Using the known rate of change in radio-active elements (radiometric dating), some Earth rocks have been shown to be billions of years old, while the oldest solar system rocks are dated at 4.6 billion years.
Astronomers use the distance to galaxies and the speed of light to calculate that the light has been traveling for billions of years.
This method can even be used on dead trees that fell in a forest long ago.
For example, the last 200 rings in the dead tree might match up with 200 rings early in the life of the living tree, so the two trees together can count back many years.
Some isotopes are unstable, and over time these isotopes “decay” into isotopes of other elements.
For example, Potassium-40 is unstable and decays into Argon-40.
Some isotopes have short half-lives of minutes or years, but Potassium-40 has a half-life of 1.3 billion years.
In this way, multiple trees can be used to build a master chronology for a forested region.
European oak trees have been used to build a 12,000-year chronology.
Once the rock hardens, however, all the Argon-40 is trapped in the sample, giving us an accurate record of how much Potassium-40 has decayed since that time.
So, if we find a rock with equal parts Potassium-40 and Argon-40, we know that half the Potassium-40 has decayed into Argon-40, and that the rock hardened 1.3 billion years ago.