The level of atmospheric C is not constant due to human activity, in part because of human combustion of fossil fuels and in part because of above-ground testing of the largely defensive weapon of the thermonuclear bomb.
Therefore dates must be calibrated based on Radiometric dating in general, of course, poses a huge problem for people who believe that the universe is 6000-odd years old.
Radiometric Dating Technologies are presented to the public by evolutionists as utterly reliable clocks for dating earth rocks or biological materials.
These include the starting conditions, the constancy of the rate of decay, and that no material has left or entered the sample.
Furthermore, if a sample has been contaminated, scientists will know about it.
The older an organism's remains are, the less beta radiation it emits because its C-14 is steadily dwindling at a predictable rate.
So, if we measure the rate of beta decay in an organic sample, we can calculate how old the sample is. Question: Kieth and Anderson radiocarbon-dated the shell of a living freshwater mussel and obtained an age of over two thousand years.