The date printed on a package of food in the United States is usually a manufacturer's judgment about quality, not a regulatory statement about safety. The distinction explains why the wording varies so much from one product to another.
The labels mean different things
Best if used by refers to flavor and texture at their peak. Sell by is an instruction to the retailer about stock rotation and is not addressed to the shopper at all.
Use by is the closest to a safety statement and appears on more perishable items, but even that phrasing is not defined uniformly across categories.
Infant formula is the exception. Its date is federally regulated because nutrient content declines over time and the product may be a baby's only source of nutrition.
Manufacturers set the date through shelf-life testing
A company stores samples under expected conditions and evaluates them at intervals for taste, appearance, texture and microbial growth.
Accelerated testing at elevated temperature is often used to estimate long shelf lives quickly, with results mapped back to normal storage conditions.
The date chosen is typically well inside the point where quality actually falls off, because a company would rather a shopper discard a good product than eat a disappointing one.
Spoilage and pathogens are separate processes
Spoilage organisms change how food smells, looks and tastes. They are the reason a product becomes unappetizing and they are largely harmless.
Pathogens generally do not announce themselves. Contaminated food can look and smell entirely normal, which is why a date cannot certify safety.
Handling matters far more than the printed date. A product held at the wrong temperature can become unsafe long before its date, and one kept properly can remain fine after it.
Storage conditions move the real limit
Dates assume a specific storage regime, usually unopened and at a stated temperature. Opening a package restarts the clock on entirely different terms.
Refrigerator temperature varies by shelf and by how often the door is opened, so two identical items in the same appliance can age at different rates.
Freezing largely stops microbial activity while quality still degrades slowly through moisture loss and fat oxidation, which is why frozen food stays safe but does not stay good indefinitely.
The confusion has a cost
A substantial share of household food waste involves items discarded because of a date rather than because anything was wrong with them.
Industry groups have pushed toward standardizing on two phrases, one for quality and one for safety, which would make the distinction visible on the package.
Until wording is consistent, the practical approach is to treat most dates as quality guidance, judge shelf-stable and frozen items on condition, and be conservative with high-risk perishables.