I suppose you can also store values as integer data. if the user enters, 15, 30, 45 or 60 then at the end you just sum up and divide by 60 and that is total time in hours. You take the remainder and multiply by 60 to get the minutes, i.e., 3.3333 hours is 3 hours and (.3333 * 60) = 20 minutes.
This kind of data entry is error prone since someone might enter .5 instead of 30 for half an hour (.5 hours instead of 30 minutes).
Normally, it's best to have user select start time and end time. Just like you do with a time clock.