Our very own J.R. Shirt has been running an experiment using Stone's Enjoy By 4/20 in which he drinks one a week past it's expiration date to see how it holds up. The men on the ground drinking actual beer are a great inspiration to those of us throwing poorly formatted SQL queries at a database trying to find truth about something as subjective as 'How good is this beer?' Full disclosure: I'm drinking an expired Lunch by Maine Beer Company while writing this. It's still good even one month past it's best-by date; resiny and plenty of aroma, although it doesn't quite live up to the fresh one I had on tap three months ago.
I also used Stone's Enjoy By 4/20 for my experiment, using a 3-day grouping of check-ins and tracking it over time. So the data for 4/20 encompassses 4/19, 4/20, and 4/21 all rolled up into one number. This isn't going to be perfect, because Untappd saves your previous rating so if you check in to Enjoy By a second time after rating it well you might not change the rating even if it doesn't taste quite as fresh.
The chart above is from 3/17 to 4/30. Before and after the check-ins drop off precipitously and I ignored those for the time being. Overall you can see that it's a slight decline and seems to really drop off after the 4/20 highlighted best-by date. It's not quite a steady decline, but people checking into the staler beer definitely rate it lower. Now let's add in the ratings around the fringes.
The first thing that jumps out to me is those low values for the first couple of dates, but the first day, 3/14, had only 2 check-ins and 3/15 only had 24 so it's likely just a statistical quirk. The current overall rating for this beer is 4.33 and it's clear from these graphs that it started out higher than that. After the best-by date the ratings get very volatile and all over the place, but as a whole they're lower as the beer gets older.
This reflects that the beer itself is of lower quality the longer it ages, which is what Stone was trying to tell us in the first place. The check-in volume does reveal that as a whole we're mostly listening to that instruction. The check-in volume peaks at 309 on March 30th, a good three weeks before the expiration date. From there it drops off, dipping below 100 on April 14th. Beer drinkers are clearly procrastinators and the check-ins spike again on 4/20 when people open the fridge and realize "crap, I'm supposed to drink this by today", and then do so. The 182 and 185 on April 20th and 21st are the highest marks since April 4th. It drops down again after that, with only about 6% of total check-ins occurring after the 21st of April.
The numbers here basically confirm what's obvious. Not only is the Enjoy By date on the bottle a valid suggestion of quality while it's fresh, most consumers are listening and getting the best out of that beer.