The connection between roast date and quality varies — coffee to coffee, roast style to roast style, storage container to storage container. Anybody who gives you a single number is simplifying.
So here is the simplification, because it is a useful one.
Seven to ten days from the roast date is the best window for almost any coffee. Your coffee will probably still be good at fourteen, and possibly at twenty-one, if you have kept it as whole beans and stored it properly.
Why not straight off the roaster
Because coffee needs a few days to degas. Fresh off the roast it is still pushing out CO₂, which gets in the way of the water and makes extraction unpredictable — you will see it as a bloom that erupts and a brew that never quite settles. Give it a week.
The only rule that matters
Buy what you will drink in a reasonable amount of time.
That is the whole thing. Two people getting through twelve ounces in a fortnight should buy twelve ounces, not a five-pound bag because it worked out cheaper per ounce. It did not work out cheaper. It worked out stale.
Coffee is perishable. It does not belong in a bomb shelter on a shelf next to the Green Giant.
Storing it
Whole bean, always, until the morning you are drinking it — ground coffee has vastly more surface area exposed to air and goes flat in days rather than weeks. Airtight, out of the light, at room temperature. Not the fridge, which is damp and full of smells your coffee will helpfully absorb.
And look for a roast date on the bag in the first place. A “best before” twelve months out tells you the roaster either does not know when it was roasted or would rather you did not.