To explain how this cycle works, let us denote both the stems and the branches by their numbers. We denote 1 by (1,1) or (甲,子), 2 by (2,2) or (乙,丑) and so on up to (10,10) or (癸,酉). But now we have run out of stems, so we denote 11 by (1, 11) or (甲,戌) and 12 by (2, 12) or (乙,亥). Now we have run out of branches, too, so 13 becomes (3, 1) or (丙,子). We continue in this way through 6 cycles of stems and 5 cycles of branches up to 60, which is (10, 12) or (癸,亥). The next number is then (1,1) or (甲,子), which starts a new sexagenary cycle.

Great importance was attached to the celestial observation in making a new calendar during ancient China. The gnomon (a column for measuring the sun's meridian altitude) and sundial were the timekeeping instruments with the sun as the object for observation. Although simple structures, the two instruments could serve many purposes, and were said to be China' oldest astronomic instruments.

By the middle of the Spring and Autumn Period (770-446 BC), to use the gnomon for making a calendar had become an important calendar-making method, through which the conclusion of a tropical year being 365.25 days long was reached.

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Chinese lunar calendar
The Chinese lunar calendar is a traditional calendar in China and it is also called Xia Li  or Yin Li. It is based on exact astronomical observations of positions of the sun and moon. Thus in essence it is a combined solar/lunar calendar.

In the calendar, one year is divided into 12 months and the months have either 29 or 30 days, always beginning on days of astronomical new moons. So each year has 354 or 355 days. To make the average length of the years equal to a tropical year, an intercalary month is added every two or three years.

As a result, an ordinary year has 12 months while a leap year has 13 months. And an ordinary year has 354, or 355 days, and a leap year has 384 or 385 days.

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Jiéqì -Solar Terms
Chinese months follow the phases of the moon. The solar-based agricultural calendar is made up of twenty-four points called jieqi. They are essentially seasonal markers to help farmers decide when to plant or harvest crops, as the lunisolar calendar is for obvious reasons unreliable in this respect.