I had a number of videos of varying duration. I wanted to find out if I watched all the videos one after another, how long they'd all take. I needed to sum the duration of the videos. At first I put the values in Excel in the format mm:ss (minute:second) but Excel didn't like that. I found that it works if you enter the times as hh:mm:ss (hour:minute:second).
Enter the values hh:mm:ss - even if your values are shorter than one hour, as mine are, make sure you enter them as shown here with a preceding 00.
Select all the cells including an extra one, this will be the total.
Format the cells as Time (as shown in the above screen shot).
Click in the empty cell at the bottom and click AutoSum
You should end up with something similar to the following: