Which unit of geologic time is the oldest

From oldest to youngest, these are the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. Note that in the United States, the Carboniferous is divided into two separate periods: the Mississippian and the Pennsylvanian. The seven periods of the Paleozoic era.

14 days is the half-life of phosphorus-32