A tyre's sidewall carries a moulded code that identifies when it was manufactured. That date matters because rubber changes with time as well as with use, and a tyre with plenty of tread can still be old.
Where the code sits and how it reads
The manufacturing date appears at the end of a longer identification sequence on the sidewall, as a four-digit group inside an oval or set apart from the surrounding characters.
The first two digits give the week of the year and the last two give the year itself, so a tyre marked with a code ending in a recent year was moulded in that week.
The full sequence is often moulded on one side only, so a tyre may need to be viewed from the inner face to find it.
Why rubber changes with age
Tyre rubber is a compound of polymers, oils and other additives, and those oils migrate and evaporate slowly over years.
Oxygen and ultraviolet light break down the polymer chains, a process that continues regardless of whether the tyre is rolling.
The result is a compound that hardens and becomes less able to flex, which reduces grip and makes small cracks more likely to form.
What ageing looks like
Fine cracking in the base of the tread grooves and around the sidewall is the visible sign, sometimes described as crazing.
Hardening is harder to see but affects wet grip first, because a wet surface depends on the rubber deforming quickly around the texture of the road.
A tyre can therefore look almost unused and still perform noticeably worse than its tread depth suggests.
Why storage conditions matter
Heat accelerates the chemistry, so tyres stored in a hot space age faster than those kept cool.
Direct sunlight and ozone from electrical equipment both attack the surface, which is why tyres are best stored away from windows and running motors.
A tyre held flat and unloaded ages more evenly than one left standing under the weight of a stationary vehicle for a long period.
Where the date is most worth checking
Spare wheels are the common case, since a spare can sit untouched for the entire life of a car and then be fitted at the worst possible moment.
Trailers, caravans and seasonally used vehicles accumulate very little mileage and can wear out their tyres by time long before tread depth becomes the limit.
It is also worth checking on a newly bought tyre, because stock that has sat in a warehouse for a long period has already spent part of its usable life before being fitted.