vb.net - Time Difference between 2 datetime fileds, only working hours

Question

I have 2 fields datetime (TaskStartDate and TaskEndDate). I created another field and called it Duration.

I need to update Duration to reflect how many business hours and minutes between the 2 fields (TaskStartDate and TaskEndDate) noting that we work only 8 hours a day.

Example:

TaskStartDate 2014-01-01 07:30
TaskEndDate   2014-01-03 15:30       Duration should read 24 Hours not 72hours