Excel 2007 : Calculations on a cell containing hours and minutes

I'm working on a time sheet application. I have the start time and end time. I also have a place for "non billable time". That would include lunch or any other time off.

I want to be able to calculate the time difference between start time and end time AND subtract the non-productive time from it.

I have no problem with the calculation of hours and minutes between the start time and the end-time. But when I try to use the non-billable time in any calculation, it gives me a #VALUE error.

Can anybody help?