Difference between 2 dates, incl weekends, with variable work days

[babryanton]

I am creating a problem report spreadsheet for work. In col C I have a
date/time that the problem started, eg, 23-Jun-2006 15:20 and in col D
I have a date/time for when the problem was resolved, eg, 26-Jun-2006
15:40.

I need to calculate the amount of hours that we were not available for
use (due to the problem). This is based on a list of available hours.
Eg, on Mondays we are available 19.5 hrs, T-F 22.5 hrs, Sa 14.5 hrs,
and Sun 0 hrs.

If I do a straight subtraction, it uses 24 hrs for each day - not
taking into account our available hrs. If I do NETWORKDAYS it
automatically doesn't consider Sat or Sun - and i need it to - and it
does have a DayEnd and DayStart but it assumes they are constants.

I found the day corresponding to the date and thought I could determine
the series of days between it, e.g., if the start day was Friday & the
end day Monday, the series in between is Sat & Sun. Then I could do a
lookup on all 4 days in a table & add them all together. But I can't
figure out how to do this!

Can anyone please PLEASE help!?

Thanks so much!