Calculating elapsed hours, respecting work hours and weekends

Hello,

I am trying to create an excel formula that appropriately calculates the elapsed time from when a ticket was created to when it has been closed out. This is also respecting work hours from 8AM to 5PM, so a 9 hour day, and also respecting weekends.

So, take

OPENED
4/20/11 7:53 PM
.
CLOSED
4/26/11 2:55 PM

Now the opened time is at 7:53PM, so since this is outside of work hours would become 4/21/11 8:00AM to respect the start of work hours. Along with that, it was opened for 5 days after moving it to 8:00AM, so the hours elapsed was 51 hours and 55 minutes, or 51.9 hours in decimal format (which is what I want).

This calculation would be converted into days, also, for different priorities, so hopefully it's possible to calculate the days based on business hours too. So, for an elapsed time of 18 hours, it took 2 days since business hours are 9 hours a day.

Thanks for the help.

Work hours are Monday to Friday, 8AM to 5PM - 9 hours.

Edit: This is also for Excel 2010. For some reason it says 03 up at the top right.