Numbers on the time scale represent time in millions of years before present. Numerical dates were added long after the time scale was established using relative dating techniques.
Evidence from radiometric dating indicates that Earth is about 4.54 billion years old. The geology or deep time of Earth’s past has been organized into various units according to events which took place. Different spans of time on the GTS are usually marked by corresponding changes in the composition of strata which indicate major geological or paleontological events, such as mass extinctions.
For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous–Paleogene extinction event, which marked the demise of the non-avian dinosaurs and many other groups of life. Older time spans, which predate the reliable fossil record (before the Proterozoic eon), are defined by their absolute age.