If an impact event was large enough, its effects were global in reach.For example, the Imbrium impact basin on the Moon spread ejecta all over the place.
The chronostratigraphic scale is an agreed convention, whereas its calibration to linear time is a matter for discovery or estimation. We can all agree (to the extent that scientists agree on anything) to the fossil-derived scale, but its correspondence to numbers is a "calibration" process, and we must either make new discoveries to improve that calibration, or estimate as best we can based on the data we have already.
To show you how this calibration changes with time, here's a graphic developed from the previous version of Fossils give us this global chronostratigraphic time scale on Earth.
This all has to do with describing how long ago something happened. There are several ways we figure out relative ages.
The simplest is the law of superposition: if thing A is deposited on top of (or cuts across, or obliterates) thing B, then thing B must have been there already when thing A happened, so thing B is older than thing A.
In fact, I have sitting in front of me on my desk a two-volume work on is not light reading, but I think that every Earth or space scientist should have a copy in his or her library -- and make that the latest edition.
In the time since the previous geologic time scale was published in 2004, most of the boundaries between Earth's various geologic ages have shifted by a million years or so, and one of them (the Carnian-Norian boundary within the late Triassic epoch) has shifted by 12 million years.The Geologic Time Scale is up there with the Periodic Table of Elements as one of those iconic, almost talismanic scientific charts.Long before I understood what any of it meant, I'd daydream in science class, staring at this chart, sounding out the names, wondering what those black-and-white bars meant, wondering what the colors meant, wondering why the divisions were so uneven, knowing it represented some kind of deep, meaningful, systematic organization of scientific knowledge, and hoping I'd have it all figured out one day.Paleontologists have examined layered sequences of fossil-bearing rocks all over the world, and noted where in those sequences certain fossils appear and disappear.When you find the same fossils in rocks far away, you know that the sediments those rocks must have been laid down at the same time.The more fossils you find at a location, the more you can fine-tune the relative age of this layer versus that layer.