The original patent (found on Twitter)...

Inventor of the clock: "There will be 12 numbers on it."
Friend: "So the day will be divided into 12 segments?"
Inventor: "No, 24."
Friend: "So will the day start at 1?"
Inventor: "The day will start at 12, which is at night."
Friend: "Err..."
Inventor: "The 6 means 30."

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I hate to be that guy, but I am pretty sure the first mechanical clocks had 24 hours.  The 12 hour clocks came later...

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martini

I hate to be that guy, but I am pretty sure the first mechanical clocks had 24 hours.  The 12 hour clocks came later...

Yeah, but his story is funnier!!!