Mercury II has two modes of clock, GMT on or off. If I turn GMT on and set my time zone, it is 20 minutes ahead. If I turn GMT off and just set the clock like any stand alone digital clock, it is very inaccurate. It runs slower and it is about 5 minutes behind in just 12 hours. Has anyone with a mercury II noticed this problem? How exactly does its GMT on mode work and why does the GMT mode have an offset you can program?