The Heavenly Stems are associated with Yin Yang and the Five Elements. Recent 10-year periods began in 1984, 1994, and 2004. The Earthly Branches are associated with the twelve signs of the zodiac. Each Earthly Branch is also associated with an animal, collectively known as the Twelve Animals. Recent 12-year periods began in 1984, 1996 and 2008.
Within the Heavenly Stems system the year is advanced up by one per year, cycling back to year one after the last (year ten). Similarly the Earthly Branches also advances by one per year, cyclically. Since the numbers 10 (Heavenly Stems) and 12 (Earthly Branches) have a common factor of 2, only 1/2 of the 120 possible stem-branch combinations actually occur. The resulting 60-year (or sexagesimal) cycle takes the name jiǎzǐ (甲子) after the first year in the cycle, being the Heavenly Stem of jiǎ and Earthly Branch of zǐ. The term "jiǎzǐ" is used figuratively to mean "a full lifespan"—one who has lived more than a jiǎzǐ is obviously blessed. (Compare the Biblical "three-score years and ten.")
At first, this system was used to mark days, not years. The earliest evidence of this were found on oracle bones dated c.1350 BCE in Shang Dynasty. This system of date marking continues to this day, and can still be found on Chinese calendars today. Although a stem-branch cannot be used to deduce the actual day in historical events, it can assist in converting Chinese dates to other calendars more accurately.
Around the Han Dynasty, the stem-branch cycle also began to be used to mark years. The 60-year system cycles continuously, and determines the animal or sign under which a person is born (see Chinese Zodiac). These cycles were not named, and were used in conjunction with regnal names declared by the Emperor. For example: 康熙壬寅 (Kāngxī rényín) (1662 AD) is the first 壬寅 (rényín) year during the reign of 康熙 (Kāngxī), regnal name of an emperor of the Qing Dynasty The months, days and hours can also be denoted using Heavenly Stems and Earthly Branches, though they are commonly addressed using Chinese numerals instead. In Chinese astrology, four Stem-Branch pairs form the Eight Characters (八字 bāzì).
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