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.

The 12 times table was taught because of the pound, 20 shillings to the pound, 12 pence to the shilling. I used to remember the 13, 14 and 15 times tables as well, but they've died for lack of use ...