How is the date of Jihai or Jiazi calculated? thank you

Don't count, there is a ready-made perpetual calendar query.

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For example, April 4th of the lunar calendar 198 1 year is the Gregorian calendar (1981May 7th), which is the first day of the fourth month of the lunar calendar.

There are ten heavenly stems, namely, A, B, C, D, E, Ji, G, Xin, Man and Ghost, and twelve earthly branches, namely, Zi, Ugly, Yin, Mao, Chen, Si, Wu, Wei, Shen, You, Xu and Hai.

The ancients put them together in the order of Jiazi, Ugly B and Bingyin (that is, the sky turns six times and the earth turns five times, which is just a cycle). There are 60 pairs from Jiazi to Guihai, which are called Jiazi.

The ancients in China used these 60 pairs of branches to represent the serial numbers of years, months, days and hours, and this is the chronology of branches.

For example: Gregorian calendar 198 1 April 16 is Jiazi Day, and April 17 is Ugly Day; In another 60 days, June 15 will be Jiazi Day and June 16 will be ugly days respectively.