for the formula I'm pretty sure you have to have it like this

=SUM(D7-E7)
or
=SUM(D7+E7)

if you don't understnad what D7-E7 means, then do this...

In the cell that the answer is suppose to be type =SUM then click on the 3:15 cell.. You'll see something like =SUM(D7 show up in the answer cell.. then type either a - or a + into the answer cell... Then click onto the 7:30 cell... You'll then see something like this =SUM(D7+E7) in your answer cell.. it should auto do the math for you after that..

BUT I also recommend you change the way you input your hours 7:30 = 7.50 & 3:15 = 3.25 Its the BEST way to get the proper answer

Hope by breaking it down a little helped.

I did this for my dad last summer & it was a life saver for him, as it auto did all the math.

If you still cannot get it to work... can you send me a copy (remove/change any personal data) & I'll input the math & you can see how I did it just by looking at it