Dates in this era are designated Anno Domini (Latin for in the year of the Lord), abbreviated AD, or the secular common era, abbreviated CE.
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.
Cognates are German Jahr, Old High German jār, Old Norse ár and Gothic jer (Gothic e is always a long vowel), all the descendants of the Proto-Indo-European noun *yeh₁rom "year, season".
Cognates also descended from the same Proto-Indo-European noun are Avestan yārǝ "year", Greek "year, season, period of time" (whence "hour"), Old Church Slavonic jarŭ, and Latin hornus "of this year".
Latin annus (a 2nd declension masculine noun; annum is the accusative singular; annī is genitive singular and nominative plural; annō the dative and ablative singular) is from a PIE noun , which also yielded Gothic aþn "year" (only the dative plural aþnam is attested).