The number of days that have passed since a coffee was roasted. Specialty bags often print a roast date instead of an expiry date, and a coffee at "day 14 off-roast" was roasted exactly two weeks ago.
Off-roast is the single most predictive number for how a coffee will brew that day. The bean is degassing the entire time after roast, and the cup it produces shifts measurably as the days tick over.
The peak window
Most filter coffees taste best between days 7 and 21 off-roast. Earlier than that, gas-driven channeling makes brews unpredictable. Later than that, aromatic intensity has started to fade.
Light roasts often peak slightly later than dark roasts because their cell structure traps gas longer. Some Nordic-style ultra-light roasts only really open up around days 14–28.
Why the date is the gold standard
A "best by" date eight months out tells you nothing about the cup. A roast date tells you everything that matters: which day of the curve you're on. Specialty roasters print it because the difference between day 5 and day 21 is bigger than the difference between two different farms.
Buy bags with a visible roast date, drink them in the 7–21 day window, and you've already eliminated one of the biggest variables in home brewing. If a bag has no roast date, treat it as old until proven otherwise.