The 24 > 12 hour time shifting problem has happened off and on to my 5.2 and 5.3 over the last couple of years, both with Tiger and Leopard. Seems to be random, and my fix is to quit NUD, reselect 24 hour time format in System Preferences (International - same as it was before), and relaunch NUD. Usually works - if not, I just do it again, and wait for the next hiccup, which usually occurs once every two months or so.
Your post caused us to look at this again. I confirmed from before that if I change the format (System Preferences->Date & Time->Open International, Formats), then set the time to be the 24 hour clock, NUD is not using that format.
However, if I first set the Region to be "United Kingdom", in NUD the 24 hour clock is supported. If I don't like the UK default settings, I can further customize them to use the US Dollar, reformat to month/day/year (rather than day/month/year), etc. If I start with the US region and then change to the 24 hour clock, NUD doesn't recognize the format change.