So, we have a “clock” which starts ticking the moment something dies.Obviously, this works only for things which were once living.Familiar to us as the black substance in charred wood, as diamonds, and the graphite in “lead” pencils, carbon comes in several forms, or isotopes.
Raw (i.e., uncalibrated) radiocarbon ages are usually reported in radiocarbon years "Before Present" (BP), with "present" defined as CE 1950.
Such raw ages can be calibrated to give calendar dates.
The most common form of carbon in the atmosphere is carbon-12, which is a stable isotope of carbon.
An unstable form of carbon isotope, carbon-14, also known as radiocarbon, is formed when cosmic rays hit atoms in the upper atmosphere of the earth.
After plants die or are consumed by other organisms, the incorporation of all carbon isotopes, including 14C, stops.