Milk goes off mostly based on 'time from opening', if you're using milk at the estimated rate that the supermarket uses to set their best before date, then it's

Milk goes off mostly based on 'time from opening', if you're using milk at the estimated rate that the supermarket uses to set their best before date, then it's no surprise that it goes off at around the right time. OTOH, if you buy a bottle of milk and let it sit unopened for 5 days, then start consuming it, you'll usually find that it lasts roughly 5 days past the best before date.