Instantly determine whether any year is a leap year under the Gregorian calendar. Explore the February calendar, view key year statistics, and dive into the rich history of calendar reform — from Julius Caesar to Pope Gregory XIII.
A leap year is a calendar year that contains an extra day — February 29 — to keep the calendar year synchronized with the astronomical year or seasonal year. Because Earth's orbit around the Sun takes approximately 365.2422 days (more precisely, 365 days, 5 hours, 48 minutes, and 45 seconds), a calendar of 365 days would drift by about one day every four years. The intercalary day added every four years (with exceptions) compensates for this drift.
The Gregorian rule for leap years:
A year is a leap year if it is divisible by 4, except for years divisible by 100, unless they are also divisible by 400.
leap = (year % 4 == 0) && (year % 100 != 0 || year % 400 == 0)
The Earth's orbital period — the tropical year — is the time it takes for the Sun to return to the same position in the sky as seen from Earth, which defines the seasons. This period is about 365.24219 days. If we used a 365‑day calendar without adjustment, the seasons would drift by almost one day every four years. After 100 years, the calendar would be off by about 24 days; after 700 years, summer would begin in December in the Northern Hemisphere. The leap year system prevents this drift, keeping the vernal equinox around March 20 each year, which is crucial for agriculture, religious observances (like Easter), and seasonal planning.
The Julian calendar, introduced by Julius Caesar in 45 BCE, used a simple rule: every year divisible by 4 is a leap year. This added an average of 365.25 days per year, which is about 0.0078 days (11 minutes) longer than the actual tropical year. Over centuries, this small error accumulated. By the 16th century, the calendar had drifted by about 10 days relative to the equinoxes.
To correct this, Pope Gregory XIII introduced the Gregorian calendar in 1582. The reform had two parts: (1) drop 10 days from October 1582 (October 4 was followed by October 15), and (2) modify the leap year rule to exclude centurial years not divisible by 400. This reduced the average year length to 365.2425 days — very close to the tropical year of 365.24219 days, requiring only a one‑day correction every 3,300 years or so.
The Gregorian calendar is now the international standard for civil use. Most countries adopted it gradually: Catholic countries in 1582, Protestant countries in the 18th century, and some Eastern Orthodox countries as late as the 20th century (e.g., Russia in 1918, Greece in 1923).
divisible by 4, but not by 100, unless by 400.
| Year | Leap? | Reason | February | Total Days |
|---|---|---|---|---|
| 2024 | ✅ Yes | Divisible by 4, not by 100 | 29 | 366 |
| 2000 | ✅ Yes | Divisible by 400 (century exception) | 29 | 366 |
| 1900 | ❌ No | Divisible by 100 but not by 400 | 28 | 365 |
| 2023 | ❌ No | Not divisible by 4 | 28 | 365 |
| 1600 | ✅ Yes | Divisible by 400 | 29 | 366 |
| 2100 | ❌ No | Divisible by 100 but not by 400 | 28 | 365 |
When Pope Gregory XIII introduced the new calendar, the immediate problem was the 10‑day drift accumulated since the Council of Nicaea (325 CE), which had established the date of Easter. To restore the vernal equinox to March 21, the reform decreed that October 4, 1582, would be followed by October 15, 1582 — effectively skipping 10 days. This caused public confusion and resistance in some regions, but the new calendar was gradually adopted. Our calculator reflects the Gregorian rules, which are the global standard today. Interestingly, the United Kingdom and its colonies (including the future United States) did not adopt the Gregorian calendar until 1752, by which time the adjustment was 11 days.
While the Gregorian calendar is the global civil standard, many cultures have their own calendar systems with intercalary mechanisms:
These diverse systems reflect humanity's universal need to track time and seasons, each with its own cultural and religious significance.