The geologic time scale is the calendar of events in Earth's history. It subdivides all of time into named units of abstract time called – in descending order of duration – eons, eras, periods, epochs and ages.
It is is a representation of time based on the rock record of Earth. It is a system of chronological dating.
It is used primarily by Earth scientists (including geologists, paleontologists, geophysicists, geochemists, and paleoclimatologists) to describe the timing and relationships of events in geologic history.