Calculation of hours between start time and end time

hey guys…
i am a bit stuck in trying to get a hour registration to work as i want it to…

so i try to make a registrations of hours used on a project… but when the user pick start time and end time i want it to calculate how many hours spendt by using fields as starttime hours:startime minutes and endtime hours:endtime minutes

but i cant figure how to calculate that when hours and minutes comes from 2 different fields… can anyone help me out here?