You may take the numerical difference between the two timestamps. Multiply integer part by 11 and fractional part by 24 to get the hours. This assumes that both the timestamps are in the 7:00 - 18:00 window of time. The attached qvw file shows this simple implementation.
If you have to take into account the non-working hours of holidays and weekends, and if you have to check if the timestamps fall in the working hours, some additional development is needed.
Can you help me here, i have used your calculation which works great. however some start dates are at weekends, i want to be able to say if the start date is say on a sunday start the calculation at 8am monday to the close date?