Help: Determining TAT based on business hours... weekends included

Hello,

I've been trying to figure out a way to calculate the TAT between 2 timestamps which in turn would produce the difference in business hours based on my company's hours of operation.

Here's what I'm looking for:

Case Created Initial Response	Initial Response	TAT (business hours)
10/8/12 6:41 AM	10/8/12 7:59 AM	????????

Hours of Operation:

Mon - Thu: 9am - 11pm
Fri: 9am - 9pm
Sat + Sun: 10am - 8pm


Any help you could provide would be much appreciated.

I'm sorry... it's 8 business hours

Thanks again