Geochronologic units are also called ‘geologic time units’ or simply time units. They are time division units for geological periods. From large-scale to small-scale, they can be categorised into eons, eras, periods, epochs, ages and chrons, and the corresponding chronostratigraphic units are eonothems, erathems, systems, series, stages and chronozones, which are internationally used geological time units. Ages and chrons (stages and chronozones) are adopted both internationally and regionally. A geochronologic unit is not a stratigraphic unit but a time duration unit of the respective stratigraphic unit.