Title says it all basically, the game is Slipstream5000

This depends on the rules the community has set. Check the rules for the category you're submitting to for details on how to time the run.

Basically, an external timer (like Livesplit) is almost never required for verification. Each game and category should have definite rules on when the time starts and ends, and the timing should be done with those rules. That timing is done by the frame-to-frame rule, you take the frames of time start and end and calculate the total time of a run.

For games with in-game timer it's usually enough to submit that timer, but not always. For example, a case where you have an in-game timer in each level but you run a full game category.