Just another post which will help you to convert your date and time to UTC and give you the response
LocalTimeFromUtcTimeRequest timeReq = new LocalTimeFromUtcTimeRequest();
//app_date is date field value which you want to update.
timeReq.UtcTime = app_date;
timeReq.TimeZoneCode = timezone;
// Execute the LocalTimeFromUtcTimeRequest
LocalTimeFromUtcTimeResponse timeResp = (LocalTimeFromUtcTimeResponse)service.Execute(timeReq);
DateTime actualdate= timeResp.LocalTime;