This is the problem.

As follows, mainly look at the third paragraph.

The combination of heavenly stems and earthly branches is a 60-year cycle, that is to say, 1990 in your calendar can be used in 2050 after 60 years. Note: The lunar calendar is marked here.

Heavenly stems and earthly branches, referred to as Ganzhi for short, originated from the observation of astronomical phenomena in ancient China. Ten dry words: encounter, enlightenment, gentleness, firmness, harmony, Tu Wei, Shang Zhang, heavy light, mystery, sunshine. The twelve earthly branches say: sleepy head, red heart, taking photos, one hand, holding beard, falling flower, Dunyi, Qi Xie, Qitan, and Dayuanxian. ?

The simplified heavenly stems and earthly branches: "A, B, C, D, E, Ji, G, Xin, Man and Ghost" is called the ten heavenly stems, and "Zi, Ugly, Yin, Mao, Chen, Si, Wu, Wei, Shen, You, Xu and Hai" are called the twelve earthly branches.

Ten heavenly stems and twelve earthly branches are matched in turn to form sixty basic units, and they cooperate with each other in a fixed order to form the chronology of the stems and branches. Heavenly stems and earthly branches's invention had a far-reaching impact, and heavenly stems and earthly branches still uses it today, for calendar, technology, calculation, naming and so on.