When I go buy my beer, I will always look for the some sort of date indicating when it expires, or when it was bottled, or something of that sort. Occasionally, I run into undecipherable codes instead of dates and I am at a lost to know when the beer was produced. Is it a company by company basis, or is there some general way to decipher the codes so I know when the beer was produced, or when it will expire, or whatever other information they care to give me?
TLDR answer: Unfornately, no.
From my own experience, this varies a lot from company to company. It also depends on country's legislation or even the state, when it comes to USA. As far as I know, expiring dates for beers are even not obligatory by law in many american states.
Besides that, some breweries inform the bottling date, others the expiration date, which is less useful, because if you are a experienced drinker you know how old (or not) you want your beer to be, despite what the producer has estimated for it. So, in those cases, first you need to figure out the given shelf-life of it, and count backwards, to know when it was bottled. Pretty messy. I age beers and I'm quite used to it.
So, in general, when that information is not clear, your best (and maybe only) friend is the web - the brewery's website or forums.
For example, Anchor Brewery has a very particular (and weird) code standard for bottling date, but it is well explained on their website.
The date code currently being used by Anchor Brewing Company (post-October 1991) replaces the clock-face that used to show the bottling month as one of 12 small notches around the main label. A three character code is now included on the new back label of the bottle. The code works like this:
The first character is always numeric and represents the last digit of the year. The second character is always alpha and represents the month by using the first letter of the month unless that letter has already been used:
January: J, February: F, March: M, April: A, May: Y, June: U, July: L, August: G, September: S, October: O, November: N, December: D
The third character in the code is either alpha or numeric and tells the day of the month. The first 26 days are represented by the alphabet with the remaining days listed as:
27th through 29th = 7 through 9, 30th = 3, 31st = 1
An example of a date code would be: January 20, 2012 = 2JT
Not all breweries are so transparent like that, though. I have had a lot of trouble trying to figure out bottling date of some European beers, specially from some small, old breweries. Some american ones just won't tell anything about it as well, even on bottles.