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Geologic Time Scale
The following is a geologic time scale developed by Andrew MacCrae of the University of Calgary.  In each column the oldest dates are at the bottom, and the black numbers on the left of each column represents the number of years ago ( in millions of years). I have added the red numbers to the right of the second and third column to show the beginning and ending  times for each of the geological period, also in millions of years ago. 

The left column gives us an idea of the total of geologic time on earth, with the majority of that time being the Precambrian.  The last 550 million of the first column is expanded out (blue area between the left and center column) and represented in the center column.
The center column shows us the time period that life has existed on earth.  The Pennsylvanian geological period lasted from 325 million years ago to about 286 million years ago.
The top 65 million years of the center column has been expanded out (green area) to form the right column.