Doesn't that get you the number of working versus non-working days?
The OP wants to calculate the number of working hours between two complete DateTimes.
Start date = 11/12/12 16:05
End date = 11/13/12 17:08
And I don't know of any built-in method that will do that because there is no way of saying whether or no the hour from 1700-1800 on Friday is a scheduled working hour. Yes it is an hour on a standard work day. But is the worker schedule until 1700hrs, or 1800hrs?
Or do you just assume that all the hours on a weekday are working hours, then from 1700 to 2400 would be 7 working hours?
Personally I think the OP needs to write a function that:
Checks the number of weekdays - easy, standard function.
Multiple by 24 to hours
Subtract the past hours off the beginning of the start day.
Subtract the remaining hours off the end of the end day.
That would get the net AVAILABLE weekday hours. But it won't be the *working* hours without factoring in the worker's schedule.
In this case verbiage is everything. Working hours verses weekday hours.