So I've figured out a weird trick. Basically, because the game creates a 'finish' auto-save when you complete a day, if you start another day and run out of time, you swap to the 'finish' auto-save.

Example: I play through Day 1 Locusts and do all the stuff needed for a perfect day without timing it. I then start a proper timed run and select Locusts. I then find a way to kill myself as quickly as possible without clicking 'End Day'. After I die to time loss, the game reloads my perfect auto-save instead. I can then end the day and get a perfect day.

Note, you can also swap from any scenario to the saved scenario. Currently, there is no way to swap over to days, only to different or the same scenario on the same day.

If this was legal, it would save so much time on all routes. However, EzPiig and I have talked to Oreo321 and we've concluded that it is to be banned. I am posting this here as an explanation of what it is.