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.
Major boundaries in Earth's time scale happen when there were major extinction events that wiped certain kinds of fossils out of the fossil record.
This is called the chronostratigraphic time scale -- that is, the division of time (the "chrono-" part) according to the relative position in the rock record (that's "stratigraphy").
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.
We have no idea how much older thing B is, we just know that it's older.
Nowadays, age-dating of rocks has established pretty precise numbers for the absolute ages of the boundaries between fossil assemblages, but there's still uncertainty in those numbers, even for Earth.
When you talk about the Precambrian, Paleozoic, Mesozoic, and Cenozoic on Earth, or the Noachian, Hesperian, and Amazonian for Mars, these are all relative ages.
Relative-age time periods are what make up the Geologic Time Scale.
A few days ago, I wrote a post about the basins of the Moon -- a result of a trip down a rabbit hole of book research.
Here's the next step in that journey: the Geologic Time Scales of Earth and the Moon.