It's too bad wms didn't put flipper codes in the games with midnight madness so you could try it out without the clock being an issue.
How does it keep time if you set the clock and leave the game on for a bit and then check the time? None of my wpc games keep accurate time when they're off. None have a midnight madness mode though and the only one that would concern me is the AFM rule the universe time and date, being inaccurate.... and in the grand scheme of things, not very important.