February 29 takes approximately 6 hours longer than 365 whole days. A leap day compensates for this lag, realigning the calendar with the Earth's position in the Solar System; otherwise, seasons would occur later than intended in the calendar year. The Julian calendar used in Christendom until the 16th century added a leap day every four years; but this rule adds too many days (roughly 3 every 400 years), making the equinoxes and solstices shift gradually to earlier dates. By the 16th century the vernal equinox had drifted to March 11, and the Gregorian calendar was introduced both to shift it back
Middle-earth calendar more generally: The Elves also have a regular 365-day solar year called "coranar" meaning "sun-round" or more commonly "loa" meaning "growth". The Elven year, which began near the northward equinox, is divided into six seasons or 'months' which consist of four 54-day months and two 72-day months. Five or eight extra days outside the seasons make the length of the "loa" 365 or 368 days. Most years are 365 days, but every twelfth year is 368 days, resulting in an average year of 365.25 days with the additional suggestion that the 'Reckoning of Rivendell' made a further correction by omitting