You would use the functions Time2Num and Num2GMTime to do this. In the
calculate event of the total time field you would put something like (in
formcalc):var startT = StartTime.formattedValuevar endT =
EndTime.formattedValuevar differenceT = Time2Num(endT, "HH:MM:SS") -
Time2Num(startT, "HH:MM:SS")$ = Num2GMTime(differenceT, "HH:MM:SS")- the
"HH:MM:SS" bit should be whatever time pattern you have used.