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A new calendar system, introduced in 1582 was named as Gregorian calendar in honour of Pope Gregory XIII, who initiated the calendar transition. Gregorian calendar approaches the tropical year much closer that its predecessor - Julian calendar, see Julian and Gregorian calendars. The calculator below converts a Julian date to the Gregorian date for the common era dates.
The common era is a contemporary starting point for years numbering worldwide. In the past, different countries chose different calendar eras for the chronology. Sometimes, there were several era in use simultaneously in a single country. Therefore, to interpret correctly a historical date you should clarify which era is this date belongs to. The following calculator converts a Julian date to the Gregorian date, taking in account the era.
Roman Empire started to number years from 1, whenever a new ruler comes to power. Diocletian era (years started from Roman Emperor Diocletian reign in 284 CE) was very popular for a few centuries until adoption of Common Era (Anno Domini) in Western Europe countries.
Anno Domini era becomes one of many chronology starting points in VI century thanks to Dionysius Exiguus, Rome monk, who calculated the easter tables in 525 CE. Dionysius did not want to number years from the reign of Rome Emperor Diocletian (most common era in those times), instead, he introduced a Christ birthday year as Year One of the new Anno Domini era. Two centuries later, the Anno Domini era became dominant in Western Europe.
Eastern Europe influenced by Patriarchate of Constantinople went their own way: years numbering started since creation of world (Annus Mundi). According to this era the Year One started on September 1st 5509 (or earlier on Match 1st 5508). In Russia this year numbering system existed till 1700, this year Tzar Peter ordered to use Julian calendar in conjunction with the Anno Domini era and January, 1st as first day of the year. See Old Russian (Byzantine) system of chronology for more details.
Iberian peninsula countries used their own era (Spanish era) before domination of the common era. Spanish era Year One corresponds to 38 B.C . Spanish era was gradually replaced by Christian era during 12-16 centuries.
The calculator uses the era starting year and day for date conversion. Information about the eras, exploited by the calculator, is collected in the Calendar Era directory. Edits and corrections of the directory are welcome.