Timestamps in Numbers are only partially recognized as time?

I have a column with timestamps minutes, seconds and milliseconds. Originally they looked like this 6:24,120 (it is output from the time command) Pasting it into Numbers causes weird results. I can add them using =A1+A2 but using =sum(A1:A2) returns 0 and if I use average I get an error message "Numbers can’t be divided by zero." (which is odd, because even if the sum doesn't work and returns 0, you can still perform 0/2 - it is not an invalid operation). I have tried changing the format to 6m24s120ms, which Numbers reformats to 6m 24s 120ms but the result is the same as when using colons and commas as separators. How do I make average and sum work in these data?