China used to have 5 different time zones during the Nationalist era. However, in 1949, Mao Zedong, the paramount leader of China at the time, decided that all of China was to use Beijing Time. Mao implemented this policy as a way to unify the new nation after years of division under various local warlords. 

While it is strange for a country as large as China to have a single time zone, it hasn't cause major economic or social problems so far, other than annoyances such as the strangeness of having the sun be out at 10PM in the westmost regions of China.
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