Sagan's Cosmic Calendar scales the universe's age to one year.
The analogy compresses roughly 13.8 billion years of cosmic history into a single calendar year. The Big Bang occurs at midnight on January 1, while the present moment arrives just before midnight on December 31. This makes immense stretches of time easier to visualize: the Solar System appears in September, while human history occupies only the final moments of the final day.
Sagan popularized the Cosmic Calendar in his 1977 book The Dragons of Eden and made it especially famous through the 1980 television series Cosmos. It is a teaching device rather than a claim that the universe literally follows a calendar. At this scale, one cosmic day represents about 37.8 million years, illustrating why recorded human history is almost invisible beside cosmic time.