Each cell is a time capsule of sorts.
Neurobiologist Kirsty L. Spalding of the Karolinsky Institute in Sweden commenting on the way the C14 carbon isotope left its timestamp on human DNA cells as the result of nuclear bomb testing, in this morning's New York Times.
By way of explanation the piece goes on:
The method for dating human cells takes advantage of an effect caused by above-ground nuclear bomb testing that took place from 1955 to 1963.
When the bombs were tested, their radioactivity created a spike in the amount of a carbon isotope, C14, in the atmosphere. The C14 made its way into plants and animals that ate the plants.
When people ate those plants and meat from the animals, the C14 was incorporated into their human DNA. After the nuclear test ban, C14 levels started to drop. The result is that every cell has a C14 level that reflects the level in the atmosphere at the time the cell was born.