The solar calendar in usethroughout mostoftheworld, sponsored byPopeGregoryXIIIin 1582 as a correctedversion of theJuliancalendar. See table at calendar.
There were three rival claimants to the papal throne at that time - the Roman PopeGregory XII, the Avignon Pope Benedict XIII, and the Antipope John XXIII.
The so-called Meridian Hall, created to verify the accuracy of the calendar PopeGregoryXIII promulgated in 1582, is in the Vatican Secret Archives, which hold some 10m documents stored by the papacy over the past 1, 200 years.
Before the Revolution, Russia had not yet adopted the Gregorian calendar, introduced by PopeGregoryXIII in 1582 to mitigate the errors of the Julian calendar, which owes its name to the Roman emperor who had chosen it 46 years before the birth of Jesus Christ.