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 the earth is divided, longitudinally, into 24 one-hour time zones (15° 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° longitude) five hours before it does in Washington, D.C. (75° west of London) and eight hours before it does at San Francisco (120° west of London). When it is noon in London it is midnight 180° to the west.
On either side of the 180th meridian the time is the same. But you would lose a day if you crossed it from the east and gain one if you travelled across it from the west.
The Date Line has some variations from the 180th meridian to allow for land areas or islands. The line bulges eastwards through Bering Strait to Take in eastern Siberia and then westward to include the Aleutian Islands with Alaska. South of the equator bulges east again to allow various island groups to have the same day as New Zealand.
Picture credit: google