Why 18 hours?
We can make 24 hour clocks, and we will if you want, but most people will never see the nighttime section - they’ll be in bed asleep. And if they did look for it, well, it’s dark at night, so…
…instead, we can put (optional) glow-in-the-dark dots on the six nighttime hours, and on the pointer. The pointer moves slowly back to the left during the night. You can actually read the time in the middle of the night without turning on the light!
Another point:
On an 18-hour face, if the pointer is halfway across, it's 3 PM. Your day is half done. Sensible.
On a 24 hour face, the pointer halfway is 6PM .. so your day is 2/3 over. Not quite so sensible.
It’s not obvious, but the 18-hour layout is a much better idea for most people.
(We can actually program any pattern of day/night hours. Want it to start at noon and go to 6 AM? OK. You rock star, you...)