In short - you could literally close your eyes, spin around, and point in a grocery store and find a product that lasts longer than its expiration date suggests. Why?
In general, the FDA does NOT require food manufacturers to put expiration/use by/best by dates on food. While food manufacturers cannot print misleading or false information on their packaging (something that IS regulated by the FDA and is a prohibited practice), the dates that are placed there are not, for the most part, based on an exact science.
Manufacturers use these labels to ensure that the end consumer is receiving the product that meets the manufacturers desired quality and flavor, rather than a guidance on when the food will be adequate for consumption. Consider the alternative: the manufacturer produces a product without a quality/expiration date, the consumer opens the product and eats it, and finds that the product does not meet their expectation for quality or flavor; while the consumer experience could vary depending on many factors based on the food composition, water activity (the amount of moisture in the food to support growth of spoilage microorganisms and bacteria), the age of the food, the packaging type, etc, etc, etc. Not printing a expiration date would be a bad business decision on the part of the manufacturer because it would be unlikely that the consumer would purchase the product again if they had a poor experience that could have been mitigated with an expiration date for the consumer to follow.
How long can that grocery item actually stay good for?
Most foods do not have an exact science based approach to the determination of an expiration date that is required by the FDA. The exception to this is infant formula; infant formula is REQUIRED by the FDA to have a use by date as the manufacturer has to guarantee that the product has at least the minimum nutrient amount stated on the label and will be acceptable quality - this is something that is NOT REQUIRED of other food manufacturers.
There are many factors that could make food "go bad", having to do with the composition and properties of the food (all based around supporting other life that may exist on the food like spoilage microorganisms and bacteria - water activity, a nutrition source, competing microorganisms, acidity, oxygen environment), the packaging of the product (clear, opaque, vacuum packaging, ability for oxygen to diffuse), and storage of the product (in sunlight, at varying temperatures, varying humidity, refrigeration, non-refrigeration). Many manufacturers try to control for these variables and make a determination on the best storage and packaging for the product to ensure the product reaches the consumer in the way that it was intended.
Control studies or stability tests can be conducted by the manufacturer to test for quality and flavor, in both lab settings and real-world retail settings as well as microbial challenge studies (testing the microbial growth in a product after a period of time and comparing it to a threshold value set by the company which is usually based on previously established challenge studies) to help make the determination of the expiration date. Additionally, predictive modeling can be used to determine the expiration dates - this is also based on established challenge studies. In summary, the dates are based on a scientific approach, but it's not an exact science. It can be reliable for quality and flavor, but the process of determination is still subjective.
Aside from printed expiration dates, what signs should grocery buyers look for to indicate that an item has "gone bad"?
The industry is moving forward with the FDA to develop more uniform practices when it comes to date labeling. Right now, it can be confusing for consumers because food manufacturers are using all kinds of different labeling interchangeably (expiration, best by, use by, sell by) as there is no standard that has been set yet for any of those labels. The FDA and industry are working towards the term, "Best if used by" if it related to optimum quality and not food safety. More information can be found here:
https://www.fda.gov/media/125114/download. This is all part of a White House initiative called, "Winning on Reducing Food Waste", and is a collaboration between the FDA, EPA, and the USDA to help inform consumers about reducing food waste while also ensuring the food they are eating is free from spoilage or potential to cause illness.
There are some food producers that are already using that terminology well, but it may take some time for alignment. In general, with the exception of infant formula, all foods have this rather subjective expiration date placed on it for quality, rather than food safety - so literally thousands of products; but rather than singling out any specific product or food family expiration date to ignore (and also accepting any liability with that) that could have had many MANY things happen to it prior to reaching the consumer and also in the consumers possession (I drank straight from the milk jug this morning in fact, and probably introduced bacteria from my mouth that will love the sugars in the milk and the water rich environment to grow and reproduce, even under refrigeration- sorry, I'm gross, but it will reinforce my point), this is where the advice for the consumer comes in to use their sense of sight to look for color or texture cues and smell comes into play. Using the combination of sight and smell with the expiration date will help to guide them in the right direction when it comes to making these decisions. As there is no food that is untouchable by bacteria, especially when a seal has been compromised (opened), or a can has been dented, these expiration dates can help us make good decisions.
So no, there is no food expiration date that should be ignored, rather, the dates printed are based on quality concerns, not food safety, and customers should use their sight and smell along with the expiration date to make the best judgement for themselves, their families, or their customers if purchasing/using the food items for a business.
What about perishable products, like milk and eggs?:
Milk will develop a lumpy texture and sour smell when it has gone bad; if stored correctly (under 41°F), it can last up to a week after the expiration date - but there are other factors that play into this: if it has been stored correctly during transport, sale, and by the consumer (under 41°F or below), what type of processing method was used to make it, the fat content, exposure to light an heat; As some of these factors are unknown prior to reaching the consumer, it is best for the consumer to use the expiration date as well as their senses to determine if they can continue using the product past the expiration date.
Eggs: most eggs are processed with mineral oil since the shells are porous. This helps to seal the shell and helps to keep out bacteria. If kept refrigerated at 41°F or below, eggs may last up to three weeks past their expiration date. However, if they have been washed or cooked (removing the mineral oil) they will only last about a week. Eggs are usually packed 1-2 days after the hen lays them. You can tell what date the eggs were packed on by looking at the USDA grade shield - it will have a 3 digit number representing the consecutive day of the year with January 1 having a code of 001 and December 31 with a code of 365. The USDA notes that sell by dates cannot exceed 45 days past the pack date (this is still just for quality, not food safety), and suggests to crack the egg into a bowl and examine it for an off-odor or unusual appearance before deciding to use or discard it. A spoiled egg will have an unpleasant odor when you break open the shell, either when raw or cooked.
The main theme: It's all about context on how long foods will last, what happened to them before it got to you and while you were storing it. For that reason, using the expiration date along with sight and smell will be the best method for determining acceptability (and may also help to reduce food waste too!).
Until then, consumers can make a determination about the food using the date printed to help guide them; consumers should take into account any foods that have changed noticeably in color, consistency, and texture - they may want to avoid eating these foods. Additionally, consumers can reach out directly to the manufacturer if they have concerns about the food. Finally, the USDA and Cornell University created an app that consumers can use: FOODKEEPER app. it is a great tool for consumers to use to guide them on storing foods properly and understanding expiration dates.