Hi,

You can refer how the stop time is calculated for an event using start time and event duration in the calendar module.In this case this will be simple because there are 2 datetime fields and a duration field - time in your case.

date_result = date_n + timedelta(minutes=round((time or 1.0) * 60))

Either you can compute total time if it is not a field of type float by the following method by storing the start time and end time.

tot_sec = (end_time - start_time).total_seconds()
tot_hours = round((tot_sec / 3600.0), 2)

Regards