is 3 week old milk that old? usually when I buy milk the best by date is 2-3 weeks away

Those dates assume unopened products. After being breaking the seal, they don’t last as long.

I generally trust the due dates printed on the items. But what would I know? Maybe it's a conspiracy to force us to drink old milk ;)

On a more serious note, UHT milk can be stored for months, given it wasn't opened.