The geologic time scale we use today has absolute ages assigned to it. The answer lies in radioactivity and the natural decay of radioactive isotopes. Nature provides three schemes, all involving the nucleus of the atom, and specifically the neutrons and protons in the nucleus. A proton and an electron transform to a neutron, decreasing atomic number by 1, but leaving atomic mass unchanged. This occurs as a stage in the decay of 238U to 206Pb.
Radioactive decay is probabilistic.
Every atom of an isotope has an equal likelihood of decaying that is not a function of temperature, or pressure, or chemistry or time. This is crucial to the success of the radioactive method. If the clock ticked slower or faster in the past, we would get badly biased estimates of little worth. When an atom decays, it produces a new isotope. We refer to the product as the daughter and the original isotope as the parent .