Origin of dating system
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.When the reckoning from Jesus' incarnation began replacing the previous dating systems in western Europe, various people chose different Christian feast days to begin the year: Christmas, Annunciation, or Easter.
"Anno an xpi nativitate" (in the year before the birth of Christ) is found in 1474 in a work by a German monk.For example, the Islamic calendar begins not from the date of the Hegira, but rather weeks later, on the first subsequent occurrence of the month of Muharram (corresponding to 16 July AD 622).It has also been speculated by Georges Declercq that Dionysius' desire to replace Diocletian years with a calendar based on the incarnation of Christ was intended to prevent people from believing the imminent end of the world.Two major theories are that Dionysius based his calculation on the Gospel of Luke, which states that Jesus was "about thirty years old" shortly after "the fifteenth year of the reign of Tiberius Caesar", and hence subtracted thirty years from that date, or that Dionysius counted back 532 years (the period during which the dates of Alexandrian Easter repeat) from the first year of his new table.It is convenient to initiate a calendar not from the very day of an event but from the beginning of a cycle which occurs in close proximity.The Gregorian calendar is the most widely used calendar in the world today.
For decades, it has been the unofficial global standard, adopted in the pragmatic interests of international communication, transportation, and commercial integration, and recognized by international institutions such as the United Nations.
Its endorsement by Emperor Charlemagne and his successors popularizing the use of the epoch and spreading it throughout the Carolingian Empire ultimately lies at the core of the system's prevalence.
According to the Catholic Encyclopedia, popes continued to date documents according to regnal years for some time, but usage of AD gradually became more common in Roman Catholic countries from the 11th to the 14th centuries.
The last year of the old table, Diocletian 247, was immediately followed by the first year of his table, AD 532.
When he devised his table, Julian calendar years were identified by naming the consuls who held office that year—he himself stated that the "present year" was "the consulship of Probus Junior", which was 525 years "since the incarnation of our Lord Jesus Christ".
It was believed that, based on the Anno Mundi calendar, Jesus was born in the year 5500 (or 5500 years after the world was created) with the year 6000 of the Anno Mundi calendar marking the end of the world.