The specific dates of the three volts are determined by matching the dates of the festivals with the dates of the dry branch chronology. The traditional method of calculation in China stipulates that the third and fourth geng days after the summer solstice are the beginning of the first (first) and middle (second) volt, respectively, and the first geng day after autumn is the first day of the last (third) volt. Geng day is the dry branch of the day with the word “G”, such as Geng Zi, Geng Yin, Geng Chen …… because the third G day, the fourth G day that is the first day of the first and middle volt, and each G day is separated by 10 days, so the time of the first volt for 10 days, the last volt provisions are also 10 days. The mid-volts can be long or short, maybe 10 days or 20 days. Each year, the third G day after the summer solstice (the first ambush) appears late or early, so the number of days in the middle ambush is not the same, so there is a difference of 30 days in some years and 40 days in others.