Mental arithmetic is in fact not universal. People who grew up on 24h time have the benefit of rote memorizing the relationship between 17 and 5 from childhood.

Like, the fact that 9:00 (or 21:00) points leftward / “west” with the hour-hand is something that kids just have to learn and remember. At first it is a lot of mental overhead. But I bet it was immediate for you when you read this, no?