You would think, but in my experience you try to use 24 hr time and people suddenly can't comprehend numbers larger than 12, and its not worth explaining every time that 17:00 = 5pm. I just do the adjustment myself since 9/10 times they'll ask for clarification anyway

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?