Most types of beer are better fresh, and freshness is particularly important for the hoppy American style IPAs and double IPAs. Sometimes over just a few months the acids that come from the hops start to break down changing the flavor profile of the beer (usually for the worst). When shopping for beers for the recent IPA blind taste test we ran into difficulty finding and interpreting some of the freshness dates. Some brewers use the date the beer was bottled/canned, others use a best by date, some don’t give anything at all. Further confusing things some brewers use a Julian calendar (I guess so only the beer geeks and distributors can figure it out).
Here is a link to an awesome site where someone has compiled a list of the dating system used by each brewery (that uses one). It’s a good reference that might save you from buying some stale beer.