You don't.

That being said, the rules state that time starts on the first frame of selecting the level, and ends on the first frame of touching the last trophy. You can just use one of the many video retimers out there.

Edit: oh yeah that's right, game rules exist. Probably should be fixed so that they match the category rules. I'm curious if this discrepancy affects any positions on the leaderboard

Timing starts when L1 fades in and ends when you touch the trophy in L10. Sorry for not changing the rules, it's been like that for 2 years and we used the timing convention that was most convenient.