Compare and contrast relative dating and radioactive dating
And OE Christians (theistic evolutionists) see no problem with this dating whilst still accepting biblical creation, see Radiometric Dating - A Christian Perspective.
This is the crucial point: it is claimed by some that an old earth supports evolutionary theory and by implication removes the need for biblical creation.
Mathematically, P = Po exp(-lambda T), where lambda = the decay constant and T = the period of decay. a zero initial number of D atoms, where P(or D) = the current number of P(or D) atoms, it follows that the age of a rock or mineral is computed as T=(1/lambda)ln(1 D/P).
Similarly, annual lake sediments can be used to estimate relative age and conventional interpretation for the Green River varves suggests they have been formed over some 20 million years.
One problem with earth dating is that the original earth surface is assumed to have eroded long ago.
But assuming the earth was formed at the time of the rest of our solar system, then recovered moon rock and meteorites can also be used to estimate the age of the earth.
The earth precesses (wobbles like a spinning top) around the sun in a series of cycles.
These cycles affect sunlight and hence long-term can form layers in rock.