Two bags of the same coffee tasted completely different — how do I read the codes on the packaging?

The reason age hits ground coffee so hard is worth understanding, because it changes what you buy.

Coffee goes stale through oxidation and through the loss of volatile aromatic compounds. Grinding increases the surface area enormously — a huge multiple of whole beans — so a ground coffee stales in weeks where whole beans take months.

What you taste as it declines is fairly specific: the aromatics go first, so it smells of less; then the acidity flattens; and eventually you get a dull, cardboard-like quality. It is not usually sour or rancid, it is absent, which is why people describe stale coffee as tasting of nothing much.

A three-month difference in shelf time for pre-ground coffee is easily enough to produce the gap you are describing.

Also worth checking: whether the bags have a one-way valve, and whether either had been opened or was stored somewhere warm. A bag left in sunlight in a shop window ages far faster than one in a stockroom.

The practical conclusions, in order of how much difference they make:

Buy whole beans and grind before brewing. This is by far the largest single improvement available to anyone buying supermarket coffee, and it costs the price of a hand grinder.

Look for a roast date rather than a best-before date. Roasters who care print the roast date. A best-before date eighteen months out tells you the producer is not thinking about freshness, and that is itself information.

Buy smaller quantities more often. A large bag that lasts two months is stale for the second month.

Store it sealed, cool, dark and dry — and not in the fridge, where it picks up moisture and odours. The freezer works for long-term storage of unopened bags, and repeatedly taking a bag in and out is worse than a cupboard.

Almost certainly age, and probably not for the reason you are assuming.

Those codes are usually a production stamp: a letter or digit for the year, something for the month, something for the day, and often a factory or line identifier. They are not standardised across companies and are not really meant for consumers, which is why they need deciphering at all. Comparing the codes on two bags from the same producer will usually show you which fields change together over time, and that is enough to work out the date positions without any documentation.

But here is the thing about your case: the two bags may well have been roasted within a day of each other. Large producers roast continuously, and a shop restocks on its own schedule. Two bags bought three months apart can come from nearly identical production and have spent very different amounts of time on a shelf.

So the variable is usually not roast date. It is how long the bag sat, and under what conditions.

On returning the bags: worth mentioning to the shop rather than assuming they will not care.

A good retailer wants to know that stock is sitting too long, because it means their ordering is wrong for that line. I have had a shop rotate their coffee shelf and start ordering smaller quantities after exactly that conversation.

Bring both bags and the codes. It is much more persuasive than a description of a taste.