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闰年判断工具

判断指定年份是否为闰年，并说明公历历法的置闰规则。

Is this year a leap year?

Any year the JavaScript engine supports (about −271,821 to 275,760).

计算原理

  1. The Gregorian rule is applied exactly: a year is a leap year if divisible by 4, except century years (1700, 1800, 1900…), which are not, unless the century is divisible by 400 (1600, 2000, 2400).
  2. The tool states which rule decided the outcome, so the answer is auditable rather than a bare yes/no.
  3. The nearest leap years on both sides are shown, along with the year’s day count (365 vs 366) and whether Feb 29 exists in it.

使用提示

  • The Gregorian calendar averages 365.2425 days per year (three leap days are skipped every 400 years), which keeps the calendar aligned with the seasons.
  • Same-numbered years repeat forever: 2000 is a leap year and so is 2400; 1900 and 2100 are not.
  • For ages and date math around Feb 29 birthdays, the Age Calculator documents its clamping convention (Feb 28 in non-leap years).
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常见问题解答

What are the leap year rules?
A year is a leap year if divisible by 4, except century years (1700, 1800, 1900), which are not, unless the century is divisible by 400 (1600, 2000, 2400).
Why is 2100 not a leap year?
Century years are skipped unless divisible by 400. 2100 ÷ 400 leaves a remainder, so it is a common year with 28 days in February.
How often is there a February 29?
Roughly once every four years, minus three times per 400 years. The Gregorian calendar averages 365.2425 days per year, keeping it aligned with Earth seasons.