You could do this a few ways… Depends on how your time values are represented/stored in your repeater.
I would recommend you use an absolute time value with units in seconds to represent time in your repeater cell. It makes the “time math” a snap. For instance, in your repeater sheet, your “Time” column would have values of 18 and 72 for your two time examples. You would need to convert the time value from seconds to “standard colon-delimited time display”, e.g, from 72 to 01:12, everywhere you show it, like in the repeater’s OnItemLoad code. Here is a good description and formula for that:
https://forum.axure.com/t/how-to-generate-a-time-duration-from-number-of-minutes/28443/5?u=mbc66
If you want to keep your time values represented as time-formatted text, you’ll need to do the inverse of that formula in your summation to get the number of seconds in each cell then convert back to colon-delimited strings, which is a bit of extra converting, but probably keeps your spreadsheet easier to read/edit/import.
Here is an example that does both. The Home page has a repeater with time stored as “elapsed seconds” and Page 1 has time stored with colon-delimited strings.
TimeSummation.rp (92.0 KB)