Since there are 10 heavenly stems and 12 earthly branches, it should be 120 combinations intuitively.
But it's not true. Because of the greatest common factor, there are only 60 combinations.
This tiny program helps you to check if a combination truly exists, just enter it.
karakarasuuuu/sexagenary-cycle-checker
a.k.a. 干支。It is a program to check if a combination is valid.
Python