Of course an accurate date measurement (if there was one) wouldn't class as as being the end/start of the 201st decade, in fact it would be a somewhat higher figure.
The CE/A.D. numbering system for years was invented by Dionysius Exiguus, a Scythian monk, in the year 525CE. His motivation was not to exalt the alleged year of Jesus's alleged birth, and in fact almost all scholars agree that if Jesus was a real person he was born at least a few years before 1CE and the friar got it wrong. He simply wanted to do away with the Diocletian system that was in use at the time, because it honored the memory of a tyrant who had persecuted Christians.
Over the next few centuries, as the stranglehold of Christianity tightened over European culture in what are now affectionately called the Dark Ages, influential historians and scholars, including the Venerable Bede, a major figure in Anglo-Saxon history, adopted Dionysius's system because at the time it was thought to measure time precisely from the start of the Christian Era. (A careful reading of the New Testament makes it clear that Jesus was also not born anywhere near New Year's Day, when it was snowing in Bethlehem and the tax collectors would hardly have been out traipsing around, but that's another issue.)
Europe was virtually synonymous with Christendom for more than a thousand years. (Nobody cared much about the Church of the East, often incorrectly referred to as "the Nestorians," which spread all the way to India before the Muslims overran the region.) So the Christian calendar became the standard way of rendering dates. As the Reformation, Renaissance, Enlightenment and Industrial Revolution propelled Europe into its Fifteen Minutes of Fame and European nations colonized much of the rest of the world, the calendar came with them.
Today even the Japanese, Chinese, Jews, and other peoples with their own calendars uncomplainingly use C.E. dates when communicating with us and with each other, for the exact reason that it is a
de facto international standard. However, the identifier C.E. ("Common Era," or "Christian Era" to the diehard Christians), which has always been used by the Jews, is increasingly replacing A.D.
(Anno Domini or
Anno Domine, "Year of (our) Lord") in other non-Christian nations, and is even making inroads in the Euro-American sphere, for example in secular universities.
But it doesn't really count anything so don't worry about it.
It also doesn't have a zero, making it a nightmare for arithmetic calculations. 1BCE (or B.C. to the diehard Christians) is immediately followed by 1CE.
