Confusion stemming from the date printed on the packages of food and when to actually dispose of it is a serious problem in the United States: Reportedly, around 90 percent of Americans throw out food prematurely. Despite this, and the efforts of consumer groups and processors in the 1970s and '80s, the federal government doesn't regulate dates on any food, or drink, outside of infant formula. Twenty states do have some rules with dating for milk products, but they're all slightly different.
So what chance do you have when you're staring at your week-old milk in the fridge? A really good one, in terms of survival: If you drink clearly sour and expired milk, you might get sick, but it's unlikely that you'll die. That date on the carton of milk, however, could mean very different things depending on what state you’re in. Some states require a sell by date, which indicates the last day a store can legally sell the milk; it's calculated to give the home consumer a reasonable amount of time to enjoy. Other states have a use by date that’s for consumers—it indicates the date that the milk is believed to be at peak flavor. So while milk cartons in Montana are labelled with a sell by date 12 days after pasteurization, nearby Washington requires a use by date that’s 21 days after pasteurization. Confused?
While the pasteurization of milk kills most of the harmful bacteria, precautions always need to be made by the consumer to keep the milk from going bad. It's mostly obvious stuff: Don't leave the milk out on the counter for a long time; don't expose it to light that can make it can lose its vitamins; and close the carton when you're done to prevent the absorption of flavors from other foods in your refrigerator. Another way to keep milk as fresh as possible is to always keep it on a shelf—never in the inside of the door of your fridge, where the temperature fluctuates the most. Depending on who you ask, the fridge temperature should be 34-38 °F or 38-40 °F. Warmer temperatures give bacteria more of a chance to develop. You could theoretically freeze your milk for up to three months, but it'll turn lumpy and yellow and yucky looking (though it's still safe to drink).
The accepted rule of thumb is that if you're properly refrigerating it, your carton of whole milk's expiration date is five days after the "sell-by" date. If it's "non-fat," "skim," or "reduced fat," you’ll have a little less time, and no one is quite sure why (although it seems that whole milk tends to go sour while skim milk tends to go bitter). "Ultra pasteurized" milk has a longer shelf life than other types of milk. If you happen to be particularly young, old, or, as Drew Harris, the chair of the New Jersey Public Health Institute put it, "immunocompromised," you should maybe cut a day or two from those estimations and proceed with more caution.