Tree rings in similar ecosystems will follow similar patterns. There's more to it than this, but imagine if each ring is thick or thin, indicating a rainy year

Tree rings in similar ecosystems will follow similar patterns. There's more to it than this, but imagine if each ring is thick or thin, indicating a rainy year or a dry year. Now, imagine you cut down a tree. You see that it has 122 rings, meaning it was born 122 years ago, or 1900. Then you go look in the same forest and you find a dead tree, you don't know when it was born or when it died, but you see that the 20 outermost rings thick-thick-thin-thick-thin-thin-etc ring pattern matches the innermost pattern on the tree you know the date range of. So that means the last 20 years of the unknown tree's life overlapped with the first 20 years of the known tree's life. So that means your unknown tree died in 1920. You count the unknown tree things and see there were 140 rings, so that means that tree was born 140 years before 1920 - in 1780. Then, you find another tree that has overlaps with this now known tree, and you keep daisy chaining your information backwards. With this method, you can go back thousands of years, if you have enough preserved trees or wood available to you. Then, you can take any random piece of tree, and if enough rings are present, you can tell exactly when that tree was growing. If you see that every tree your building is made out of stopped growing in the year 1300, then you can be fairly confident that the building was built around or slightly later than 1300(since most people aren't going to let good lumber just sit around for centuries).