The day actual doesn’t start at midnight. Officially the astronomical day starts at noon, when the sun is highest in the sky. This is exactly at opposite side, of the earth, of Meridian 0^ in Greenwich. So it is 12 midnight in Greenwich, and exactly 12 noon on the other side of the world. Midnight is the start of our what we consider a ‘day’, and high noon is what is traditional considered the start of the astronomical day.