Time zone
What is a time zone?
A political stripe painted on a rotating ball. Noon is where the Sun is overhead. A zone is an argument about how far from that line a country is willing to sit, so clocks in that stripe all say the same hour. See it on the world clock and the day/night map.
Noon is a place
If Earth had one clock, noon would travel west around the planet every day and the date would never know where to jump. Zones freeze a whole stripe at one hour so neighbours can agree.
The stripes are not honest meridians. China is one zone. India is a half-hour off everyone. Those are political choices, visible on the world clock.
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The map is the reason
The day/night map is why zones exist: the terminator is a moving line, and every zone is a bet about how far from the overhead Sun a country will let its clocks sit.