Edit: I don’t mean how the consumers know when the product has expired but how those who create the products also create the expiration dates that we see written.
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There have been numerous studies done regarding shelf life of products under different conditions. Producers know when a product will start to spoil when stored under appropriate conditions. There are also [FDA guidelines](https://www.fda.gov/drugs/pharmaceutical-quality-resources/expiration-dates-questions-and-answers) that say how long a product can stay on the shelf before becoming dangerous to consume.
In general a date is chosen based on the amount of time that the product will remain at or close to the quality that it was designed to have. Usually the rule of thumb is 90% of the “freshness” that is present at the time of production. Manufacturers will often tie this to the amount of time they’re willing to stand behind or guarantee their product.
The “expiration” date or “use by” date doesn’t mean that the product has gone bad, and usually the product hasn’t gone bad – it simply means that the manufacturer will no longer stand behind the quality of the product.
As with most things these days, the sales and marketing departments can impact the determination of expiration dates. Sales may go up if people are throwing out perfectly good products because they’re past the expiration or use by date marked on the packaging – some manufacturers will shorten the time before expiration to exploit this. If they make it too short however, they may end up with product that doesn’t sell fast enough, so the store that keeps it in stock may have larger amounts of unsellable products. If this occurs frequently, they may choose to source products from other manufacturers. Consumers may also switch to other products if they find them to “go bad” or “expire” too soon before they can use them.
Medications are generally easier to test, as the natural decay or breakdown of the chemical constituents can often be measured in a laboratory. But this doesn’t mean that this knowledge is used to dictate the expiration marked on the bottle. All of my prescription medications expire 1 year after the prescription date – it is highly unlikely that this is accurate or based on analysis.
For foodstuffs you can usually use your senses of sight, smell, and taste to determine if something is bad. If it smells sour or rancid, it’s likely bacterial growth causing it. For eggs, check and see if they float (fresh eggs sink, ok eggs float halfway up, rotten eggs float at the very top).
Keeping food frozen prevents pretty much any food-borne bacteria from living. Keeping it close to freezing (in a refrigerator) will significantly slow bacterial growth. Letting food sit out can inoculate food with bacteria, as well as allow bacterial to grow at more hospitable temperatures where it may thrive. Cooking food thoroughly will kill most bacteria, but after it comes out of the oven new bacteria can try to colonize. All of these factors should be considered to have a more significant impact on whether it’s safe to eat than a date stamped on the package.
Oxygen is also a concern, which leads to things getting stale, or otherwise losing their initial quality. Oxygen is one of the most reactive natural substances, so keeping containers sealed without air inside offers protection, but as soon as you break that seal can impact how long the quality of the product will last.
Usually when a food product is shipped a few samples are kept in storage as Remains. Over time the remains are analyzed for things like bacterial/fungal growth, preservative concentration, pH, etc. By tracking how these qualities change over time the best by date can be adjusted for future product runs.
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