The "epoch" serves as a reference point from which time is measured.
In a more gradual change, a deciding moment is chosen when the epoch criterion was reached.
An epoch in astronomy is a reference time used for consistency in calculation of positions and orbits.
A common astronomical epoch is J2000, which is noon on January 1, 2000, Terrestrial Time.
An epoch in Geochronology is a time period, typically in the order of tens of millions of years.