Why is the yellow calendar different from the calendar?

Gregorian calendar is an ancient calendar in China, which takes the position of the sun in the zodiac as the benchmark and uses its branches to represent the year, month and day. It was used at the beginning of the founding of People's Republic of China (PRC), but now it is only for people. This calendar, also known as the AD calendar, is a calendar from the birth of Jesus to 20 10 this year, and now it is an international calendar.