invented Anno Domini years to date Easter.]]Anno Domini May also be spelled "Anno Domine." ( by speakers of English; ), Blackburn & Holford-Strevens p. 782 abbreviated as AD or A.D., defines an epoch based on the traditionally reckoned year of the conception (Annunciation) or birth (Nativity) of Jesus of Nazareth. Similarly, Before Christ (from the Ancient Greek "Christos" or "Anointed One", referring to Jesus), abbreviated as BC or B.C., is used in the English language to denote years before the start of this epoch.The designation is used to number years in the Christian Era, conventionally used with the Julian and Gregorian calendars.The approximation of the year in the old Persian calendar attributed to Omar KhayyƔm is 365.2424 days, which is very close to the vernal equinox year, but requires a 33-year cycle. The definition by Milutin Milankovi?, used in the "revised Julian calendar", is 365.2422 days, which is very close to the mean tropical year, but uses unequal long-period cyc
Source:
http://en.wikipedia.org/wiki/AnnoDomini