Time zones grew out of railway scheduling problems in the 1800s. North American railroads adopted standard zones in 1883, and an 1884 conference in Washington set Greenwich as the world's zero meridian.
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The seven-day week has no single agreed origin. It likely draws on Mesopotamian sevens, the Jewish Sabbath cycle and the Roman planetary week, and Constantine made it official in AD 321.
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Pope Gregory XIII decreed the reform in 1582, dropping ten days and cutting leap years. Catholic countries switched at once, while others took until 1752 or the 20th century.
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The usual explanation is that early clocks copied the shadow of a sundial in the northern hemisphere. The link is plausible but not firmly documented, and there is a notable objection.
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The 24-hour day comes from ancient Egyptian and Greek practice, while 60 minutes and seconds come from the Sumerian and Babylonian base-60 system, kept for its convenient divisibility.
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