Anno domini system of dating
In this same history he also used another Latin term, "ante vero incarnationis dominicae tempus" ("the time before the Lord's true incarnation"), equivalent to the English "before Christ", to identify years before the first year of this era, even though he used zero in his computus.Both Dionysius and Bede regarded Anno Domini as beginning at the incarnation of Jesus, but "the distinction between Incarnation and Nativity was not drawn until the late 9th century, when in some places the Incarnation epoch was identified with Christ's conception, i.e., the Annunciation on March 25" (Annunciation style)."Anno an xpi nativitate" (before the birth of Christ) is found in 1474 in a work by a German monk.In 1627, the French Jesuit theologian Denis Pétau (Dionysius Petavius in Latin), with his work De doctrina temporum, popularized the usage ante Christum (Latin for "Before Christ") to mark years prior to AD. This calendar era is based on the traditionally reckoned year of the conception or birth of Jesus of Nazareth, with AD counting years after the start of this epoch, and BC denoting years before the start of the epoch.
Since BC is not derived from Latin it is placed after the year number (for example: 68 BC, but AD 2011).
Tiberius began to reign with his father, Augustus, in AD 12.
The 15th year of his reign would then be 26 or 27 AD, placing Jesus' birth about 5 or 4 BC (because there is no year 0).
Alternatively, the secular abbreviations CE and BCE are used, respectively.
There is no year zero in this scheme, so the year AD 1 immediately follows the year 1 BC.