From the Wikipedia article on leap years, it appears that calendar systems that predated the Romans accounted for leap days, but do we know which calendar was the first to introduce a leap day, and if so, which year in human history was truly the first leap year?

  • 1
    If the first leap year existed in another calendar system (possibly one with a different number of days and a different start to the year), how would you map that to the current calendar in order to determine which year in human history was truly the first leap year?
    – Steve Bird
    Feb 26, 2019 at 21:14
  • 2
    The Ancient Sumerians had leap years (as per your title), but they added an extra month when needed, rather than a day, to keep the lunar year and solar year in step. Feb 26, 2019 at 22:07

1 Answer 1


Julian calendar was introduced in 45 BC. Leap years are those whose number is divisible by 4. However, there is a catch here. Of course Julius did not count the (negative) years BC (as we do).

(There is a joke: "Archeologists found a coin with the date inscribed: 45 BC":-)

To preserve the pattern of divisibility by 4, one needs to count from zero. But historians do not like zero, so in our common count year 1 is preceded by year 1 BC. (Which is more reasonable to call 0). So what is commonly known as 45 BC is really year -44. Which is divisible by 4. So this year was the first leap year.

By the common count (used by historians) it is 45 BC.

Source: Wikipedia.

  • 1
    According to the Wikipedia article the Julian Calendar started with a leap year every three years. There seems to be some disagreement about exactly when these were applied.
    – Steve Bird
    Feb 26, 2019 at 22:10
  • leap days were used before Ceasar although they were not regular. Every year the Pontifex Maximus decided whether to have leap days and how many, usually for political reasons since most public offices would start from March 1st. Cesar decided to end this mess.
    – Rad80
    Feb 28, 2019 at 14:25

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