The strength of the Earth's magnetic field affects the amount of cosmic rays entering the atmosphere.
A stronger magnetic field deflects more cosmic rays away from the Earth.
This is the “half-life.” So, in two half-lives, or 11,460 years, only one-quarter of that in living organisms at present, then it has a theoretical age of 11,460 years.
Anything over about 50,000 years old, should theoretically have no detectable C.
It cannot be used to date volcanic rocks, for example.
The rate of decay of N in 5,730 years (plus or minus 40 years).
This also has to be corrected for. Second, the ratio of C in the atmosphere at that time to be estimated, and so partial calibration of the “clock” is possible.