I am using EWS 1.2 to send appointments. On creating new Appointments, TimeZone is showing properly on notification mail, but on updating the same appointment, it's TimeZone reset to UTC.

Could anyone help me to fix this issue?

Here is sample code to replicate the issue:

ExchangeService service = new ExchangeService(ExchangeVersion.Exchange2010_SP1, TimeZoneInfo.FindSystemTimeZoneById("Eastern Standard Time"));
service.Credentials = new WebCredentials("ews_calendar", PASSWORD, "acme");
service.Url = new Uri("https://acme.com/EWS/Exchange.asmx");

Appointment newAppointment = new Appointment(service);
newAppointment.Subject = "Test Subject";
newAppointment.Body = "Test Body";
newAppointment.Start = new DateTime(2012, 03, 27, 17, 00, 0);
newAppointment.End = newAppointment.Start.AddMinutes(30);
newAppointment.RequiredAttendees.Add("[email protected]");

//Attendees get notification mail for this appointment using (UTC-05:00) Eastern Time (US & Canada) timezone
//Here is the notification content received by attendees:
//When: Tuesday, March 27, 2012 5:00 PM-5:30 PM. (UTC-05:00) Eastern Time (US & Canada)
newAppointment.Save(SendInvitationsMode.SendToAllAndSaveCopy);

// Pull existing appointment
string itemId = newAppointment.Id.ToString();

Appointment existingAppointment = Appointment.Bind(service, new ItemId(itemId));

//Attendees get notification mail for this appointment using UTC timezone
//Here is the notification content received by attendees:
//When: Tuesday, March 27, 2012 11:00 PM-11:30 PM. UTC
existingAppointment.Update(ConflictResolutionMode.AlwaysOverwrite, SendInvitationsOrCancellationsMode.SendToAllAndSaveCopy);