Every element of the clock you are reading this on was invented by somebody, somewhere, for a reason that usually had nothing to do with what it became.
Cairo gave us the day. Egyptian astronomers divided the sky into 36 star groups — decans — and used them as a night clock: twelve were visible across a full night, so the night got twelve hours and the day got twelve to match. That is where the 24-hour day comes from, and it has never been revised. The same system produced a 365-day civil year, which Egypt then declined to give a leap day, so its calendar wandered backwards through the seasons and took about 1,460 years to realign with the rising of Sirius.
Athens keeps the machine. The Antikythera mechanism, pulled from a shipwreck in 1901 and recognised as a geared device in 1902, was built around 150–100 BC. It predicted eclipses, tracked the moon's irregular motion, showed the planets, and counted down the Olympic cycle. Nothing of comparable complexity appeared for 1,400 years.
Rome gave us the calendar. In 1582 Gregory XIII deleted ten days and, more importantly, fixed the leap year rule: divisible by four, except centuries, unless divisible by four hundred. That rule is running silently inside every device on Earth.
Chicago gave us the time zone. Before 18 November 1883, North America ran on roughly three hundred local solar times. The railways simply imposed five zones on a continent (four in the United States) without asking any government. Chicago's clocks were set back nine minutes and thirty-two seconds, and the city had noon twice that day.
London gave us the zero. In 1884, delegates from twenty-five countries voted that longitude zero would run through the Airy Transit Circle at Greenwich. Every time zone on Earth is measured from that line — which, as GPS later revealed, is about 102 metres from where the modern reference meridian actually falls.
Paris gave us the metre, by measuring an arc of the meridian from Dunkirk to Barcelona through a revolution and a war. It then kept the word "Greenwich" out of its time law altogether: from 1911 legal time was "Paris mean time, retarded by 9 minutes 21 seconds", until UTC replaced the phrase in 1978.
Wellington gave us daylight saving, in the form of a paper read to the Wellington Philosophical Society in 1895 by an entomologist who wanted more evening light to catch moths. He was laughed at.
Berlin gave us daylight saving in the form that actually happened: an order issued on 6 April 1916, which took effect on 30 April, to save coal in wartime. Within two months most of Europe had copied it, including Britain, France and Portugal, all at war with Germany.
Copenhagen gave us the most ambitious clock ever designed by one person — Jens Olsen's Verdensur, twelve movements, 15,448 parts, wound by hand every week, with a gear that completes one revolution every 25,753 years.
Zurich gave us the most disciplined one: a station clock whose second hand is deliberately wrong for 58.5 seconds out of every 60, arriving early and waiting, so that it can be exactly right at the only instant that matters.