BC refers to after A.D.. Because this method of dating BC was not invented by China, it came from the west, and it was in A.D., so one year BC refers to the year after A.D.. The first year of A.D. was the reign of the Emperor of the Han Dynasty. So it was the emperor who began to adopt this chronology. Speaking of the invention of this chronology, there is another allusion. At that time, the emperors of western countries wanted to hold a grand ceremony to commemorate Jesus' birthday, but the emperors made a mistake. Then at this time, someone invented a new commemorative method to solve this problem, that is, the chronological method of BC. They call the year when Jesus was born the first year of AD, but we call the time when this method was introduced into China BC.

The 5,000-year history of China is calculated by chronology. But every emperor changed his title when he ascended the throne. They used his title to calculate their time. Every country has a different year number, but they calculate the time in the same way. They all use their own year numbers and their own time. So in the past, our country adopted the chronological method.

During the period of the Republic of China, China began to formally use the method of AD chronology. In fact, the original chronological method was introduced into China, but we didn't formally adopt it, but absorbed their experience to use our own chronological method. After the founding of the Republic of China, because we didn't have an emperor, there wouldn't be a title. They decided to change this method of chronology, then adopt the method of AD chronology used in the west, and then began to calculate time with this method of AD chronology.

In a word, the method adopted in our country belongs to our own chronological method, which was adopted in the late Republic of China.