In a practical way (instead of just, why not be polite?), the information for permanently displaying to the public may be different than the information relevant to you at a single moment.
If you just ask it without context, "today we'll close at 16:00" is a perfectly reasonable response.
Hopefully a few of them will realize it is easy and so start updating their own information as it changes. Everyone wins - their customers get up to date information and so come when they are open.
If you just ask it without context, "today we'll close at 16:00" is a perfectly reasonable response.