Take two hours from 00:00 and show 10:00 not ######

Hi all,

I have a formula that takes a variety of production times for our manufacturing business (i.e. 06:00 equals 6 hours production time) and subtracts them from a finish time to derive start time.

For example, if finish time is 02:00 and production time is 06:00 then finish time less production time should equal 20:00.

However when I attempt this formula I get a ##### answer. Any way around this?

Thanks.