Calculate time in Excel 2003

Question: I need to be able to calculate the amount of time in hours:minutes between one date/time and another. I can subtract one from the other and format the output but I need to also subtract weekend days and evenings (a 8-5 workday).

I can get the net working days (networkdays) but that's not what I want.
Is there an easier way than writing a custom function? If not, can someone provide general pseudocode for the algorithm. Thank you.

Example: If the start value is 12/17/2004 16:00 and the end value is 12/21/2004 10:20 the result should be 12:20.