In a related article on geologic ages (Ages), we presented a chart with the various geologic eras and their ages.
In a separate article (Radiometric dating), we sketched in some technical detail how these dates are calculated using radiometric dating techniques.
Another method is to make age measurements on several samples from the same rock unit.
This technique helps identify post-formation geologic disturbances because different minerals respond differently to heating and chemical changes.
Radioactive decay rates have been measured for over sixty years now for many of the decay clocks without any observed changes.