<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. There are different periods of geologic time that we use to describe the history of the Earth. For example, the Paleozoic era is the period of time between the Cambrian explosion and the Permian extinction, which is between approximately 541 million years ago and 250 million years ago.