Geologic time is the timescale of the history of the Earth, divided into periods of time millions of years long.
Geologists estimate that the Earth is at least 4.5 billion years old. Such an expanse of time is hard to grasp, but it is fundamental to understanding basic geologic processes, such as weathering and erosion, that have shaped the Earth over millions of years.
One way to think about geologic time is to condense the 4.5 billion years of Earth history into a single calendar year of 12 months. Using this scale, the oldest rocks discovered at the Earth's surface would date from mid-March. Very primitive life forms first appeared in the Precambrian seas in late November. Dinosaurs became dominant in mid-December, but disappeared the day after Christmas, at about the same time that the Rocky Mountains were uplifted. Creatures resembling humans appeared sometime during the evening of December 31st. The last of the continental ice sheets began to recede from the Great lakes area about one minute and 15 seconds before midnight, and Columbus discovered America three seconds before midnight.
In order to talk about the 4.5 billion years of earth history, geologists break it up into several eras – the Paleozoic, Mesozoic, and Cenozoic Eras. The eras are then subdivided into periods. Each of these spans a different length of time. For example, the Paleozoic Era, which runs from 545 million to 248 million years ago, is much longer than the Cenozoic Era, which covers the time from 65 million years ago to the present. And the 26 million years of the Silurian Period (443 to 417 million years ago) is much shorter than Devonian Period, which covers approximately 63 million years (417 to 354 million years ago).
So far, we've been talking about time units. When we talk about rocks deposited during a general time period – for example, the Permian Period, we call them Permian rocks. But a single time period can include many different types and layers of rocks. The Pennsylvanian Period, for example, covers the time from 320 to 290 million years ago, and a rock deposited early in that period may be entirely different from a rock deposited later on.