My phone, along with 2 other people who have the exact same phone have had at least 3 different times in the past month where this occurs. We have the Motorola Droid Razr Maxx HD (XT926M). For the one questioning on whether we mean at half past two in the afternoon it displays 14:30, yes that is what we mean. We go in we turn off the setting, which I agree is too many steps to take for any of us totally different individuals to have done this ourselves. (Pull down, press top right corner square, press setting, scroll down to date and time, and uncheck 'Use 24-Hour format'.
Something keeps turning this back on sporadically, and Googling shows it's not just our phone or the OP's phone, but I see no reason as to why, or what's going on. I have surmised it has something to do with an update, but have not figured out which update, or why.