The International Date Line is an imaginary line extending from the North Pole to the South Pole. It passes through the Pacific Ocean where, according to an international agreement, the date changes. As the Earth rotates, each day begins and ends on this line.
The date line is a consequence of the worldwide use of time-keeping systems arranged in such a manner that the local noon corresponds approximately to the time at which the Sun crosses the local meridian of longitude. The line is necessary because the Earth is divided longitudinally into 24 one-hour tie zones (15 degrees longitude each) which make one full day on Earth. Since Earth rotates eastward, the time on the clock progresses westward round the world. Thus 12 O’clock noon occurs in London (0 degree longitude) five hours before it does in Washington D.C. (75 degrees west of London), and eight hours before at San Francisco (120 degrees west of London). So, when it is noon in London, it is midnight at the place 180 degrees to the west.
On either side of the 180th meridian, the time is the same. Let us take the example of a traveller going completely around the Earth, carrying a clock that he advanced or retarded by one hour whenever he entered a new time zone, and a calendar that he advanced by one day whenever his clock indicated midnight. When he returns to his starting point he would find that the date according to his calculations was different by one day from the date kept by persons who had remained at the starting point. That’s why a traveller moving eastward across the line sets his calendar back by one day than the one travelling westward. It has a strange effect. That means if you cross the date line going eastward, you gain a day while those travelling westward lose a day.
The date line, however, has some variations from the 180th meridian to allow for land areas or islands. The line bulges eastward through the Bering Strait to take in eastern Siberia and then westward to include the Aleutian Islands with Alaska. South of the equator, it bulges eastwards again to allow various island groups to have the same day as that of New Zealand.