Calculating working hours time span

Does anyone have a function to calculate working hours time span between 9am and 5.30pm using the 12 hour clock

I want to calculate averagewaitingtimes using the format below.

 I want  the calculation to exclude bank holidays and weekends and should not be the 24 hour format
The working hours are between 9am  to 5.30pm
I want the calculation to stop at 5.30pm

e.g  
datepricequoterequested -24/10/2011 at 15.00
datepricequotetodistributor  – 24/10/2011 at 16.00
datepricequotefromdistributor – 24/10/ 2011 at 17:00
datepricequoteprocessed  -25/10/2011 at 10:00am
Stop counting from  17:30 and 09:00

e.g
15:00-16:00 1 hour at company
16:00-17:00 1 hour at distributor
17:00 – 17:30 ½ hour at company
09:00 -10:00 1 hour at company
Gives the company 2 ½ hrs


Format is:
datepricequoteprocessed -  (datepricequotetodistributor  - datepricequotefromdistributor)- datepricequoterequested

e.g
25/10/2011 at 10:00am-(24/10/2011 at 16.00-– 24/10/ 2011 at 17:00)- 25/10/2011 at 10:00am