Every decade is called a "decade", for example, 2x years is the twenties, 9x years is the nineties, and so on. But the first twenty years of every hundred years are generally not called "ten years", but the beginning of a century.
Every hundred years is called a century, but the first century BC and the first century BC are 99 years, that is, the first year of AD ~ 99 is the first century, the second century is 100 ~ 199, and the first century BC is 99 ~ 1, and so on.
China began to adopt the AD calendar from 1949.
Nowadays, textbooks actually convert various chronology methods into AD chronology to express historical time. Therefore, AD chronology is a chronology method that students must master.
Two concepts related to advertising calendar are "century" and "year". Every century is 100 year. Theoretically, 1 ~ 100 is the first century, 10 1 ~ 200 is the second century, and so on, but according to idioms, people often refer to a certain 100.
For example, the year 100 was not regarded as the last year of the first century, but as the first year of the second century. In this way, the remaining 99 years of the first century became 1 ~99, and every century after that became 100, that is, it became 100 ~ 199 in the second century and 200 ~299 in the third century.