The first time I went to the US, barely an hour after landing, I was in whatever-the-name-the-station-close-to-the-jfk-airport, waiting for a train to get to downtown NYC and the only other guy in the station came close and asked me if I could tell him the time.

I was pretty self conscious about spoken English back then, so I said "sure!" and just raised my phone screen, showing it was 19:30.

The guy made a weird face and asked what was that about, and there, dear reader, I found out that they don't use 24h clocks in the US and that I was up for an avalanche of small cultural shocks in the next few days.