During the time between when you arrive and when you punch in, can you leave and come back? Or are you required to stay there in case you are needed? Do you work even though you are not clocked in?
Under some circumstances, it is my understanding that employers are required to pay for "wait time," but I do not know the exact criteria.
It is legal for your boss to change your shift during the shift to add a 1/2 hour to the end of it. Is that what you meant by that last part?