Leap year vernal equinox moves a day earlier in the calendar about every 131 years. The Revised Julian calendar adds an extra day to February in years that are multiples of four, except for years that are multiples of 100 that do not leave a remainder of 200 or 600 when divided by 900. This rule agrees with the rule for the Gregorian calendar until 2799. The first year that dates in the Revised Julian calendar will not agree with those in the Gregorian calendar will be 2800, because it will be a leap year in the Gregorian calendar but not in
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