You need to enter your time, say 14 minutes and 15 seconds, like
0:14:15
-note the leading 0, otherwise Excel treats 14:15 and 14 hours and 15 minutes.
Then simply sum, and format the sum cell as time. If your sum exceeds 24 hours, you'll need to format the cell for [h]:mm:ss, which
will prevent Excel from wrapping the result around to the next day.