To get the total number of hours between your two field you can use, this FormCalc in the calculate event of your hours field.
(Time2Num(TimeField2.formattedValue, "HH:MM") - Time2Num(TimeField1.formattedValue, "HH:MM")) / (60 * 60 * 1000)
If you want 10 minute increments you might be better off with a couple of dropdowns to choose hours/minutes or maybe something like this timepicker with up/down 'spinner' arrows.