When I was a kid, I was such a nerd, that I invented my own decimal timekeeping system.

Even wrote a little macOS menubar clock for it — I was dead-serious.

Edit: omg the website still works, even though I never put any real content there …

http://yreality.net/UJD/

Edit 2: Found this old explanation I apparently put together in July 2010, according to my image archive:

12 1 2 3 4 5 6 7 8 9 10 11

The reason for 12-hour clocks is most cultures worldwide have variable length hours of over a year. For Western times this comes from Greeks who had 12 day and 12 night hours. Early water clocks in antiquity would attempt to make that adjustment automatically.