Fix Mono rounding error on DateTime conversion. Issue #44

Author: yfakariya

src/MsgPack/MessagePackConvert.cs

@@ -111,7 +111,7 @@ public static string DecodeStringStrict( byte[] value )
 		/// </returns>
 		public static DateTimeOffset ToDateTimeOffset( long value )
 		{
-			return _unixEpocUtc.AddMilliseconds( value );
+			return _unixEpocUtc.AddTicks( value * _ticksToMilliseconds );
 		}
 
 		/// <summary>
@@ -125,7 +125,7 @@ public static DateTimeOffset ToDateTimeOffset( long value )
 		/// </returns>
 		public static DateTime ToDateTime( long value )
 		{
-			return _unixEpocUtc.AddMilliseconds( value );
+			return _unixEpocUtc.AddTicks( value * _ticksToMilliseconds );
 		}
 
 		/// <summary>

test/MsgPack.UnitTest/MessagePackConvertTest.cs

@@ -109,7 +109,7 @@ private static void AssertIsUnixEpocDateTimeOffset( DateTimeOffset actual, long
 		{
 			Assert.That( actual.Offset, Is.EqualTo( TimeSpan.Zero ) );
 			var epoc = new DateTimeOffset( 1970, 1, 1, 0, 0, 0, TimeSpan.Zero );
-			Assert.That( actual, Is.EqualTo( epoc.AddMilliseconds( millisecondsOffset ) ) );
+			Assert.That( actual, Is.EqualTo( new DateTimeOffset( epoc.Ticks + millisecondsOffset * TicksToMilliseconds, TimeSpan.Zero ) ) );
 		}
 
 
@@ -214,7 +214,7 @@ private static void AssertIsUnixEpocDateTime( DateTime actual, long milliseconds
 		{
 			Assert.That( actual.Kind, Is.EqualTo( DateTimeKind.Utc ) );
 			var epoc = new DateTime( 1970, 1, 1, 0, 0, 0, DateTimeKind.Utc );
-			Assert.That( actual, Is.EqualTo( epoc.AddMilliseconds( millisecondsOffset ) ) );
+			Assert.That( actual, Is.EqualTo( new DateTime( epoc.Ticks + millisecondsOffset * TicksToMilliseconds, DateTimeKind.Utc ) ) );
 		}
 
 		private static void AssertIsUnixEpocDateTime( DateTime expected, DateTime actual )