|八字 BāZì: The eight characters. “八 Ba” means eight, and “子 Zi” means character. The eight Chinese characters are derived from a lunisolar calendar, which is known as “夏历 XiàLì Summer/Solar Calendar, the 万年历 WànNiánLì Ten Thousand Year Calendar, or "Farming Calendar,” which indicates the season’s transition time and planetary cycles at a given time.
The 夏历 XiàLì calendar is based on a sexagenary cycle (六十花甲 LiùShí HuāJiǎ) also called (干支 GānZhī) the Stem-Branch cycle. Aligning the (Shí TiānGān 十天干) the 10 Heavenly Stems cycle with the (Shí'èrDìzhī 十二地支) 12 Earthly Branches cycle in a specific order results in 60 cycle combinations.
Each of the 60 cycles is recorded in a stack of two characters; the upper character is taken from the 天干 10 Heavenly Stems cycle, and the lower character is taken from the 地支 12 Earthly Branches cycle. These stacked characters for the four time units; year, month, day, and hour—are known as the four pillars.
Since each time unit or pillar consists of two characters, the total of the four pillars consists of eight characters; therefore, this system for timekeeping is called 八字 BāZì the eight characters.