Good question! Different brewers have different codes. As simple as Budweiser's "born on date", and as complex as Asahi's letter A-M for month, next two digits are the day, final digit is the year. B139 would translate to February 13, 2009.
Weihenstephaner's code is four digits, first one is day of month, second two are week of year, final is year. 7218 would be the 27th of May, 2008. Some brewers put a "best before date that you can actually read. Most have a secret code different from each other, so that only insiders can understand it. Stale beers, especially the wheat beers, Weizen, Weisbeers, Hefeweizen.. go bad without refrigeration in 8 weeks or so. If kept under 45F they will keep 8-9 months. They taste sour and bitter after time.
If you have ever had a Heinikens that tasted "skunky" or like old tires, it is too old. Same goes for Corona, even though fresh it is a little skunky.
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The dates on the beer bottles does not point towards the expiration dates unlike other edible products. It can mostly mean the date that it was instructed to be taken off from the shelf.