Ganzhi Calendar & the Sixty Jiazi Cycle

What Is the Stem-Branch Calendar?

The Stem-Branch Calendar (Ganzhi Jinian) is the traditional Chinese calendrical system in which the ten Heavenly Stems and the twelve Earthly Branches are paired in sequence to form 60 combinations (the Sixty Jiazi), cyclically marking years, months, days, and hours. The system was first used to record days and later extended to recording years, continuing in use to the present day.

In Bazi, a person's birth year, month, day, and hour are all expressed in Stem-Branch notation, forming the Four Pillars. The Stem-Branch calendar is the foundational coordinate system for casting the Four Pillars.

The Sixty Jiazi — The 60-Year Stem-Branch Cycle

The ten Heavenly Stems (Jia, Yi, Bing, Ding, Wu, Ji, Geng, Xin, Ren, Gui) and the twelve Earthly Branches (Zi, Chou, Yin, Mao, Chen, Si, Wu, Wei, Shen, You, Xu, Hai) are paired sequentially. Their least common multiple is 60, producing the "Sixty Jiazi" cycle:

The Sixty Jiazi forms the foundation of China's traditional timekeeping system. A complete cycle spans sixty years — also the origin of the expression "Sixty Flower Jia" (liùshí huā jiǎ), the traditional term for a person's sixtieth birthday.

Calculating the Year Stem-Branch

Start of Spring as the Year's Beginning

In Bazi chronology, the Start of Spring (Lichun) (approximately February 3–5 each year) marks the year boundary, not the first day of the lunar new year. Those born before Lichun have their year Stem-Branch belonging to the previous year; those born after Lichun switch to the new year.

Deriving the Year Stem-Branch from the Gregorian Year

Heavenly Stem: Take the Gregorian year, divide by 10, and use the remainder to look up the stem (remainder 1 = Jia, 2 = Yi … 0 = Gui). Simplified formula: (Gregorian year - 4) mod 10 yields the stem index (1 = Jia, 2 = Yi, …, 0 = Gui).

Earthly Branch: Take the Gregorian year, divide by 12, and use the remainder to look up the branch (remainder 4 = Zi, 5 = Chou …). Simplified formula: (Gregorian year - 4) mod 12 yields the branch index (0 = Zi, 1 = Chou, 2 = Yin …).

Quick reference examples:

Calculating the Month Stem-Branch

Month Branches Are Fixed to Solar Terms

The Earthly Branch of each month is determined by the Solar Terms (the twelve Jie) and remains fixed:

Month Stem Determined by Year Stem — "Five Tigers Chase the Year"

The Month Stem is determined by the Year Stem of that year, using the mnemonic "Five Tigers Chase the Year" method:

Day Stem-Branch and Hour Stem-Branch

Day Stem-Branch

The Day Stem-Branch cycles every 60 days. Historically derived from a known reference date, in practice it is simply looked up from a perpetual calendar.

Hour Stem-Branch

The Earthly Branch of the Hour Pillar is fixed to the twelve two-hour periods of the day (Zi, Chou, Yin, Mao …). The Hour Stem is determined by the Day Stem, using the mnemonic "Five Rats Chase the Day" method:

Relationship Between the Stem-Branch Calendar, Lunar Calendar, and Gregorian Calendar

These three systems operate in parallel and are not interchangeable. Bazi uses Stem-Branch chronology as its sole coordinate standard; chart casting does not depend on the lunar month start or Gregorian year start.