> However if you check the expiration dates in the US, milk in plastic tends to expire much more quickly than the milk in paper cartons. So there is more going on than it simply being different containers.

Without further details, I'm guessing that "milk in paper cartons" has been UHT-treated, which means it could last even longer than the published expiry date.

It'll still go bad in a few days or weeks once it has been opened, depending on how it has been stored.