Both BC and AD are foreign chronological symbols. Why did China use them in ancient history? (For example, Qin Shiwang used BC22 1 year to unify the six countries? )

I think it may be your misunderstanding about this problem. BC and AD are international chronological systems, not foreign chronological symbols. AD is often represented by a.d. (Latin abbreviation of Anno Domini, meaning "the year when the Lord was born"), while BC is represented by b.c. (abbreviation before Christ, meaning "before Christ").

This chronological system began to be adopted by European countries. China adopted the Gregorian calendar month and day from the year after the Revolution of 1911 (19 12), 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 through consultation 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 A.D. calendar is A.D. 1 year, and there is no "A.D. 0", 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 year.