Is this legal?

Confirm on your next check that you are getting paid for all hours worked.

It's possible that the manager overrides the time on the POS.

If you aren't getting paid, find a new job. And request that you be paid the missing money. If probably have to do this in writing. If they still don't pay, file a claim with your state or the federal wage and hour board.

Unpaid extra hours are definitely not legal.

Another question is what can you do against this. Despite being illegal, those practices are widespread in this sector, and kind of tolerated by the states. Moreover, it's the kind of jobs in which you can easily lose your job if you complain.

In theory you could (anonymously) call some "labour inspector" (I don't know which country you're in but I guess they exist everywhere, that's a job recognised by the ILO). Ask him to come after closure on some busy day, so that he can witness the fraud by himself. It would teach your boss a lesson and he would think twice before stealing your hours again. Unfortunately, the labour inspectors are usually very few and don't have much time to help.

Another possibility is to try to talk with a trade union, and possibly to try to establish a union branch in your restaurant. First, they'll give you practical advise. Then, if you complain to your boss it's much better to complain as a group instead of doing it alone. Both because you'ld give more power to your voice and because you'ld make it more difficult for him to answer with sanctions.

No, not legal. They are legally required to keep accurate records of the hours you work and to pay you for ALL of those hours.

One of those things too that if you say something they’ll probably just fire you soon. You should report them.

It's not really legal but you try doing anything. They all ignore laws and get away with it most of the time with only a few token prosecutions so it looks like something is being done about it. Another law they break very often is the gaps between work, like the time you need to be off before another shift starts, virtually every company breaks that even the leading ones. You try doing anything about it and they'll make an excuse to get rid of you or even set you up to make a critical mistake so it's easy to dismiss you.