, 0, 0, 0, 0, 0)]        // UTC+8 to UTC+3: subtract 5 hours, result is MinValue boundary
+        public static void ConvertTime_DateTime_NearMinValue_PositiveOffsetZones(
+            int sourceOffsetHours, int destOffsetHours,
+            int inputHour, int inputMinute, int inputSecond,
+            int expectedHour, int expectedMinute, int expectedSecond)
+        {
+            TimeZoneInfo sourceTimeZone = TimeZoneInfo.CreateCustomTimeZone($"UTC+{sourceOffsetHours}", TimeSpan.FromHours(sourceOffsetHours), $"UTC+{sourceOffsetHours}", $"UTC+{sourceOffsetHours}");
+            TimeZoneInfo destTimeZone = TimeZoneInfo.CreateCustomTimeZone($"UTC+{destOffsetHours}", TimeSpan.FromHours(destOffsetHours), $"UTC+{destOffsetHours}", $"UTC+{destOffsetHours}");
+
+            DateTime earlyDate = new DateTime(0001, 01, 01, inputHour, inputMinute, inputSecond);
+            DateTime converted = TimeZoneInfo.ConvertTime(earlyDate, sourceTimeZone, destTimeZone);
+
+            DateTime expected = new DateTime(0001, 01, 01, expectedHour, expectedMinute, expectedSecond);
+            Assert.Equal(expected, converted);
+        }
+
+        [Theory]
+        [InlineData(-4, -3, 22, 46, 45, 23, 46, 45)]   // UTC-4 to UTC-3: add 1 hour
+        [InlineData(-3, -4, 23, 46, 45, 22, 46, 45)]   // UTC-3 to UTC-4: subtract 1 hour
+        [InlineData(-3, -8, 23, 0, 0, 18, 0, 0)]       // UTC-3 to UTC-8: subtract 5 hours
+        public static void ConvertTime_DateTime_NearMaxValue_NegativeOffsetZones(
+            int sourceOffsetHours, int destOffsetHours,
+            int inputHour, int inputMinute, int inputSecond,
+            int expectedHour, int expectedMinute, int expectedSecond)
+        {
+            TimeZoneInfo sourceTimeZone = TimeZoneInfo.CreateCustomTimeZone($"UTC{sourceOffsetHours}", TimeSpan.FromHours(sourceOffsetHours), $"UTC{sourceOffsetHours}", $"UTC{sourceOffsetHours}");
+            TimeZoneInfo destTimeZone = TimeZoneInfo.CreateCustomTimeZone($"UTC{destOffsetHours}", TimeSpan.FromHours(destOffsetHours), $"UTC{destOffsetHours}", $"UTC{destOffsetHours}");
+
+            DateTime lateDate = new DateTime(9999, 12, 31, inputHour, inputMinute, inputSecond);
+            DateTime converted = TimeZoneInfo.ConvertTime(lateDate, sourceTimeZone, destTimeZone);
+
+            DateTime expected = new DateTime(9999, 12, 31, expectedHour, expectedMinute, expectedSecond);
+            Assert.Equal(expected, converted);
+        }
+
         [Fact]
         public static void ConvertTime_DateTimeOffset_VariousSystemTimeZones()
         {