Well, it's pretty simple if you don't given them the task until they agree they're ready to start, and then record when you receive the submission.
Any candidate capable of actually inventing a time machine should certainly be hired.
Any candidate capable of actually inventing a time machine should certainly be hired.
No candidate wants to be entered in the Hunger Games for who has the most time to sink into a take-home.
Also, such a system does not prevent cheating, which is really the worst issue here.