eon

Deep time is written in this unit under two aliases: astronomers write Gyr, geologists write Ga, giga-annum, and both are spoken "billion years". Stellar lifetimes, galaxy ages and the radiometric dates of rocks all land between a tenth of an eon and 13.8 of them, so the unit's entire working range spans barely two orders of magnitude, which is rare for a unit and part of why the round billion sits so comfortably.

The clocks that measure it are half-lives, and the two great ones bracket the unit nicely. Uranium-238's half-life is 4.47 eons, almost exactly the age of the Earth, so about half the planet's primordial U-238 is still here; potassium-40, at 1.25 eons, has been through three and a half halvings since the planet formed and still dates nearly every volcanic ash bed geology cares about. The half-life solvers on this site run on the same constants at any scale, from these down to seconds.

For orientation inside an eon: the sun circles the galaxy about once per quarter eon, a galactic year of roughly 230 million years, so the solar system is about 20 galactic orbits old, and the whole of animal life since the Cambrian has happened within the last two and a bit of those orbits.