The calendar’s nearly six-hour surplus
A year in the calendar is 365 days, but Earth takes about 365 days and six hours to return to the same seasonal position in its orbit. Adding February 29 every four years compensates for those extra hours: four roughly six-hour leftovers make about one day.
But the match is not exact. The orbital year is a little shorter than 365 days and six hours, so adding a full day every four years adds slightly too much time. Left unchecked, that small surplus would gradually shift calendar dates away from the seasons.
Why most century years lose February 29
The Gregorian calendar corrects the overcorrection with a simple rule: a year divisible by 100 is not a leap year, unless it is also divisible by 400. So 1900 skipped February 29, while 2000 kept it.
Skipping a leap day removes one day from the calendar’s accumulated surplus. Keeping it every 400 years prevents the correction from swinging too far the other way. Across a 400-year cycle, the calendar has 97 leap days rather than the 100 that a strict every-four-years pattern would produce.
A small adjustment with a seasonal payoff
The rule does not make the calendar perfectly match Earth’s orbit, but it keeps the difference small enough that seasons do not steadily migrate through the months. Without the century exception, spring’s calendar dates would drift earlier relative to the season; without the 400-year exception, the calendar would gradually drift in the opposite direction.
That is why an ordinary century year can be common while a year such as 2000 is a leap year: the calendar is trimming a tiny error, then occasionally restoring a day so the seasons stay close to their expected place.
