"Starts when the level loads" when is that exactly? is it when the screen is black?

Why don't we video just the time itself? If you look at https://www.speedrun.com/sdd , they have it so you just need to have a convincing video or photo even of the time.

Why don't we use the in game timer that says how long the runs are instead of having to records screens and count frames and worry about loading times? If theres a proper reason I'm not thinking of, please inform me, otherwise, why don't we switch?