Most of us who use the 24-hour clock as the norm have no trouble (or even consciously think) about converting it into "o'clock". Like, now it's 19:21 hours, which is 21 minutes past 7. Don't care which it is.

Agreed. If I'm at the PC and my wife asks me the time, I will read off the PC's display, in 24-hour format. Just to add to the fun, we also have a "left-handed clock, in which the definitions of clockwise and anticlockwise are the opposite of those considered the norm.