[volemo]

I was also confused why the outer ring must only show zero or one, and then it struck me — twelve hour system.

[ray__]

The author wanted it to retain some practical value, hence the discussion of the four “layers” of time—departing from the 12-hour system completely, even if there is a better way to represent time outside of it, would make the clock difficult to use.

[wongarsu]

I struggle to see how this clock would be less practical to use with a 24 hour system

Or if we want to keep the numbers small to make them easier to read in this clock's numerals, why not a 6 hour system?