Time formula going back to zero?

So i am basically making a way to track payroll. Here is the formula i have in my E column

=IF(D2<C2,D2+1,D2)-C2

and so forth with the times entered into the c and d columns.

In the last e column where i want the total for the week i have:

=SUM(E2:E15)

problem is that when i enter more than 3 or more start and stop times the sum cell reverts back to zero, when all the other e still have the number of hours.

what is going on?