What Is a Leap Year?

Without this regular calendar adjustment, you might see snow in summer (eventually).

One of the first facts you memorize in school is that a year has 365 days. But what about when it doesn’t? Nearly every four years, a bonus day is quietly tacked onto the end of February. Usually a 28-day dash, February in a leap year offers 24 extra hours to tie up loose ends, celebrate, or fit in some extra work time. Since it’s so unusual, there are many quirky traditions associated with Leap Day. Though a random extra day every four years might seem silly, there’s a scientific reason for leap years.

Why do we need leap years?

There is a purpose behind this year’s 366-day calendar. February 29, also known as Leap Day, helps keep our calendars aligned with the Earth’s rotation around the sun.

Mike Lombardi, who leads the Time and Frequency Services Group at the National Institute of Standards and Technology (NIST) and is a timekeeping expert explains, “A year is about 365.25 days long, so adding one extra day every four years is necessary to keep the calendars accurate.”

Nixing Leap Days from calendars would be problematic, he says, adding that “the date would soon be incorrect and would fall further and further behind.”

It might seem like a quarter of a day—only six hours!—doesn’t make much difference. But after ten years without a Leap Day, your calendar would be off by 2.5 days. Add a few more decades and the weather might feel more like one of these Octoberfest celebrations in December.

When were leap years added to the calendar?

Before the modern Gregorian calendar, people used the 12-month, 365-day Julian calendar, which History.com reports was created by Roman emperor Julius Caesar in 46 B.C. The emperor even included a leap year every four years. But Caesar’s math wasn’t quite right, so a tiny sliver of time—11 extra minutes per year—gradually shifted the calendar off course.

In 1582, Pope Gregory XIII’s astronomers realized the time discrepancy and implemented the leap year system that we still use today. “To bring the date up to date, (the Pope) had to remove ten days from the calendar, so October 4, 1582, was followed by October 15th,” says Lombardi.

Our modern Gregorian calendar looks a lot like the Julian calendar, with one important difference: To keep the dates as exact as possible, timekeepers skip Leap Day in centuries that are not divisible by 400. For instance, if you were alive in 2000, you experienced Leap Day in 2000, but it was skipped in the years 1700, 1800, and 1900.

So is the calendar is correct now?

Not quite. Since it takes 365 days, 5 hours, 48 minutes and 45 seconds for the Earth to travel around the sun, there is still a small margin of error. Timekeepers such as Lombardi spend their workdays making sure the world’s clocks, which are set according to Coordinated Universal Time (UTC), stay as exact as possible.

That means that once in a while, timekeeping experts add a leap second to our clocks. “By international agreement, leap seconds are always scheduled on either the last day of June or the last day of December, but the interval between them has been as short as six months and as long as seven years,” Lombardi says. “The first leap second occurred in 1972 and there have been 27 so far, with the last one occurring at the end of 2016.”

Unlike leap years, leap seconds are not scheduled in advance. They are added to UTC as needed, and since they’re here and gone so quickly, leap seconds don’t affect anyone’s daily routine.

Birthdays for Leap Day babies

For most of us, leap days and leap seconds are fun bonuses. But if you were lucky enough to be born on February 29, Leap Day is also an unusual opportunity to celebrate the date on which you were actually born. Leap Day babies, often called leaplings, celebrate non-leap year birthdays on either February 28 or March 1. There’s even a club and honor society for leaplings to connect and celebrate birthdays together. Do you know how many people around the world share your birthday?

Being a leapling can cause legal complications. For instance, leaplings may wonder whether they can legally drink alcohol in the United States on February 28 or March 1 of the year of their 21st birthdays. Or maybe they wonder what day they can join the military or potentially vote in a primary election.

In 2012, John Reitz, professor of Law at the University of Iowa answered these questions in an interview with a newspaper called the Globe-Gazette. Reitz suggested that March 1 should be leaplings’ legal birthday since that is still one day after February 28. Next, find out why February is the chosen month to be shorter in the first place.

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Leandra Beabout
Leandra Beabout is a freelance journalist and branded content writer with a BA in English education from Indiana University. She writes about travel, health, and literature for Reader's Digest, Lonely Planet, CNN, and Literary Hub, among other publications. She is also a regular contributor to Greatist.com. Leandra is based in Indiana. Follow her on Instagram @LeandraBeabout and LinkedIn Leandrabeabout