Carbon-14 is not a constant in the atmosphere (as required by the mathematical formulas used in dating samples) due to the well known fact that the earth’s magnetic field is decaying or weakening. The magnetic field protects life from harmful radiation of the sun. The weaker the protection of the magnetic field gets—the more Carbon-14 that is created in the atmosphere. The stronger the field around the earth, the fewer number of cosmic rays are able to reach the atmosphere. Therefore, in the past we can presume there was much less carbon-14 in the atmosphere which would give fossils a false older dates calculation.

https://www.geo.arizona.edu/Antevs/ecol438/radiocarbon.html