Calculate 0h durations in a timesheet?

My job requires us to turn in timesheets for each pay period and I am trying to turn it digital to make it a little easier. In the process, I have gotten Numbers to calculate the durations I need unless there is no time present. If I don't have a clock in/out present, the calculation is 0 rather than 0h and it throws everything off. I tried changing the cell format to duration but when I click off the cell and go back to it, it changes back to automatic. I tried doing an if statement in the function but if I try to type any duration into the function (i.e. 0h) it simply won't let me. Is there a way to get the calculation to default to 0h if there isn't a time present to calculate?