I am having the same problem with LM12. I've been using Cinnamon for a while now. The problem showed up a few days ago, perhaps after the most recent update for Cinnamon showed up in the Update Manager. I verified that the setting had not been switched. I clicked it over to 24 hour, closed, and then went back to 12 hour but still no change. It's only a minor thing but a fix would be nice.
I was a bland little boy until I discovered the joys of weirdness. - John Kricfalusi in a blog entry dated August 29, 2007