You can add times using the
=SUM worksheet function. Just enter all of your times as HH:MM:SS, and then use SUM to add them up. You may leave off the :SS if you prefer. By default, Excel will display the sum of times in "time-of-day" format, meaning that adding 12:30 + 12:45 will yield 01:15. You can prevent Excel from "rolling over" at 24 hours by formatting the result cell as [h]:mm which will cause it to display 25:15 rather than 01:15.
If you want to add up minutes and seconds, you must include a leading "0:" in your data.
For example, enter "0:10:20" to indicate 10 minutes, 20 seconds. When you sum these times, Excel will display the sum in "time-of-day" format, meaning that adding 0:40:10 and 0:30:20 will yield 1:10:30. You can prevent Excel form "rolling over" at the hour by formatting the result cell as [m]:ss which will cause it to display 70:30 rather than 1:10:30.
Another method of adding times is to use the TIME function. To add 1 hour, 35 minutes, 10 seconds to a time in A1, use the function:
=A1 + TIME(1,35,10)