Can anyone help me with a formula that gives the working hours between two dates?

Sample is below:
Create Date Fixed Date
8/16/2012 19:29 8/23/2012 22:15

You have Holiday List
8/13/2012
8/14/2012

And working hours:
DayStart 9:00:00
DayEnd 17:00:00

Thanks a lot for your help!