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").
The science of paleontology, and its use for relative age dating, was well-established before the science of isotopic age-dating was developed.
On Earth, we have a very powerful method of relative age dating: fossil assemblages.
Paleontologists have examined layered sequences of fossil-bearing rocks all over the world, and noted where in those sequences certain fossils appear and disappear.
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.
With this kind of uncertainty, Felix Gradstein, editor of the For clarity and precision in international communication, the rock record of Earth's history is subdivided into a "chronostratigraphic" scale of standardized global stratigraphic units, such as "Devonian", "Miocene", " ammonite zone", or "polarity Chron C25r".
Conveniently, the vast majority of rocks exposed on the surface of Earth are less than a few hundred million years old, which corresponds to the time when there was abundant multicellular life here.
There are absolute ages and there are relative ages. We use a variety of laboratory techniques to figure out absolute ages of rocks, often having to do with the known rates of decay of radioactive elements into detectable daughter products.
Unfortunately, those methods don't work on all rocks, and they don't work at all if you don't have rocks in the laboratory to age-date. They are descriptions of how one rock or event is older or younger than another.
Here's the next step in that journey: the Geologic Time Scales of Earth and the Moon.
In the science of geology, there are two main ways we use to describe how old a thing is or how long ago an event took place. When you say that I am 38 years old or that the dinosaurs died out 65 million years ago, or that the solar system formed 4.6 billion years ago, those are absolute ages.
There's no absolute age-dating method that works from orbit, and although scientists are working on age-dating instruments small enough to fly on a lander (I'm looking at you, Barbara Cohen), nothing has launched yet. Relative age dating has given us the names we use for the major and minor geologic time periods we use to split up the history of Earth and all the other planets.