The problem is this one in the picture.

As follows, mainly look at the third paragraph.

Heavenly stems and earthly branches is a 60-year cycle, which means that your 1990 calendar can be used in 2050 after 60 years. Note: The lunar calendar is marked here.

Heavenly stems and earthly branches, the main branch for short, originated from the observation of astronomical phenomena in ancient China. Ten dry words: encounter, enlightenment, gentleness, firmness, harmony, Tu Youyou, shangzhang, heavy light, mystery and sunshine. Twelve branches say: sleepy, daring, taking photos, single-handedly, Xu Bao, Yeluo, Dunyi, Qitan, Dayuanxian. ?

Simplified heavenly stems and earthly branches: "A, B, C, D, E, Ji, G, Xin, Ren, Gui" is called ten heavenly stems, and "Zi, Ugly, Yin, Mao, Chen, Si, Wu, Wei, Shen, You, Xu, Hai" is called twelve earthly branches.

Ten heavenly stems and twelve earthly branches are collocated 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, which is used in calendar, art, calculation, naming and so on.