They're half-way around the Earth from each other,
so that must be 12 hours.
The International Date Line and the Prime Meridian both separate the eastern from the western hemisphere.
12
24 Hours. When you pass the international date line which is the meridian of 180 longitude, you add a day if going west or subtract a day going east.
The second name for the prime meridian is the international date line
By definition and international agreement, the Prime Meridian is Zero longitude.All longitudes on earth are measured east and west of the Prime Meridian.
The prime meridian
The prime meridian separates the Eastern Hemisphere from the Western Hemisphere.
The Prime Meridian, which is why it is known as the Greenwich Meridian.
The Greenwhich meridian also Prime Meridian is 0 degrees longitude and International Date Line roughly follows the 180° longitude or antimeridian. The antimeridian is the meridian which is 180° east or west of the Prime Meridian.
The longitude of the Prime Meridian, by international definition and agreement, is zero (0°). .Every point on the Prime Meridian has a different latitude, and every latitude on Earthhas a point on the Prime Meridian.
The spelling is "prime meridian". That is the meridian from which counting begins; i.e., the prime meridian, by definition, has longitude zero, and degrees are counted to the east and to the west of the prime meridian. In the international system currently used, that is the Greenwich meridian - a meridian that goes through the Greenwich Observatory in London.
Prime meridian, International Date Line, time zones