<P> Imperial China continued to use ke and geng but also began to divide the day into 12 "double hours" (t 時, s 时, oc * də, p shí, lit . "time (s)") named after the earthly branches and sometimes also known by the name of the corresponding animal of the Chinese zodiac . The first shi originally ran from 11 pm to 1 am but was reckoned as starting at midnight by the time of the History of Song, compiled during the early Yuan . These apparently began to be used during the Eastern Han that preceded the Three Kingdoms era, but the sections that would have covered them are missing from their official histories; they first appear in official use in the Tang - era Book of Sui . Variations of all these units were subsequently adopted by Japan and the other countries of the Sinosphere . </P> <P> The 12 shi supposedly began to be divided into 24 hours under the Tang, although they are first attested in the Ming - era Book of Yuan . In that work, the hours were known by the same earthly branches as the shi, with the first half noted as its "starting" and the second as "completed" or "proper" shi . In modern China, these are instead simply numbered and described as "little shi". The modern ke is now used to count quarter - hours, rather than a separate unit . </P> <P> As with the Egyptian night and daytime hours, the division of the day into twelve shi has been credited to the example set by the rough number of lunar cycles in a solar year, although the 12 - year Jovian orbital cycle was more important to traditional Chinese and Babylonian reckoning of the zodiac . </P> <P> In Thailand, Laos, and Cambodia, the traditional system of noting hours is the six - hour clock . This reckons each of a day's 24 hours apart from noon as part of a fourth of the day . 7 am was the first hour of the first half of daytime; 1 pm the first hour of the latter half of daytime; 7 pm the first hour of the first half of nighttime; and 1 am the first hour of the latter half of nighttime . This system existed in the Ayutthaya Kingdom, deriving its current phrasing from the practice of publicly announcing the daytime hours with a gong and the nighttime hours with a drum . It was abolished in Laos and Cambodia during their French occupation and is uncommon there now . The Thai system remains in informal use in the form codified in 1901 by King Chulalongkorn . </P>

Where did the 24 hour day come from