Well, good luck if you ever travel outside of the USA, because you'll find that airports, train stations, and other public conveniences (not to mention convention programs, such as last years' Worldcon programming schedule (http://www.interaction.worldcon.org.uk/sched05.htm)) are mostly going to use 24-hour (mistakenly called by Americans "military") time.

I share Doug's impatience with people who can't parse 24-hour time. My wife gets nearly incandesent over it. She was sitting at the feedback session seething with frustration over people complaining about it. (She also wishes the USA would join the rest of the world on the metric system of measurements, too.)