Game's score is based on a client-side countdown. How to bulletproof it?

I'm working on a game, which has score based on a JavaScript countdown: the faster you finish the level before the countdown reaches zero, the bigger your score is.

How can I make sure it is not somehow altered when I finally receive it from client-side on server-side?

My initial idea is to make two checkpoints: one at the beginning of a level and another at the end. Checkpoint is basically a session sent via AJAX to server-side PHP script which is then timestamped. So after the game is finished on client-side, the score is verified with the one on server-side. Is this kind of protection any good?

Thank you in advance!

EDIT: I'm also open to any other ways to achieve the desired functionality.