I've been trying to get excel to calculate the difference between a start of duty time and and end of duty time.

It works ok for example if the duty starts at 07:00 and finishes at 19:00, however if the duty were to start at 19:00 and finish at 07:00 the following morning it comes up with #Value!

Does anyone know how to derive the difference between times if they are not on the same day?

Many thanks in advance.