Jeez, I should get one of your stars for this. Ok here we go -
I have a bottle of balsamic vinegar sitting in one of my kitchen cupboards right now. The sell-by date is in the late 1/2 of 2005. That doesn't mean that the bottle in my cupboard has yet to be made, I know this because it's, well, it's in my cupboard and I use it from time to time.
The sell-by date is the date reccomended by the manufacturers of the product (or maybe some outside health and safety organisation) that retailers have to sell it by, otherwise they have to remove it from the shelves or mark it clearly that it is out of date (code).
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GOVT. Warning: Do not smoke around children they will badger you for fags.