I need a zero in my cell rather than xxx after my calculation

[stuj77]

Hi,

I am trying to create a formula. I have a value in a cell that I have created by subtracting one time (hours / minutes) from a set time. For example contracted hours 60:00 is the set time and used hours is taken away from this. The problem I have is that if the time I am taking away is more than the set time I get xxxx value. If the time I am taking away is less than the set time I get the value I want. I need to have zero displayed the formula displays xxxx.

This sounds confusing and I probably have not explained it well but any help will be greatly appreciated. Thanks