Well it usually starts when you spawn in and are on the spawning platform, just look at other speedruns and look where they start.

They start on first movement (at least that's what i assume)

Timer starts on first frame of movement.

this game has a ton of mods

so game must be active

Timer starts on the first frame of movement (After reset) and timer ends on the frame that the the cutscene plays / checkpoint number changes.