We always assume that the way we speak of years in history let’s say from Before the Common Era BCE known until 30 years ago as BC (before christ) or CE common era as AD Anno Domine which is no longer used in historical research, was the way to speak of times past. Per example Julius Cesar and Cicero lived before Christ and died before him but they and society around them did not say, oh we are 45 years before Christ and next year we will be 44 years before him. The Romans and everyone else had their own calendar for the months of the year and ways of counting the months and the years which was based on the day of the foundation of the City of Rome by Romulus on 21 April.
It was the Catholic Church who started to mark time in terms of before and after Christ by using the Liturgical calendar, the birth of Christ was the year 0. The church calendar became the only calendar to use. Thus marking the break with the old pagan world and forcing everyone to accept the new Faith which did not go very well.
In Republican times Julius Cesar added January and February to the calendar and decried that years would now start on 1 January. That was in 708 which marks 708 since the foundation of Rome, the year 0 for them, previously the year started on the 1 March.
The Church would many centuries later alter 708 to 45 BCE to correspond with Christ Birth. The Church when it finally became official in 313 CE and no longer seen as a strange Jewish sect following some convicted criminal called Jesus decided to continue with the Julian Calendar. But given that the Roman administration continued in the West until 476 CE counting of the years was still based on the foundation of Rome. It changed very gradually and slowly first in the Papal States and gradually elsewhere in the Western world. In the Eastern part of the Roman Empire, Constantinople (Istanbul) falls only in 1453 to the Ottoman Turks.
Then Pope Gregory XIII who reigned from 1572 to 1585, came up with the Gregorian Calendar to correct a problem with the Julian calendar, each year was short due to a calculation mistake some 15 centuries prior. So Pope Gregory decried that in all of Christendom, the correct year would be 1582 and not 2335 under the old Roman calendar. If we still followed the Calendar of Rome and the old Julian way of counting years based on the foundation date of the City of Rome we would be in 2771 and not 2018. I wonder if we would all look a lot older?
The problem, not everyone adopted the new calendar. The Russian Orthodox Church rejected this idea because they have their own Pope or Patriarch and do not recognize the Bishop of Rome as Pope. Russia and Greece continued with the revised Julian Calendar until 1900. In Western Europe since the ruling Prince or King had to decree a change in the calendar it took many years for every one to adapt to the new Gregorian Catholic Calendar. The German States is a case in point, some Principalities were majority Catholic, i.e. Bavaria, they changed the calendar in 1582 but others like Saxony were Protestant refused to do so and used a revised Julian calendar, eventually for mercantile reasons they too rallied to the new ways. So depending where you were in the World it was either one or the other Calendar. Of course many other calendars existed or still exist all around the world, Muslims 1440 and Hebrew 5778 and the Persian Calendar 1397, in China, Chinese New Year 2018″ is actually celebrating the onset of the year 4716, commemorating the reign of the Yellow Emperor, the asserted beginning of Chinese civilization by the ethnic Hàn Chinese. With the 20th century the world all got in synch finally. Funny how we take from granted the way we look at things today.
In 1582 some 11 days were added which explains the jump from 4 to 15 October