Numbers timesheet code

Hi all I am having trouble with some code in numbers and have spent way too much time on this

trying to make a sheet that compares a start time to a end time to give a total for each job this is the easy part but my totals seem to have a 0 when no time is entered creating a error when adding my totals as some things are in h/m and others are just 0 because this is a spot with a formula in it it won’t let me format these 0s go time date. How do I make a 0 a blank