There is a bug with in game time where the in-game time and lap timers won't match up. For the time being, those runs will be timed by adding up each lap timer. This can be seen in where the in game timer is .01 seconds slower and 2 seconds by the end of the level. If it is found that the in-game timer is right and lap timers are wrong, times will be adjusted accordingly.

Make sure to add up laps properly, as the thing that seems to be a millisecond counter actually counts the frames instead (30 fps), so every .3 is actually 1 second.
For example, the run above has .12, .28, and .29 in the millisecond spot. This adds up to .69 or 2.09 seconds in the case of timing. Combined with the 56 seconds that were fully counted, this run still ends up being 58 seconds.

Most of the time, this bug only takes off a frame or two, so it is unlikely you will need to count your laps like this, and the in-game timer will be accurate most of the time.

is the bug with the mushroom at the start allowed?

@cold_oatcold_oat yeah it is since it's part of the game and not mods/dev tools, this thread was just explaining how to time runs to people, as they didn't notice the whole 30-frame thing even existed and would say stuff like "my run is 56.69 seconds, not 58.09".