You are entitled to be paid. ( favour or not ) If you continue this then the mgt will come to expect you to stay on and this will eventually lead to what appears to be poor scheduling getting even poorer.
As long as you use the time clock correctly by clocking in and OUT at the right time then the restaurant is obliged to pay you by law.
Claim you missed wages. Young americans and others a penalised for work by supporting youth rates of wage. They say if you pay peanuts you'll get monkeys working for you. Rise above it and be paid whats due to you.
McD's New Zealand is one of the few businesses in our couyntry that has pledged NOT to pay youth rates. We pat staff a base minimum of $8.41 at present.
I believe this stand is right. You work hard so you Should be paid. More so if you a in a staggered pay system.
Anyway back to the point:
Your time card becomes a legal document and to alter this would be illegal. USE IT or if you store operates the way you say then YOU'LL LOSE IT. Enjoy your work and help your mates out but don't be taken for granted.