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calendar_todayGregorian Calendar Ruleshistory_toggle_off400-Year Cycle RuleverifiedISO 8601 Standardevent_repeatFebruary 29 Leap Day

Leap Year Calculator

Is your year a leap year? Check instantly, discover the next leap day, trace astronomical drift, and explore the 400-year Gregorian cycle with mathematical precision.

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Gregorian Leap Year Confirmed

2028 is a LEAP YEAR ✓

February Length29 Days+1 Extra Calendar Day
Calendar Year Days366 Days8,784 total hours
Gregorian Rule MetRule 3: ÷ 4Divisible by 4, not 100
Season Alignment+0.7575 dKeeps spring & autumn on time
HOW THIS YEAR IS CHECKED
2028 ÷ 400Does not divide evenly (Remainder 28)
2028 ÷ 100Does not divide evenly (Remainder 28)
2028 ÷ 4Divides evenly by 4 without remainder ✓

2028 divides evenly by 4 and is not a special century year. This makes 2028 a standard quadrennial leap year (every 4 years).

arrow_back Prior Leap Year2024
4 years prior
Click to examine →
Subsequent Leap Year arrow_forward2032
in 4 years
Click to examine →
THE 3-STEP LEAP YEAR RULE

How the Leap Year Rule Works Step by Step

Path highlighted for year 2028
Step 1 Checked

Divisible by 400?

Century years like 1600, 2000, 2400 bypass all further checks and are immediately classified as leap years.

Year % 400 === 0false (Remainder: 28)
Step 2 Checked

Divisible by 100?

If not divisible by 400, century turnarounds like 1700, 1800, 1900, 2100 are common years (365 days).

Year % 100 === 0false (Not century)
Step 3 • TriggeredRule 3 Applied: Leap Year!

Divisible by 4?

Every year evenly divisible by 4 that is not an exempted century year is a standard quadrennial leap year.

Year % 4 === 0true (Divisible by 4)
DID YOU KNOW?

The Century Rule: Why 2000 Was a Leap Year, but 1900 Was Not

Most people think every 4th year is automatically a leap year. But century years (years ending in 00) must also divide evenly by 400. That is why 2000 had February 29th, while 1900 was a regular 365-day year. The next century year, 2100, will also not be a leap year!

1900Common Year1900 ÷ 400 has remainderFeb had 28 days
2000Century Leap Year2000 ÷ 400 divides evenlyFeb had 29 days
CALENDAR FACTS • 400-YEAR CYCLE146,097 Days

The 400-Year Calendar Pattern

Every 400 years, the calendar completely resets and repeats the exact same dates and weekdays.

Leap Years in Cycle
97/ 400 years
Exactly 24.25% of all years
Common Years
303/ 400 years
75.75% of calendar
Total Days in Cycle
146,097days
Exactly 20,871 weeks (divisible by 7)
Average Gregorian Year
365.2425days
Only drifts by 26 seconds per year from Earth's seasons
Displaying 400 years in cycle
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analytics
Selected Year Inspector: 2028Standard Leap Year (366 Days, Feb 29). Divisible by 4, not 100.
EXPLORE MULTIPLE YEARS

Count Leap Years Between Two Years

Find out how many leap years occur between any two dates and get a complete list.

Presets:
TOTAL YEARS201 Years
LEAP YEARS49 Leaps
COMMON YEARS152 Common
EXTRA DAYS ADDED+49 Days
ALL LEAP YEARS IN THIS PERIOD (49 TOTAL)
1904190819121916192019241928193219361940194419481952195619601964196819721976198019841988199219962000200420082012201620202024202820322036204020442048205220562060206420682072207620802084208820922096
Historical Chronology

Gregorian vs. Julian Calendar Systems

Julius Caesar introduced leap years in 45 BCE with a simple rule: every 4th year without exception. Over 16 centuries, this caused a 10-day discrepancy against the seasons.

Century YearGregorian Rule (÷400)Old Julian Rule (÷4)DAYS DRIFTEDWHAT HAPPENED
1600Leap Year (366d)Leap Year (366d)10 Days AheadGregorian Papal Bull in 1582 cut 10 days
1700Common (365d)Leap Year (366d)11 Days DriftJulian fell 1 additional day behind equinox
1800Common (365d)Leap Year (366d)12 Days DriftBritain adopted Gregorian in 1752 (lost 11 days)
1900Common (365d)Leap Year (366d)13 Days DriftRussian Empire ran 13 days behind Europe
2000Leap Year (366d)Leap Year (366d)13 Days DriftRare century alignment; both added Feb 29
2100Common (365d)Leap Year (366d)14 Days DriftNext century anomaly; 2100 will NOT be leap
2400Leap Year (366d)Leap Year (366d)16 Days DriftNext century year divisible by 400
info
What is the Proleptic Gregorian Calendar?

