What it does

For the nerds: The old Ultrakill timer worked by adding deltaTime every frame:
timer += dT*GameSpeed
This works fine at a small scale but not at a large scale, especially when dealing with frame jitter, it would get up to 0.4 seconds off within 5m 30s.

This new timer works by using Unity's native Time.time function and timestamps in order to get the accurate time. Some changes had to be made [changing cutscene code to subtract timestamp instead of this.seconds, adding an accumulator so it remains consistent with 8-1's fake exit, etc.]
The code goes something like:

timer = Time.time-levelStart
This is accurate to the millisecond for up to 9 hours.
Note that this mod conflicts with mods that edit StatsManager such as FrankenToilet or mods that edit CutsceneSkip.