Leap Year Checker
Check if any year is a leap year. Find leap years in a range.
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Find Leap Years in Range
How Leap Years Work
Divisible by 4? Usually a leap year.
Divisible by 100? Skip it (not leap).
But if divisible by 400? It IS leap!
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The Complete Guide to Leap Years
We often take our calendar for granted, but it's actually a complex mathematical system designed to keep humanity in sync with the cosmos. The Leap Year is the critical adjustor that prevents our seasons from drifting out of alignment.
Without this simple correction of adding February 29th every four years, our calendar would drift by about 6 hours every year. After 100 years, we would be off by 24 days. After 700 years, summer in the Northern Hemisphere would begin in December!
The Mathematics: It's Not Just "Every 4 Years"
Many people believe the rule is simply "every 4 years," but that's not statistically accurate enough. The solar year is actually 365.2422 days long.
If we just added a day every 4 years, we'd add 0.25 days per year (365.25), which is slightly too much compared to 365.2422. To fix this over-correction, we have a 3-step algorithm:
Rule #1
Divisible by 4?
Example: 2024, 2028
Rule #2
Divisible by 100?
Example: 1900, 2100
Rule #3
Divisible by 400?
Example: 1600, 2000
A Brief History
The Julian Calendar
Julius Caesar introduces the concept of a leap day every 4 years. It was a huge improvement but overcorrected by about 11 minutes per year.
The Gregorian Calendar
By the 16th century, the calendar had drifted 10 days! Pope Gregory XIII introduced the "Century Rule" (Rule #2 and #3 above) to fix the drift, creating the modern calendar.
Fun Facts About Leap Days
- 💍Women Proposing: An old Irish tradition (The Bachelor's Day) states that women are allowed to propose to men on leap day for good luck.
- 🐸Leaplings: People born on Feb 29 are called "leaplings". There are about 5 million leaplings worldwide. The chances of being born on a leap day are 1 in 1,461.
- 🌍Not Just Earth: Mars has more leap years than Earth! Because a Martian year is 668 sols (Martian days), it adds a leap sol roughly every two years.
Frequently Asked Questions
What is a leap year?
A leap year is a year containing 366 days instead of the usual 365 days. The extra day is added to February, giving it 29 days instead of 28. This adjustment keeps our calendar synchronized with the astronomical year.
Why do we need leap years?
Earth takes approximately 365.2422 days to orbit the Sun, not exactly 365. Without leap years, the extra roughly 6 hours each year would accumulate, shifting our seasons by about 24 days every 100 years. Leap years correct this drift.
What are the 3 rules for calculating a leap year?
- The year must be evenly divisible by 4.
- If the year can also be divided by 100, it is NOT a leap year.
- UNLESS the year is also divisible by 400. Then it IS a leap year.
Was the year 2000 a leap year?
Yes. Although 2000 is divisible by 100 (which usually disqualifies it), it is also divisible by 400, triggering the special exception rule. Therefore, it was a leap year.
Will the year 2100 be a leap year?
No. The year 2100 is divisible by 100 but NOT by 400. So, despite being divisible by 4, it will be skipped as a leap year to keep the calendar accurate.
When is the next leap year?
Assuming the current year is 2024 (a leap year), the next leap year will be 2028, followed by 2032, 2036, and 2040. Leap years typically occur every 4 years.
What happens if you are born on February 29?
People born on February 29 are called 'leaplings'. Since their actual birthday only exists once every four years, they usually celebrate on February 28 or March 1 in non-leap years.
Is there ever a Leap Second?
Yes! Separate from leap years, 'leap seconds' are occasionally added to Coordinated Universal Time (UTC) to adjust for irregularities in Earth's rotation speed, though plans are in place to potentially abolish them by 2035.
Who invented the leap year?
The concept was introduced by Julius Caesar in 45 BCE with the Julian calendar. However, his calculation was slightly off. Pope Gregory XIII refined it in 1582 (creating the Gregorian calendar we use today) by adding the 'divisible by 100 and 400' rules.
Are there leap years in other calendars?
Yes. The Jewish calendar adds a 'leap month' (Adar II) seven times every 19 years. The Chinese calendar also adds an intercalary month periodically to stay aligned with solar and lunar cycles.