中国古代的纪年法是帝王年号与“干支”并用，其中“干支纪年法”从史书上有明文记载的公元前841年（庚申年），一直沿用到现在。
“干”是“天干”，有十个字组成，这十个字是：甲、乙、丙、丁、戊、己、庚、辛、壬、癸。“支”是“地支”，由十二 个字组成，这十二个字是：子、丑、寅、卯、辰、巳、午、未、申、酉、戌、亥。把天干的十个字和地支的十二个字按顺序配合起来，可以得到60种排列，如：甲 子、乙丑、丙寅……
这60种排列周而复始，循环使用，每六十年叫做“一个甲子”。如公历的2001年，是农历的辛巳年，公历的2002 年，是农历的壬午年；而六十年后，2061年又是辛巳年，2062年又是壬午年。从东汉（25——220）时开始，人们又用鼠、牛、虎、兔、龙、蛇、马、 羊、猴、鸡、狗、猪12种动物来配十二地支，组成了十二生肖，也叫十二属相。这就是子鼠、丑牛、寅虎、卯兔、辰龙、巳蛇、午马、未羊、申猴、酉鸡、戌狗、 亥猪。这样，子年是鼠年，丑年是牛年，寅年是虎年…… 于是，每个人一出生，就有一种动物作他的属相。子年出生的属鼠，丑年出生的属牛，寅年出生的属虎，以此类推。
现在，中国人在用公历纪年和计算年龄的同时，仍然习惯用属相纪年和推算年龄。
English introduction: Twelve Symbolic Animals
It is traditional in China that when a person is born, one animal (shuxiang)is used to symbolize this year. Shuxiang,also called shengxiao(any of the 12 animals representing the Earthly Branches),is a traditional way in China to number the years and to record a person's age.
The ancient Chinese people invented the method to designate the years by the Heavenly Stems consist of the characters: jia, yi, bing, ding, wu, ji, geng, xin, ren, and gui. And the Earthly Branches are composed of 12 characters: zi, shu, yin, mao, chen, si, wu, wei, shen, you, xu, hai.Combining each of the 10 Heavenly Stems with one of the 12 Earthly Branches in sequence creates 60 chronological symbols. For example jiazi, yichou, bingyin, etc. These 60 symbols are used in circles and thus each year has a chronological symbol.
For example,2001 corresponds to xinsi, 2002 to renwu in the lunar calendar; after 60 years, 2061, once again, corresponds to xinsi, and 2062, to renwu. Later, people used 12 animals to correspond to the 12 Earthly Branches, forming the 12Symbolic Animals, namely Earthly Branch One Rat, Earthly Branch Two Ox, Earthly Branch Three Tiger, Earthly Branch Four Rabbit, Earthly Branch Five Dragon, Earthly Branch Six Snake, Earthly Branch Seven Horse, Earthly Branch Eight Sheep, Earthly Branch Nine M0nkey, Earthly Branch Ten Rooster, Earthly Branch Eleven Dog, Earthly Branch Twelve Pig. Thus the zi year is the Year of the Rat, and the chou Year is the year of the Ox, and the yin Year is the Year of the Year of Tiger, etc. Therefore, when a person is born, he has an animal as his aymbolic animal. The year 2002 was a renwu year under lunar calendar, also the year of Horse, and so children born in this yea are all Horse babies.
Even though the Chinese people now number the years and their age under the Gregorian calendar, the still continue to use the symbolic animals.As long as people know a person' probable age and his symbolic animal, people can infer his exact age and year of birth.
