Calculate between two date/time fields?

Hello,

How do I calculate the time in hours or days between two fields (startDate and FinishDate in Date/Time format 8/12/2008 14:41) less weekend and holiday?

Example:
StartDate FinishDate Result
8/12/2008 12:09 8/12/2008 14:41 92 mins or 1.53 hrs or 0.936 days

Thanks

=A1-A2 formatted as Time
HTH
Ola