The Cosmic Calendar is a method to visualize the chronology of the universe, scaling its currently understood age of 13.8 billion years to a single year in order to help intuit it for pedagogical purposes in science education or popular science. A similar analogy used to visualize the geologic time scale and the history of life on Earth is the Geologic Calendar.

In this visualization, the Big Bang took place at the beginning of January 1 at midnight, and the current moment maps onto the end of December 31 just before midnight.^[1] At this scale, there are 438 years per cosmic second, 1.58 million years per cosmic hour, and 37.8 million years per cosmic day.

The Solar System materialized in Cosmic September. The Phanerozoic corresponds only to the latter half of December, with the Cenozoic only happening on the penultimate day on the Calendar. The Quaternary only applies to the last four hours on the final Cosmic Day, with the Holocene only applying to the final 23 Cosmic Seconds. On the other hand, relic radiation is dated at the first fifteen minutes of the very first Cosmic Day; even if we stretch the Cosmic Calendar to 100 years, the relic radiation would still happen just after the start of the second Cosmic Day.

The concept was popularized by Carl Sagan in his 1977 book The Dragons of Eden and on his 1980 television series Cosmos.^[2] Sagan goes on to extend the comparison in terms of surface area, explaining that if the Cosmic Calendar were scaled to the size of a football field, then "all of human history would occupy an area the size of [his] hand".^[3] The Cosmic Calendar was reused in the 2014 sequel series, Cosmos: A Spacetime Odyssey.^[4]