It sounds like you want to subtract instead of add.

If A1 has 6:30 AM and A2 has 4:30 PM then =A2-A1 will return 10:00 AM.
Number formmating can be chosen to your taste.

If you are going across midnite (input 11PM and 1AM, expecting the result 2) then things get very dependant on your situation. The answer that I choose in that case (mostly) is to include date information when entering time. That has its problems, but other methods do as well.