For most of history, noon was when the Sun was highest — so every town’s clock was slightly different. Travelling 100 km east or west shifted local time by several minutes. Nobody minded, until trains arrived.
Railway chaos
Railway timetables had to list departures in dozens of local times. In Britain, railways adopted “railway time” based on Greenwich in the 1840s, and towns gradually followed. In North America, railways introduced standard time zones on 18 November 1883 — “the day of two noons”, when many clocks were reset.
Greenwich becomes zero
In 1884, delegates at the International Meridian Conference in Washington chose the meridian through the Royal Observatory at Greenwich as 0° longitude. Countries then gradually adopted standard times as offsets from Greenwich.
Not all whole hours
Time zones follow politics as much as geography. India uses UTC+5:30 (here is why), Nepal UTC+5:45, and China — about as wide as the continental US — uses a single time zone.
UTC today
Today the reference is Coordinated Universal Time (UTC), kept by atomic clocks, and computers store times in UTC and convert to local time for display — the reason Excel and databases sometimes show times shifted by 5½ hours.
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