The Gregorian calendar was legislated on October 15, 1582. When calculating dates prior to 1582 (such as the year 1200 or 44 BCE), computer science and the ISO 8601 specification apply the modern Gregorian rules retrospectively. This mathematical projection is termed the Proleptic Gregorian Calendar. If modeling historical records in England before 1752 or Russia before 1918, Julian dates must be reconciled.

WHY LEAP YEARS EXIST

The Earth's Orbit Around the Sun

Why can't a year be a simple 365 days? Earth takes a little over 365 days to circle the Sun.

Mean Tropical Year
365.24219
Mean solar days per complete orbit around the sun.

Earth takes 365 days, 5 hours, 48 minutes, and 45.2 seconds to pass between two successive vernal equinoxes.

Excess: +0.24219 days / year
Julian Formula (45 BCE)
365.25000
Average days per year under pure 4-year leap rule.

Overshoots the true orbit by 11 minutes and 14.8 seconds each year, accumulating 1 full superfluous day every 128 years.

Error: +1 day every 128 years
Gregorian Reform (1582 CE)
365.24250
Omits 3 leap days every 400 years (−0.0075d).

Brings the annual discrepancy down to just 26.8 seconds per year. The Gregorian calendar will require no manual adjustment for 3,236 years.

Accuracy: 99.9992% aligned

Historical Calendar Evolution

753 BCE – 46 BCEAncient Roman Calendar

Only 10 months and 304 days. Officials added irregular leap months by hand, which threw calendars off by months.

45 BCEJulian Calendar Reform

Julius Caesar created a straightforward 365-day calendar with one extra day every 4 years in February.

1582 CEGregorian Calendar Reform

Pope Gregory XIII skipped 10 drifted days and added the rule to skip 3 century leap years every 400 years.

1988 – PresentModern Global Calendar (Everyday Standard)

Used worldwide by international agreements, phones, computers, and everyday life in standard YYYY-MM-DD format.

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Authoritative Chronometry & Astronomical Standards

Explore official astronomical calculation models and historical calendar reform archives maintained by global observatories.

NASA Calendar Ephemeris Guideopen_in_newRoyal Museums Greenwich Historyopen_in_new
Interactive ChallengeScore: 0 / 3

Can You Beat the Calendar?

Test your mastery against the century anomalies that trip up 80% of people.

1. Was the year 1900 a Leap Year?
2. Will the year 2100 be a Leap Year?
3. How many leap years exist in 400 years?
Curiosities & Culture

10 Fascinating Leap Year Facts

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Leaplings & Leap Day Babies

About 5 million people worldwide are born on Feb 29. Odds: 1 in 1,461.

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Legal Birthday Rules

In the UK and Taiwan, leaplings turn 18 on March 1. In the US, it is legally deemed February 28 or March 1 based on state statutes.

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Bachelor's Day Tradition

5th-century Irish folklore allowed women to initiate marriage proposals on Feb 29 to balance patriarchal tradition.

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Software Y2K Leap Bug

Many older algorithms in 2000 crashed because programmers forgot the ÷400 rule, falsely assuming 2000 was a non-leap century.

payments
Salaried Work Paradox

Salaried employees work an extra day in leap years without additional contractual compensation, saving employers billions.

travel_explore
Sweden's February 30th (1712)

When transitioning calendars, Sweden added a double leap day in 1712, creating history’s only documented February 30th.

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Leap Seconds Distinction

Leap years fix orbital geometry. Leap seconds fix Earth’s irregular rotational deceleration (UTC vs TAI).

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Summer Olympics & Elections

US Presidential elections and Summer Olympic Games traditionally coincide with leap years, tracing ancient Greek Olympiads.

QUICK LEARNING GUIDE

Simple Leap Year Rules Explained (No Math Degree Needed)

The 3 straightforward steps you can use in your head to check any year in seconds.

Rule 1 • Standard Years

Most years: Divide by 4

If a normal year divides by 4 without a remainder (like 2024, 2028, 2032), it is a leap year with 366 days.

Rule 2 • Century Exception

Years ending in '00': Stop!

Turn-of-the-century years (like 1700, 1800, 1900, 2100) are NOT leap years even though they divide by 4.

Rule 3 • The 400 Exception

Divide by 400: Leap Year!

If a century year divides cleanly by 400 (like 1600, 2000, 2400), it earns February 29th back!

Century Years Quick Cheat Sheet1600 (Leap) • 1700 (Common) • 1800 (Common) • 1900 (Common) • 2000 (Leap) • 2100 (Common) • 2400 (Leap)
Easy Rule of Thumb
/**
 * Canonical Gregorian Leap Year Check (ISO 8601)
 * @param {number} year - Astronomical year integer
 * @returns {boolean} True if leap year
 */
function isLeapYear(year) {
  return (year % 4 === 0 && year % 100 !== 0) || (year % 400 === 0);
}

// Assertions
console.assert(isLeapYear(2028) === true);
console.assert(isLeapYear(1900) === false);
console.assert(isLeapYear(2000) === true);
Help & Documentation

Frequently Asked Questions

Everything you ever wanted to know about intercalary calculations, calendar history, and century rules.