Calculating the chronological order (ordinal number) that someone is experiencing after birth is a traditional way of calculating age in China, and it is also popular in other East Asian countries. I was one year old when I was born, because it was the first year after birth, and then on the first day of the first lunar month, I was one year older every day. The lunar calendar used in ancient China (actually, it is the lunar calendar, not the lunar calendar) is arranged according to the moon phase cycle. However, in combination with the year calculated with reference to the position of the sun, leap months may occur due to the adjustment time error. But the ancients didn't celebrate their birthdays twice in the same year. In addition, the ancient calendar will be adjusted in terms of counting days due to special circumstances such as the change of dynasties and the accession of new emperors, so "emphasizing young days" is the biggest feature of the imaginary age algorithm. It is different from the current international age calculation method, that is, calculating the length of time after someone is born. In modern times, in order to distinguish the two age calculation methods, there have been two names: nominal age and full age.