AD, the Gregorian calendar era, is a chronological method that originated in western society. Julian calendar was originally called the Christian era, also known as the Gregorian calendar or A.D., and was reformed by Italian doctor and philosopher aloysius Lilius. 1582, the then Pope Gregory XIII approved its promulgation.

It began in the year when Jesus was born. In julian calendar and Gregorian calendars, the date after the birth of Jesus is called the principal year, Anodomini (AD) (Latin). Before the birth of Jesus, it was called "BC".

However, in order to dilute its religious color and avoid causing antipathy among non-Christians, modern scholars mostly change its name to Common era (abbreviated as C.E.) before Common Era (abbreviated as B.C.E).

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China chronology method:

1949 On September 27th, the first plenary session of China People's Political Consultative Conference unanimously adopted the AD calendar year in People's Republic of China (PRC). This chronological system began to be adopted by European countries. 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.

Because the starting point of the AD calendar is 1, there is no "AD 0", so most scientists who have a full understanding of the AD calendar and the most authoritative astronomical institutions in the world clearly support the statement that 2 1 century began at 200 1.

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