Thanks for the suggestion Support... I'll definitely do that next time.
After looking through the user guide again I realized a way to fix my error in the code. Here is what I came up with and it works great!
Appointment.StartTime=AS_TIMESTAMP(AS_STRING(Appointment.StartTimeDate,'MM/dd/yyyy')+' '+Appointment.StartTimeTime,'MM/dd/yyyy HH:mm a')
Appointment.EndTime=Appointment.StartTime+AS_NUMBER(Appointment.Duration)
