The lunisolar Chinese calendar
How it works: The first combination of Heavenly stem and Earthly branch is 甲子, then it goes 乙丑, 丙寅, 丁卯, and so on taking the next from each respective row. When you reach the end of the 10 first Heavenly stems, just go back to the beginning and you start again with 甲戌, and so on until you have all 60 and end up back at the start. Each one of these combinations is a cycle.
Generally the Japanese use the Kun yomi reading of the combination to name the concerned0