The year 2020 gets an extra day. February 29 is leap day, that oddity that strides onto the calendar once (almost) every four years.

Attempting to sync calendars with the length of the natural year baffled people for centuries, with shuffling dates that birthed civil, religious and agricultural chaos. Leap year was invented to bring the calendar in step with the natural world, making farming easier.

“Having a leap day is much more significant to agriculture than you might think,” said Peter Coclanis, agricultural history economist at the University of North Carolina at Chapel Hill and former president of the Agricultural History Society. “Farmers like as much predictability as they can get in an unpredictable world, and having leap years creates more reliable seasons for planting and harvesting.”

The root of the calendar confusion lies in how the earth rotates, said Andrew Novick, an engineer at the National Institute for Standards and Technology’s (NIST) Time and Frequency Division in Boulder, Colo. The number of earth’s revolutions about its own axis — that is, days — is not equal to how long it takes for the earth to get around the sun, said Novick.

The Western, or Gregorian, calendar has 365 days in a normal year. But the solar, or tropical, year is approximately 365.2422 days long.

That fraction, said Coclanis, doesn’t seem like it would make a big difference. But over time, the shift is dramatic.

If leap year didn’t exist and a person lived 80 years, the calendar would be displaced 20 days in their lifetime. Over hundreds or thousands of years, Coclanis said, extra days add up, meaning some of the coldest, darkest days of the year could fall in summer.

According to Novick of NIST, if there had never been a leap day since the Julian calendar was adopted, today’s calendar would say July 2021.

According to Coclanis, in agricultural societies around the world, people have labored to make nature’s schedule fit their own.

“Every developed society has tried to find ways to anchor the calendar,” said Coclanis. “It’s been a lot of trial and error, but people have come up with increasingly calibrated ways to make time work for them.”

Historical records show that some 5,000 years ago, Sumerians created calendars in which they divided the year into 12 months of 30 days each: a 360-day year.

When Egyptians adopted this, they added five festival days at the end of the year, according to the Global Egyptian Museum.

By the time of Julius Caesar’s famed affair with Cleopatra, Rome’s calendar had diverged from the seasons by about three months, according to classical scholars, but Egypt by then had established a leap-year system.

Inspired by the Egyptian example, Caesar adopted the system. To inaugurate this new measure of time, he decreed a transitional year in 46 B.C. known as the “Year of Confusion” with 445 days. From then on, leap day occurred once every four years.

But even this, according to Novick of NIST, was imperfect. Caesar’s system added 0.03124 of a day too much, making his Julian calendar run 600 seconds too fast each year.

And that “spare change” added up. According to Novick, by the time of Pope Gregory XIII in the 16th century, the mismatch was becoming a problem, causing important dates, including religious and other holidays, to drift.

In 1582, after consultation with his astronomer, Christopher Clavius, the pope adopted a newer version of the calendar with time-warp tactics.

Now, according to Novick, using the Gregorian calendar, leap years divisible by 100, like the year 1900, are skipped unless they’re also divisible by 400, like the year 2000, in which case they’re observed. The lost leap days, said Novick, keep the calendar on time.

Not every society uses a solar calendar system, said agricultural historian Coclanis. Some, including Chinese and Islamic societies, have relied on lunar-solar calendars. Even so, he said, they have had to invent their own leap days and other rules to keep the calendar in line with agricultural and seasonal rhythms.

“The bottom line is that syncing the calendar with the seasons was important to farmers throughout history in every major society,” said Coclanis. “And it’s still important today.”

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