Do you guys just use math to retime your speedruns down to the milleseconds or do you use programs? If so, which one?

Properly https://noobjsperson.github.io/speedrun-timer if im wrong tell me

Ok. I'd like to add, whats the difference bewteen that one and this one: https://somewes.com/frame-count/ (they have 1 milleseconds of difference when retiming using both of em: yours uses YT vids while this one uses the original file since with yt vids its broken).