When you go to a store today, you can always find someone studying the side of a box of food. They are trying to decide if the food is good for them. They are checking to see how many calories or how much salt is in what they want to buy. There is something else on the box that people should look at. It is the expiration date.
Almost every item in the grocery store is marked with a date. Federal law does not make the food companies do this, but they do anyway. Only infant formula and baby food have to have dates on them.
States have different laws. Most states say that milk and other items that can go bad have to be sold before their expiration date.
There isn't a system that everyone uses to date food. There are common words that are used. One of these is the "sell by" date. This tells the store how long they can keep the food and sell it. Don't buy it after the sell by date.