Since the world is approximately a sphere, and the sun is only on one side of it, in some places it's light and in some places it's dark. So people decided that every so often they'd redefine when the time is, so that "noon" is during daylight hours, and "midnight" is sometime in the dead of night. Kinda.
See the TimeNotes page for information on TimeZones as applied to your OperatingSystem.
2 pages link to TimeZone:
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