2024 is a leap year. Here's what that means — and why we still have them.

Thursday, Feb. 29 is leap day — that's something you don't hear very often. Here's what you need to know about the day that happens almost every four years.

🗓️ What is a leap year?

A leap year is when an extra day is added to our modern-day Gregorian calendar — the world's most widely used calendar, named after Pope Gregory XIII — during the shortest month of the year. February's extra day brings our usual total of 365 days to 366.

🌎 Why are leap years important?

A planet has to be aligned with its stars. It takes 365 days and about six hours — or 365.242189 days — for the Earth to orbit around the sun. Because we follow the Gregorian calendar, the number is rounded to just 365. To account for the six hours, we add an extra day almost every four years. Our last leap year was in 2020, so 2024 is the year we make up that extra time.

We also have to sync up our calendars with the seasons. The extra six hours would shift seasons by about 24 calendar days within 100 years. If we stopped leap year, centuries from now, our descendants could be celebrating Christmas in July.

📖 Who invented leap years?

Let's take it back to 46 B.C., when Julius Caesar created the Julian calendar. The Roman Empire had noticed that their calendar was drifting when Romans counted a year to be 355 days. Caesar decided to count a year at 365.25 days, adding an extra day once every four years. This was based off the rule mentioned earlier — that a year evenly divisible by four would be a leap year.

But that didn't solve the problem. That calculation produced too many leap years because Earth's trip around the sun is 365.242 days. The Julian calendar ended up being 11 minutes and 14 seconds longer than the tropical year — the time it takes for seasons to repeat.

In the late 16th century, Pope Gregory XII improved the realignment of our calendar with the seasons.

🧮 How do you calculate leap years?

Leap year usually happens about every four years — but there are some exceptions. The year has to be evenly divisible by 4 to be a leap year. It's not a leap year if the year can be evenly divided by 100 unless that year can also be evenly divided by 400, thanks to Pope Gregory XII.

If the math isn’t adding up for you, here’s what happened in 2000.

The year 2000 was a leap year, but it broke one of the rules:

2000/4 = 500 ✅ ...That completes the 1st rule.

2000/100 = 20 ❌ ...That breaks the leap year rule,

Unless…

2000/400 = 5 ✅ …Which solves that problem!

Due to the rules, there was no leap year in 1900 and there won’t be one in 2100.

🎂 So you were born during a leap year. How old are you actually?

The 5 million "leaplings" born on leap day typically celebrate their birthday on Feb. 28 or March 1 during the so-called "common years." Since Feb. 29 is actually a date, it is still used on important documents like identification and passports.

Some leaplings can run into a few issues as many government systems and some websites that require you to enter your birth date don't recognize Feb. 29.

Government system imperfections aside, leaplings can still choose to be forever young on their birthday:

1924: You could be 100 years old or 25. Both are milestones.

1940: You'll be 84 years old or 21. If you choose the latter, you’re long overdue for a sip.

1952: You're turning 72 or 18, but you would have been eligible to vote over five decades ago.

1960: You would be 64 years old or finally celebrate your sweet 16.

1984: You can be 40 years old or just starting double digits at 10.

2000: Tell them you're 24 or 6. See if you qualify for a child's discount like Brady Olsen, of Cottonwood, Calif.

2024: Welcome to the world of the Gregorian calendar and leap day madness! Still acting like a baby in 2028 would make perfect sense since leaplings born this year will be either 1 or 4 on the next leap day.

👽 Do other planets have leap years?

Yes, Martians are also thrown for a loop on Mars. "Sols" are Martian days, so a year on Mars is 668 sols. But it takes Mars 668.6 sols to travel around the sun. So, just like Earth, there needs to be an extra day added to catch up. According to NASA, in a 10-year span, the red planet has 668 sols in four of the years and 669 sols in six of the years. That gives Mars more leap years than regular years.

💒 What are some leap day traditions and superstitions?

Since leap year is considered such a unique day, the allure of taking the big “leap” is popular among couples who want to take advantage of the infrequent occurrence.

For example, traditions dating back to around 1,600 years ago in the U.S. and Europe would give women the rare chance to propose to their significant other.

This couple who didn't want to feel the pressure of having to celebrate their wedding anniversary every year decided to get married on leap day. That way, they can opt to do big celebrations every four years.

However, many Greek and Ukrainian couples believe that if you get married on Feb. 29, you'll likely end up divorced for fear of starting something new on that date.

According to California newspaper Redding Record Searchlight, historians noted that in the past, Europeans and Americans thought leaplings would "suffer later in life" for being born on leap day.

That's not how Anthony, Texas, sees it, though. The small town in El Paso County holds a four-day festival to celebrate leap year and its leaplings.