I mean, do they say "I sleep at 9pm" or more like "I sleep at 2100 hrs" even while they are talking informally? 2100 hrs sounds very formal to me, but yeah, I was just wondering if they used 24 hour format for only official and government proceedings and used 12 hour format for casual stuff.
As far as I know, no one ever says "I sleep at 2100 hrs (twenty-one hundred hours)". We say "I sleep at 9 o'clock" or "at 21 o'clock"). Then of course change o'clock with your lagnuage's equivalent such as "uur" or "Uhr" in Dutch and German for example. Pm or am is almost always derived from context, and if it's not you can add "in the morning/afternoon/evening".
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