I and trying to calculate the time of my run that was timed inaccurately. Does anyone know the exact time of the countdown (" Ready, Set, GO!" thing?).

In case anyone is wondering, I calculated it as 3.5 seconds

P.S. I would only like the time where the time starts, not when the GO! disappears, to be cauculated

For rta (which is the timing system we use generally use for these speedruns), we include the total time, not the summation of individual levels. That includes loading times and the timer "countdown". Otherwise, I believe that the time from when you enter the level to the first frame of possible movement (aka, when the timer for the level starts) is roughly 3.33 seconds.
Interestingly enough, I just found out that the timer starts 1 frame before "Go" appears.

Hope this helped.

Also, I lied before, the first frame of possible movement is 1 frame after the timer starts.

Okay, thanks for this information! 😃