Gregorian calendar
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The Gregorian calendar is the calendar used in most parts of the world. It went into effect in October 1582 following the papal bull Inter gravissimas issued by Pope Gregory XIII, which introduced it
Christopher Clavius (1538–1612), one of the main authors of the reform
Pope Gregory XIII, portrait by Lavinia Fontana, 16th century
First page of the papal bull Inter gravissimas
Detail of the pope's tomb by Camillo Rusconi (completed 1723); Antonio Lilio is genuflecting before the pope, presenting his printed calendar.
Anno Domini
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Anno Domini and before Christ qualify years in the Gregorian and Julian calendars, whose epoch is the traditional year of the conception or birth of Jesus. AD counts years since the epoch, BC the year
Gregorian calendar
…years are identified by consecutive year numbers. A calendar date is fully specified by the year (numbered according to a calendar era, in this case Anno Domini or Common Era), the month (identified by name or number), and the day of the month (numbered sequentially starting from 1). Although the calendar year…
Anno Domini inscription at Klagenfurt Cathedral, Austria
Statue of Charlemagne by Agostino Cornacchini (1725), at St. Peter's Basilica, Vatican City. Charlemagne promoted the usage of the anno Domini epoch throughout the Carolingian Empire.