Except the author of the joke is European, and 24-hour time on this side of the Atlantic is nothing special (or "military") at all, not even worth making a joke about. The problem with the sign was typographical -- the Underground people wrote 1700 rather than 17:00.
But the more I dig myself into this argument, the more I seem like the person I knee-jerked against, so, um ...
I guess it was just that phrase. "military time".Claes Wallin (韋嘉誠) at 2016-09-17T15:57:33Z