Adding duration to time

I have an application to calculate the arrival time for a time log. Ex:

Start Time Duration Est. Arrival

07:00 AM 8:25 3:25 PM (15:25 is acceptable)

I've noticed that Apple's time functions are very limited. I've tried a number of different ways to do this, but they all come up with formula errors (which is another issue with Apple, it doesn't always give you meaningful messages).