Most brewers use code dates on beer packaging. They do this to help manage inventory and to prevent very old beer from reaching the consumer’s hands. The easiest and most consumer friendly code is a date that says sell by on the package or label. The sell by date can be set anywhere from 6 months to one year after the date the beer was packaged. Generally speaking the fresher after packaging you can get a beer the closer it will taste to how the brewer intended. In some cases, the code dating isn’t as straight forward and might consist of some numbers related to the year and date packaged using what’s known as a Julian date. Julian dating is fairly simple each day is numbered. So, January first is 001 and December 31st is 365 (366 in a leap year) using Julian dating. For example a code might look like 021-001. This would tell us the beer was packaged on January 1, 2021.
In an ideal world we would consume every beer within 60 days of when it was put in a package. For the first 60 days of shelf life there should be no perceptible flavor issues with beer that has been properly stored and handled. Between two to four months the changes would be slight. After 4 months, there can be some changes. Generally beers that are malty or malt forward keep better than those that are highly hoppy (like an IPA).
The best way to extend the life of a beer is through proper storage. Modern beers are chill proofed. Placing a beer from your basement into an ice chest, removing it, drying it, putting it back in your basement and then putting it into your refrigerator and drinking it isn’t going to somehow destroy the taste or aroma. The important factor is light. A clear glass or green bottle may have chemically treated hops to avoid turning by sunlight. Brown bottles offer protection but direct bright sunlight or even fluorescent light will cause a chemical reaction with hop oils in the beer. The result will be a light struck product. The resulting aroma is distinctive and is called “skunky.” The beer literally smells like a skunk.
When you open a beer that is exceptionally old but not light struck, the problem will be due to oxidation. Commercial brewers work hard to make sure there is no oxygen in a package when it is sealed. The beer is slightly agitated to foam up pushing air out and CO2 is sometimes used to create a positive pressure during sealing. As time goes on the small amount of oxygen degrades flavor. The beer starts to stale and taste like honey, wax or cardboard.
Some beers and styles are deliberately held for long periods to condition in the cask or bottle. Some people appreciate the longer aging of certain styles which mellow and change over time like wine. But for most beers, the rules above apply. The fresher the beer, the better it will taste. There is a cushion beyond the sell by date and beer can be tasted to see if it is acceptable. Open it give it a smell and a taste. I have personally tasted old beer in a six pack where one bottle was off and the one next to it was fine. Ultimately it is a matter of the consumers taste and preference.
Code dates are a useful way of getting better tasting beer but you’re not going to die if you drink a beer one day beyond the sell by date. Relax and enjoy.