The starting point of the A.D. calendar is 1, but there is no "A.D. 0", so most scientists who have a full understanding of the A.D. calendar and the most authoritative astronomical institutions in the world clearly support the statement that 2 1 century began at 200 1. A. D. is the abbreviation of "Gregorian calendar era", which is an international calendar system. The legendary birth year of Jesus Christ is the first year of Gregorian calendar (equivalent to the first year of Emperor Ping of the Western Han Dynasty in China). The rise of Christianity in Europe has gradually become the dominant religion. In order to expand the ruling power of the church, a Christian monk named Jonesi strongly advocated the birth of Jesus as the era, which was strongly supported by the church. The year when the Bishop of Rome officially announced the birth of Jesus, the God worshipped by Christianity, was designated as the AD year, so some people called it the Christian era. Before the birth of Jesus, it was called "BC" (the abbreviation of Latin Anno Domini in A.D., meaning "the year of the Lord's birth"), and after the birth of Jesus, it was called "After A.D." (the English abbreviation of BC, meaning "before Christ"). In western countries, the AD calendar year is widely used around 1400. China adopted the Gregorian calendar month and day (19 12) one year after the Revolution of 1911, but at the same time adopted the year of the Republic of China. The calendar year of People's Republic of China (PRC) adopts the calendar year system of most countries in the world, which was decided at the first plenary session of China People's Political Consultative Conference in September 1949. At the meeting, everyone agreed to adopt the common calendar system in most countries in the modern world, that is, adopt AD as the calendar year of new China. On September 27th, the second of the four resolutions adopted at the first plenary session of the China People's Political Consultative Conference was: "The year number of People's Republic of China (PRC) adopts AD". There are many dating methods in the world, but at present the most commonly used method in the world is the AD dating method. In addition, there are chronologies of cadres and branches, astronomical chronologies, historical chronologies, and chronologies of emperors' titles. In addition, there are Islamic times, Buddhist times, Jewish times, Greek times, Japanese times and so on. In China, as early as 2000 BC, it had its own calendar. For a long period of history, China adopted the "era method of cadres and branches", that is, the ten-day cadres and the twelve-day branches were combined into one, and every 60 years was a cycle.