Anything over about 50,000 years old, should theoretically have no detectable C.
That is, they take up less than would be expected and so they test older than they really are.
A stronger magnetic field deflects more cosmic rays away from the Earth.
Overall, the energy of the Earth's magnetic field has been decreasing, so more C is being produced now than in the past.
Furthermore, different types of plants discriminate differently.
It does not give dates of millions of years and when corrected properly fits well with the biblical flood.
Obviously, this works only for things which were once living.
It cannot be used to date volcanic rocks, for example.
One rare form has atoms that are 14 times as heavy as hydrogen atoms: carbon-14, or C ratio gets smaller.
So, we have a “clock” which starts ticking the moment something dies.