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 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!
It’s not obvious, but it’s a much better idea for most people.