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Leap year checker

Leap year checker: whether a year has 366 days under the Gregorian rule, which test decides it, and the previous and next leap years.

업데이트 검증한 예제: 5

시도하기
결과
Common year
결과: Common year
Rule that decides it
Not divisible by 4
Days in the year
365
Days in February
28
Previous leap year
2024
Next leap year
2028

2026 is a common year (not divisible by 4), so February has 28 days. The previous leap year is 2024 and the next is 2028; the Gregorian rule gives 97 leap years every 400 years.

Leap years 2017–2040 shaded, 2026 outlined

201720182019202020212022202320242025202620272028202920302031203220332034203520362037203820392040
계산 방법 S

  1. Divisible by 4?

    2026 mod 4=22026 \bmod 4 = 2

    No year entered, so this checks the current year, 2026.

    No — a common year.

  2. Divisible by 100?

    2026 mod 100=262026 \bmod 100 = 26

    No — the century rule doesn't apply.

  3. Divisible by 400?

    2026 mod 400=262026 \bmod 400 = 26

    No.

Leap year checker 소개

Under the Gregorian rule a year is a leap year if it is divisible by 4, except century years, which are leap years only if they are also divisible by 400. The checker applies the three tests in order, reports the one that decides, and gives the days in the year and in February and the nearest leap years either side.

Century years are where the rule catches people out. 1900 was a common year because it is divisible by 100 but not by 400, while 2000 was a leap year because it is divisible by 400. The next skipped year is 2100, so the leap year after 2096 is 2104, eight years later.

The rule gives 97 leap years in every 400, an average year of 365.2425 days, about 27 seconds longer than the mean tropical year of 365.24219 days. Years before 1582 are checked with the Gregorian rule extended backwards.

계산 예제

2024

년
2024
결과
윤년
Rule that decides it
Divisible by 4 but not by 100
Days in the year
366
Previous leap year
2020
Next leap year
2028

검증 출처: Python calendar.isleap(2024) = True

1900, a century year

년
1900
결과
Common year
Rule that decides it
Divisible by 100 but not by 400
Days in February
28
Next leap year
1904
Previous leap year
1896

검증 출처: Python calendar.isleap(1900) = False (Gregorian century rule)

2000, divisible by 400

년
2000
결과
윤년
Rule that decides it
Divisible by 400
Days in February
29

검증 출처: Python calendar.isleap(2000) = True

Leap years skip 2100

년
2096
결과
윤년
Next leap year
2104
Previous leap year
2092

검증 출처: Python: first x > 2096 with calendar.isleap(x) is 2104

자주 묻는 질문

Why is 2100 not a leap year?

Because century years must be divisible by 400 to be leap years, and 2100 ÷ 400 = 5.25. February 2100 will have 28 days, and there will be no leap year between 2096 and 2104. The same rule made 1700, 1800 and 1900 common years, while 1600 and 2000 were leap years. The next century leap year is 2400.

Why do we have leap years?

The year of the seasons, the tropical year, is about 365.2422 days, so a 365-day calendar would slip by almost a day every 4 years and about 24 days a century. The Julian calendar's leap day every fourth year overcorrected by about 11 minutes a year; by 1582 the March equinox had drifted 10 days, which the Gregorian reform removed and the century rule prevents from recurring.

When was the Gregorian calendar adopted?

Pope Gregory XIII decreed it in the bull Inter gravissimas of 24 February 1582. Italy, Spain, Portugal and Poland switched that October, when 4 October was followed by 15 October. Britain and its colonies changed in September 1752, dropping 11 days; Russia in February 1918, dropping 13; and Greece in 1923, also dropping 13.

When is the next leap year?

2028. The leap years after it are 2032 and 2036, and 29 February falls on a Tuesday in 2028, a Sunday in 2032 and a Friday in 2036. Every year divisible by 4 up to 2096 is a leap year, because the first century year in that run that fails the 400 test is 2100.

“Leap year checker”의 정확도는 어느 정도인가요?

정확도는 입력값과 계산 방법의 가정에 따라 달라집니다. 십진 연산은 유효숫자 50자리를 사용하지만, 추정값·수치해석 방법·원본 데이터의 정밀도는 더 낮을 수 있습니다. 표시값을 반올림해도 이러한 한계는 사라지지 않습니다. 독립적인 출처의 풀이와 대조한 계산 예시: 5. 예를 들어 “2024”은 Python calendar.isleap(2024) = True와 대조해 확인합니다.

이 계산 방법의 출처는 무엇인가요?

Pope Gregory XIII, Inter gravissimas (1582) — the Gregorian leap-year rule; US Naval Observatory — Leap years.

이 계산기 소개

leap  ⟺  (y mod 4=0∧y mod 100≠0)∨y mod 400=0\text{leap} \iff (y \bmod 4 = 0 \land y \bmod 100 \ne 0) \lor y \bmod 400 = 0

출처

  1. Pope Gregory XIII, Inter gravissimas (1582) — the Gregorian leap-year rule
  2. US Naval Observatory — Leap years

출처와 대조하여 검증

이 계산기에는 독립적인 출처에서 답을 얻은 계산 예제가 5개 있습니다. 테스트 모음에서 실행되며 여기에서도 실행할 수 있습니다.

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