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When computations are done involving both years AD and years BC, it is common to use Astronomical year numbering, in which 1 BC is designated 0, 2 BC is designated −1, and so on.Other eras are also used to enumerate the years in different cultural, religious or scientific contexts.
The ISO standard ISO 80000-3, Annex C, supports the symbol "a" (for Latin annus) to represent a year of either 365 or 366 days.
As 218 out of every 900 years are leap years, the average (mean) length of this Julian year is A calendar era assigns a cardinal number to each sequential year, using a reference point in the past as the beginning of the era.
Worldwide, the most commonly used calendar era is referenced from the traditional—now believed incorrect—year of the birth of Jesus.
A leap year occurs every fourth year, or leap year, during which a leap day is intercalated into the month of February. The Gregorian calendar attempts to cause the northward equinox to fall on or shortly before March 21 and hence it follows the northward equinox year, or tropical year. It is estimated that by the year 4000 CE, the northward equinox will fall back by one day in the Gregorian calendar, not because of this difference, but due to the slowing of the Earth's rotation and the associated lengthening of the day.
The Revised Julian calendar, as used in some Eastern Orthodox Churches, currently does a better job than the Gregorian in synchronizing with the mean tropical year.