To answer this question, let's assume that history is regarded as a "number axis", with the first year of AD as the "origin", the forward number as "negative number" and the backward number as "positive number".

In detail, this problem is quite complicated, and we have to start with what is called "AD".

"AD" is the Gregorian calendar era, also known as the Christian era. The legendary year of Jesus Christ's birth is the first year (Latin abbreviation for "the year of the birth of the Lord"), which began in the sixth century.

Why should we name "the first year of A.D." after the year of Jesus' birth? This is related to the death of Jesus, who had a serious conflict with the Jewish rulers and was sentenced to be crucified. On the third day of the Jewish calendar (April 7, Gregorian calendar), Jesus Christ appeared before all the apostles.

Burn your face, also called Good Friday. But Christianity believes that Jesus died to atone for the sins of the world (from the Gospel of John) and went to heaven; At the same time, Jesus' mother is called "the Virgin Mary".

In 532 AD, Dionysius Execius, a Christian monk, put forward a suggestion that Jesus was born 532 years ago and the following year (533) should be the beginning of the calendar. This idea has been strongly supported by the church and used in the church.

By 1582, the Pope had formulated the Gregorian calendar and continued to adopt this chronology. Because of its high accuracy and international use, the Gregorian calendar is called "Gregorian calendar". From then on, the year when Jesus was born was called "the first year of A.D."

".

Every country in the world has its own chronology, so it is very inconvenient to calculate its calendar after intercommunication. For example, the Islamic era used "the origin of ambition"; Jews use the Jewish era; Buddhist countries use the Buddhist calendar era; Myanmar and Thailand use the Myanmar Calendar (Gache).

Like a car) era; Japan uses the Japanese era and so on, so all countries adopt the Gregorian calendar.

In China, from the records of the Shang Dynasty, there was a chronology method of various countries, and until the establishment of the People's Republic of China (PRC) (PRC), they all used their own chronology. After the founding of New China, China's calendar was in line with the world and unified into "Gregorian calendar".

By analogy, "the first year of A.D." is equivalent to the first year of the reign of Emperor Ping of the Western Han Dynasty in China. The year from here is called how many years BC, which becomes the "negative number" of the calendar year. Later, it was called how many years A.D., which answered our question "Why BC?"

Is the century going backwards? And the century after A.D. should follow the numbers? "

Reference: A Concise Encyclopedia of Britain by China Encyclopedia Publishing House; Editorial Manual of Heilongjiang People's Publishing House