Is it legal to put a daily penalty?

[Sami Gren]

My tenant us asking for a month extension, but to be sure she will move out at that time, I want to put a big daily penalty fee (maybe like $75 per day) for each day she stays more than a month, so I could make sure she is seriously getting out and not just playing around with me, is it legal? (State of new jersey) And what do you think in general..