In versions 1.0, 1.1, 2.0 and later versions of the .NET Framework, when this property is set to Local, DateTime objects are always formatted as the local time. That is, local time zone information is always included with the serialized data. Set this property to Local to ensure compatibility with older versions of the .NET Framework.
In version 2.0 and later versions of the .NET Framework that have this property set to Roundtrip, DateTime objects are examined to determine whether they are in the local, UTC, or an unspecified time zone. The DateTime objects are then serialized in such a way that this information is preserved. This is the default behavior and is the recommended behavior for all new applications that do not communicate with older versions of the framework.