How do I sum time values?

I have a number of formulas that calculate the difference between two times of the day. The cells are formatted to produce the result in hours and minutes.
When I put a sum formula at the end of the row to add these times it will add the first two and then it seems to revert to a smaller number. I think that it is adding the time value not the actual hours and minutes.
Thank you in advance for your help
David P.