Just to add more info:

In your version is previous the .NET 4.5, you can use the  TimeZoneInfo.ConvertTimeBySystemTimeZoneId in .NET 3.5 and 4.0, that converts a time to the time in another time zone, based on a time zone identifier.
Here’s how to use this method using different time zone identifiers like “GMT Standard Time”, using just a simple code snippet:

static void Main(string[] args)
{
    DateTime dt = DateTime.Now;
    Console.WriteLine("Time in Different TimeZones:\n");

    Console.WriteLine("Greenwich Mean Time (GMT): {0}",
                TimeZoneInfo.ConvertTimeBySystemTimeZoneId(dt,
                TimeZoneInfo.Local.Id, "GMT Standard Time"));
    Console.ReadLine();
}

With this method you can do you conversion in the moment you send the date time using the WS method.

Kind Regards,
GM