The watch movement does two things. It rotates the hands so that they complete two 12 hour cycles every 24 hours. It also advances the day and date wheels at the end of one of those 12 hour cycles. You as the watch owner have to advance the hands until the day and date click over as they will at midnight. Then you can set the hands to your local time and the watch will keep time and advance the day and date at midnight.If it is determined by just that period, then how does the watch determine if it is am or pm?