There's probably a real, scientific explanation for how long it can go, dealing with pasteurization and homogenization and all those other -ations. But truly, it just comes down to how adventurous you are. The expiration date is very much a psychological factor -- the passing of the date transforms the milk in your mind. So you smell what you expect to smell, taste what you want to taste. You can be daring, or not.

I always just throw it away. The guilt comes right after.