Finding it
Look for the letters DOT on the sidewall and read the string that follows. The date is the last three or four digits of that string. Everything before it identifies the plant, the size and the tire line in codes that mostly mean something only to the manufacturer.
The date code is required on one sidewall only. If you find DOT with nothing numeric after it, you are on the side that carries the partial code, and the full one is on the other side of the tire. That is the single commonest reason someone concludes a tire is undated.
Reading it
Four digits, used since 2000: the first two are the week of the year and the last two are the year. 2519 is the 25th week of 2019. 0824 is the 8th week of 2024.
Three digits, used before 2000: the first two are the week and the last single digit is the year, with nothing to say which decade. 228 could be the 22nd week of 1998, 1988 or 1978. Whatever it is, a three digit code means the tire predates 2000, which on any road tire is reason enough to have it inspected and replaced whatever the tread depth looks like.
What the date does not tell you
A date code is a manufacturing date, not an expiry date. A tire's serviceable life depends on how it was stored, how it was loaded, whether it was run underinflated, how much heat and ultraviolet it saw, and whether it has been damaged. A five year old tire stored in a dark garage and a five year old tire run underinflated in a hot climate are not in the same condition.
Vehicle manufacturers and tire manufacturers publish their own age guidance, and it varies. This site reads the code and stops there, because the code is a fact and the replacement interval is a judgement about a specific tire in a specific life.