That makes it real easy!
You just march in to your Sup, and tell him it was advertised as 3:30-8:30 and you will not tolerate false advertising!
You do not appreciate bait & switch!
Stare right into his eyes, speak in a firm no nonsense tone, and pound your fist for emphasis to make sure he gets the point!
That will either impress him, showing you are one not to be messed with, OR, you will be escorted out after handing over your work ID.
(of course, you might to check first and make CERTAIN that the advertised times do not have any type of disclaimer "these times are subject to change to meet the volume and the needs of our Customers"