How a food expiration date is determined varies depending on the food. Many foods, especially packaged foods, have a "sell-by" date rather than an expiration date. Sell-by dates are often determined by the amount of time a food can be stored at a given temperature without losing its nutritional value or developing harmful bacteria. Each food will have its own specific levels of temperature over a given amount of time, so sell-by dates will be different for all foods.

In the US, the Food and Drug Administration (FDA) oversees the regulation of packaged food and drugs, while the Department of Agriculture (USDA) regulates the production and packaging of fresh meats and produce. The FDA requires an expiration date only on infant formulas; the USDA requires a label for the packaging date of fresh poultry. All other food and drugs can be labeled with terms such as "best if used by," as opposed to having a clear expiration date. The FDA notes that this is because the majority of foods and drugs may spoil if they are kept to long, but they do not present a serious health danger.

Foods that are labeled with a "sell-by" date require that a store sell the product by that date, so in many cases, these labels are the closest thing many products have to an expiration. Milk also has an expiration date, as does meat, meaning they should not be sold after the sell-by date. The FDA notes that labels such as "best if used by" refer more to the quality or flavor of the food, while "use by" dates are more like expiration dates after which consumers should not consume a food or medication.

Baby formulas will usually have an expiration date or sell-by date because the nutrients in the formula can decrease over time. In the US, the FDA requires specific nutritional levels in the formula. If the formula is stored past its expiration date, it can also decrease in quality and develop lumps that clog the nipples of a baby bottle.

How a food is packaged can affect its expiration date. Canned foods can be stored for up to a year as long as they are stored at a temperature below 75° F (24° C). Dry foods, such as cereals pastas and cake mixes, require a dry storage area, and can be stored up to six months prior to use, but the may spoil faster if they are exposed to moisture. Experts note that many soft cheeses must be used within three to four days of opening because they can grow harmful bacteria.