How was the first year of AD determined?

It is based on the year when the legendary Christ Jesus was born, which happened to be the first year of the Yuan Dynasty in China. In this year, before this year, it was how many years BC, and after that, it was called how many years AD.

Extended data:

Xinyounian is one of the sixty years in the official calendar, and the corresponding Gregorian calendar years are 192 1, 198 1, 204 1 (60-year cycle). In AD, if the number of years is divided by more than 60 1, or the number of years is reduced by 2, the remainder after division by 10 is 9, and the remainder after division by 12 is1. From beginning of spring in that year to beginning of spring in the following year, it was a "heart-warming year".

Lunar calendar is a dry calendar, and solar terms are used to guide agricultural production. Branches and branches are not divided into solar terms, regardless of traditional habits or official calendars (see the perpetual calendar compiled by Purple Mountain Observatory of Chinese Academy of Sciences for details). The so-called chronology of trunks and branches means that trunks and branches are literally equivalent to trunks and branches.

In ancient China, the sky was dominant and the earth was subordinate. The connection between heaven and stem is called heavenly stem, and the connection between earth and stem is called earthly branch. Together, it is called heavenly stems and earthly branches, or "dry branch" for short. There are ten heavenly stems, namely A, B, C, D, E, Ji, G, Xin, Ren and Gui, and twelve earthly branches, namely Zi, Ugly, Yin, Mao, Chen, Si, Wu, Wei, Shen, You, Xu and Hai.

The ancients put them together in the order of Jiazi, Ugly B and Bingyin (that is, the sky turns six times and the earth turns five times, which is just a cycle). Sixty pairs from Jiazi to Guihai are called Jiazi. The ancient people in China used these 60 pairs of branches to represent the serial numbers of years, months, days and hours, and this is the chronology of branches.

References:

Baidu encyclopedia-the first year of AD.