Date Line

What is the Date Line?

The Date Line (usually called the International Date Line), is a north-south line through the Pacific Ocean where, according to international agreement, the date changes. East of the line it is one day earlier than it is to the west.

The line is necessary because earth is divided, longitudinally, into 24 one-hour time zones (15 degrees longitude each) which make one full day on the earth. Since the earth rotates eastwards, the time on the clock progresses westward round the world. Thus, 12 o’clock noon arrives in London (0 degrees longitude) five hours before it does in Washington, D.C. (75 degrees west of London) and eight hours before it does at San Francisco (120 degrees west of London). When it is noon in London it is midnight 180 degrees to the west.

On either side the 180^th meridian the time is same. But you would lose a day if   you crossed it from the east and gain one if you traveled the west.
The Date line has some variations from the 180th meridian to allow for land areas or islands. The line bulges eastward through Bering Strait to take in eastern Siberia and then westward to include the Aleutian islands with Alaska. South of the equator it bulges east again  to allow various island groups to have the same day as New Zealand.