Using the earth for radiometric dating
Radioactive parent (P) atoms decay to stable daughter (D) atoms e.g.the carbon isotope C-14 decays to nitrogen-14 and the uranium isotope U-235 decays to the lead isotope Pb-207.
One early approach was based upon ocean salinity [John Joly, 1800's].
This assumed the ocean was initially pure water and that it's salinity was derived from continental erosion.
Ice cores from Greenland and Antarctica show annual layers (varves) and can be traced up to about 40,000 years before the layers become too thin due to compaction.
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.
This implies the earth is at least 20 million years old.
Astronomical cycles can also be used to measure relative age.
Absolute dating supplies a numerical date whilst relative dating places events in time-sequence; both are scientifically useful.
This is based upon the spontaneous breakdown or decay of atomic nuclei.
The time required for half the original number of parent atoms to decay is called the half life.