2100 is not a leap year—why do leap years exist?

Four times 0.24 is almost one day. That is why we add February 29 every four years—gathering leftover scraps into a full day. Calling a leap year 「lucky」 is mostly a name-based myth; it is a correction to keep the calendar near the seasons. It is not fortune—it is the mismatch between Earth's orbit and the length of a day.

The catch is the word 「almost.」 Adding a full day every four years puts in a little too much. Over centuries that shifts when seasons begin. Treating 1900 as a leap year would have nudged spring later bit by bit.

So there is another rule. Years divisible by 100 are not leap years—unless they are also divisible by 400. That is the Gregorian rule most countries use today. It adds 97 leap days every 400 years, averaging about 365.2425 days.

2000 was a leap year; 1900 was not. 2100 is divisible by 100 but not 400, so it is a common year. We added a day in 2024; the next is 2028, and 2026 is a rest year for that correction. Memorizing only 「every multiple of four」 trips people up at 2100.

On a phone calendar, compare a February that ends on the 29th with one that ends on the 28th and the rule appears at once. People born on February 29 often celebrate on February 28 or March 1 in common years. Contracts and exam dates that say 「the same date every year」 are safer if you double-check leap years.

Think of leap years less as 「special years」 and more as a tool that keeps seasons and dates close. A calendar is not a perfect copy of nature—it is an agreement tuned toward the seasons.