Milk does not suddenly go bad on the expiration date. It could even be fine a week or two later if stored in the fridge.

Expiration dates are very conservative estimates of when the item will go bad.

For example, the method of preserving normal fresh milk - pasteurization - is to heat it to a certain temperature for half an hour.

Pasteurization will kill all or almost all the bacteria in the milk. Even if a tiny amount survives, if the milk is kept cold, it would not be able to reproduce enough to spoil the milk by the expiry date. If they get really lucky with the pasteurization, they may even kill everything and your milk will be OK 2 weeks AFTER expiry. But the manufacturers cover their asses and give you the shortest time, just in case.