The sell-by date really has nothing to do with whether any particular item is still edible. Even with perishables such as meat, the sell-by date doesn't determine when the product goes from good to bad. The sell-by date is, generally speaking, a date chosen to ensure that all or substantially all of the marketed product is fresh, which means that it is usually significantly before the date by which the average product would start to go bad. It just makes common sense to go by how the food appears, not by some relatively arbitrary date.