Month is calculated according to solar terms.

In the Book of Changes, the algorithm of each month is not based on the first day to the thirtieth day, but on festivals. For example, in the first month, beginning of spring and beginning of spring, the festivals will be counted in silver moon, even if it is surprised in mid-February, it will be in Uzuki and even in Tomb-Sweeping Day in March. . . . . The boundary of a year is the beginning of spring.