You can bury stuff like setting the time into a menu, people will accept it. They know that there will be a way to set the time. But if the 12/24 is hidden, whatever is picked as the default will drive away a noticeable fraction of those would-be buyers who prefer the other setting. For starters, pretty much everybody who does not get pulled into a sales person interaction that's sufficiently deep to touch the subject. And that sales person interaction won't be the best possibly, you don't want to bore them with nuisance settings, you want to show off The Stopwatch (or whatever was the flagship feature of the watch in question)