How to calculate difference between two date & time along with working hours and days

Hi Champs,

I have following requirement.
I want to calculate difference between two date & time considering working days and hours of an organization.

example:

1) Problem logged on : 10.08.2012 11:00:00
2) Problem solved on : 13.08.2012 16:00:00

I need to calculate number of hours taken to solve the problem where my organization's working time is 9.30 to 6.30 (Monday to Friday), 9.30 to 2.00 (Saturday) and Sunday is holiday.

My expected result should be : 24 hours

Can any one suggest me which formula should I use to get the correct result ?..