I don't have a GMT Master ii so I can't compare on the exact same movement. However, I checked the Caliber 3135 as used in my Sea-Dweller 4000, Submariner Date, and Datejust, and could confirmed the date change was precisely at 12 a.m. If you were concerned on the date change tolerance time, am pretty sure that you could send it back and Rolex would put it back on good time, all pun intended. And you would further get the Rolex green leather pouch FOC after the servicing.