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Leap years (which keep the calendar in sync with the year) should not be confused with leap seconds (which keep clock time in sync with the day).
The Gregorian calendar adds an extra day to February, making it 29 days long, in years divisible by 4, excepting years divisible by 100, but including years divisible by 400. So 1996, 2000, and 2400 are leap years but 1899, 1900 and 2100 are not.
The reasoning behind this rule is as follows:
This difference of a little over 0.0001 days means that an error of a day will accumulate in around 8,000 years. But in 8,000 years' time the length of the vernal equinox year will have changed by an amount we can't accurately predict (see below). So the Gregorian leap year rule does a good enough job.
The Gregorian calendar is a modification of the Julian calendar first used by the Romans. The Roman calendar originated as a lunar calendar (though from the 5th century BC it no longer followed the real moon) and named its days after three of the phases of the moon: the new moon (calends, hence "calendar"), the first quarter (nones) and the full moon (ides). Days were counted down (inclusively) to the next named day, so 24 February was ante diem sextum calendas martii ("the sixth day before the calends of March").
Since 45 BC, February in a leap year had two days called "the sixth day before the calends of March". The extra day was originally the second of these, but since the third century it was the first. Hence the term bissextile day for 24 February in a bissextile year.
Where this custom is followed, anniversaries after the inserted day are moved in leap years. For example, the former feast day of Saint MatthiasSaint Matthias is the Apostle chosen by the remaining eleven apostles to replace Judas Iscariot, following Judas' betrayal of Jesus Christ and suicide ( Acts 1:21 26). His feast day in the Roman Catholic Church was February 24, until it was moved in the 2, 24 February in ordinary years, would be 25 February in leap years.
This historical nicety is, however, in the process of being discarded: The European UnionFor other uses, see EU (disambiguation). The European Union or EU is a supranational organisation of 25 European states. It was established with that name by the Treaty on European Union (commonly known as the Maastricht Treaty) in 1992 but many aspects o declared that, starting in 2000, 29 February rather than 24 February would be leap day, and the Roman Catholic ChurchThe Roman Catholic Church (often called simply the Catholic Church, but see Catholicism for other meanings of the term "Catholic Church") is a worldwide body of Christians in full communion with the Pope, the Bishop of Rome, and subscribing to the beliefs also now uses 29 February as leap day. The only tangible difference is felt in countries which celebrate 'name days'.
The Julian calendar adds an extra day to February in years divisible by 4.
This rule gives an average year length of 365.25 days. The difference of about 0.0124 days with respect to the vernal equinox tropical yearA tropical year is the length of time that the Sun, as viewed from the Earth, takes to return to the same position along the ecliptic (its path among the stars on the celestial sphere). The precise length of time depends on which point of the ecliptic one means that the vernal equinox moves a day earlier in the calendar every 130 years or so.
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