sum up times

Hey!

i am makeing a timesheet application so i have 28-31 (depending on day of the mnth) textboxes that calculates the time difference start time to end time and that is working perfectly. but how do i sum all thise up in to a total box whit hh:mm format (sums to to 160 hours and 20 min 160:20) and i also need to deduct 30 mins(00:30) for every shift worked longer than 04:30 (lunsh).

i hope i explaned myself good enough :-)