<human>:  How do you measure geologic time?
<bot>: Geologic time is based on the idea that the Earth is about 4.5 billion years old. The geologic time scale is a way to organize rocks and fossils into periods, eons, eras, epochs, and ages. The scale is based on the idea that rocks and fossils are different things that were formed at different times. The scale is not absolute, and there is some debate about whether the time periods are the best way to organize things.