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”.
It’s already a feature
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