We produce and sell cheese. The best by date we put on it is required by some regulation or other. The date is mostly arbitrary. If the package is still sealed and the best by date is a year ago, congratulations. Your medium cheddar is now sharp cheddar.
If you opened the package of some fresh Colby, and the best by date is 3 months in the future (so it's been open for 3 months), you're going to have to trim a lot of mold off of it and it probably won't even taste that great anymore.
Really it would be more accurate to say: best before X time after opening or X time after purchasing, whichever is first.
But even then: if someone buys the sharp because they like it, they'll very possibly like it more as it ages to extra sharp, even though it passes the best by date.
If you opened the package of some fresh Colby, and the best by date is 3 months in the future (so it's been open for 3 months), you're going to have to trim a lot of mold off of it and it probably won't even taste that great anymore.
Really it would be more accurate to say: best before X time after opening or X time after purchasing, whichever is first.
But even then: if someone buys the sharp because they like it, they'll very possibly like it more as it ages to extra sharp, even though it passes the best by date.