Check the resources tab for a tool that may help you retime runs.

If the run is on YouTube, the YouTube Frame Timer should be used for accurate results.

For all Rooms, time ends when the first character of the text scroll appears and when the level is fully faded-out. This will be obvious.

For all Rooms, time starts on the first frame anything apart of the level is visible. This will, again, be obvious.

Between Escape 1 and Escape 2, time ends when no portion of the level is visible and the screen is fully faded-out. Time starts the same as all other Rooms.

While it would be nice if you could retime your runs yourself, it is not necessary. Moderators will retime runs if you opt not to.