Closing earlier than advertised times

We showed up at the store at 16:50 thinking we will be on time before the store closes only to find that the doors are closed. Upon enquiring the cashier said they close at 17:00 but couldn't help us as they close the doors at 16:30 in order to leave at 17:00. When asked what she thought we should do about it she said she didn't know as she was just following the rules. I asked her to call the manager who told me the same thing except that she didn't know what we had to do and she didn't even care. She closes the doors at 16:30 to close off the tills and leave at 17:00. The bizarre thing is that she doesn't care whether she loses the sale or the customer