Chronostratigraphy is the branch of stratigraphy that studies the age of rock strata in relation to time.
Stratigraphy, a branch of geology, is basically the study of rock layers and layering (stratification). ... This article is about the geologic use of the term, for other uses see Stratum (disambiguation) Interstate road cut through limestone and shale strata in eastern Tennessee In geology and related fields, a stratum (plural: strata) is a layer of rock or soil with internally consistent characteristics that distinguishes it... A pocket watch. ...

Chronostratigraphic units, with examples:

• eonothem - Phanerozoic
• erathem - Paleozoic
• system - Ordovician
• series - Upper Ordovician
• stage - Ashgill

Chronostratigraphic units refers to the rocks in quarries, drill cores, seismographic charts etc. Rocks of the Upper Jurassic Series were formed during the Late Jurassic Epoch
You can write: Tyrannosaurus rex was found in Upper Cretaceous, but if you write:the T-rex was living in Upper Cretaceous, you mean that the T-rex was living inside the rock !!
A small cinder quarry A dimension stone quarry A quarry is a type of open-pit mine from which rock or minerals are extracted. ...

Chronostratigraphy is such an important branch of stratigraphy, if not the most important, because the age correlations derived are crucial to drawing accurate cross sections of the spatial organization of rocks and to prepare accurate paleogeographic reconstructions.

See also geochronology Geochronology is the science of determining the age of rocks, fossils, and sediments. ...