I assumed that there is a regulatory process involved with expiration dates—hard and fast rules. This is not the case. According to the USDA, expiration dates on food aren't required by the federal goernment. Although some states do require it, and there are dating regulations when it comes to infant formula.
“Sell-by” “Best if used by” and “use-by” have similar, but slightly different meanings. None of these are meant to be safety dates for the food (other than infant formula), but rather describe quality.
A “Sell-by” date is geared more toward the retailer, indicating to them when they should rotate product off the shelves.
A “Best if used by” date is an indicator of quality (the food will not be “bad” after that date).
A “Use-by”date is the last day the manufacturer recommends using the product based on quality, not safety.
That depends on what it is and how it was handled. The USDA recommends using products that display the “use-by date” by that time. For sell-by dates that go past at home, you can continue to store the food for a short amount of time depending on what it is. Some common products are: ground meat and poultry (1-2 days past the date), beef (3-5 days past the date), eggs (3-5 weeks past the date).
That depends on the food. Take orange juice, for example. One cup of OJ can offer a full day’s dose of vitamin C. But after it’s been open for a week, it loses the antioxidant benefits from exposure to air and light. (And that could happen even before it reaches its expiration date.)
Some foods do lose nutrients when exposed to oxygen in the air, and your food may lose a little more nutrition when it's cooked. Aim to eat your fresh fruits and vegetables soon after purchasing them, but don't sweat nutrient losses too much.
Expiration dates refer to quality, not safety. For example, if a refrigerated product was kept below 40 degrees and was well packaged and handled, it may look and smell a little funny, but would not necessarily be considered unsafe. If it was left out on a warm counter for hours or contaminated by something else, it may harbor harmful bacteria that could cause food-borne illness. (Perishable foods should never be thawed on the counter for longer than 2 hours because, while the center of the food may remain frozen, the outer surface may enter the Danger Zone, the range of temperatures between 40° and 140°F, in which bacteria multiply rapidly.) For this same reason, you should never eat meat, poultry, eggs or sliced fresh fruits and vegetables that have been left out for more than 2 hours (1 hour in temperatures hotter than 90°F). But this could happen to any food and is not related to expiration dates.