unit) were deposited during the Silurian Period (a geochronologic unit).

The age of a geochronologic unit can be refined and changed by improved dating techniques. However, the equivalent chronostratigraphic unit boundary remains unchanged. For example, in early 2022, the base of the Cambrian Period (a geochronologic unit) was revised from 541 Ma to 538.8 Ma but the rock definition of the boundary (GSSP) at the base of the Cambrian, and thus the boundary between the Ediacaran