Greenwich Mean Time

By international agreement, the time of day at the prime meridian (0°), called Greenwich mean time (GMT), sets the standard for the rest of the world. The Sun moves 15° each hour, making the time one hour earlier every 15° west of the prime meridian. When it is 3 pm at Greenwich (longitude 0°), at longitude 75° west (near New York City) the time is five hours earlier (10 am), and at longitude 120° west (near Los Angeles, California) it is eight hours earlier (7 am). Where longitude 180° west meets longitude 180° east in the Pacific Ocean is the international date line. There, the date changes by one day. When it is 10 am Monday on the east side of the international date line, it is 10 am on Tuesday on the west side of the line.