Payment for DJ's

I'm getting ready to sign our contract with our DJ. It states that 50% is due 5 months prior to event, that I understand. But then it states that remaining balance is due 21 days prior to event. Does anyone know, is that standard to pay a DJ in full prior to your wedding?

Thank you.

We did pay our balance in advance, it was 10 years ago now so I can't remember how early, but 21 days seems reasonable - if you were paying by check, he would need time for it to clear.