I agree it should be added. Based on what I saw in a submitted run for Switch:
The run starts when you choose the chapter's "From the Beginning" option. The run ends when the animation for the blue ring ends at the end of the level, right before the screen goes black and then shows the score screen.
Written rules would be helpful for consistency. I watched some other submitted runs tonight and not every submitted run's timer is synced to the game's start and end.
My best estimate is individual levels end when the blue circle fades right before the credits (both Chapter 1 recordings from @bobdedbuilder), except for the last level which ends when the confetti is fully rendered after the last text box is closed at the end of the Princess' speech (Any% recording from @TheCowness).
Im also assuming cuz this hasent been done much due to a capture card probably being a requirement if not i would probably send my sub 2:30:00 for chapter 3
