This last December was the 10th anniversary of Carl Sagan's death. One of his most popular episodes of Cosmos was titled The Dragons of Eden where he first described his Cosmic Calendar. This website from discovery.com has a simple image showing the Cosmic Calendar as Carl described it. A few websites are selling posters of the Cosmic Calendar, like AllPosters.com.

The premise is that if you compress the entire history of the universe into a calendar year, homo sapiens only exist in the last 6 minutes, and the last second represents the last 400+ years of human history.

You can see Cosmos, and hear Carl describe it on YouTube here: