The International Date Line is an imaginary line of longitude on the Earth’s surface located at about 180 degrees east (or west) of the Greenwich Meridian. The imaginary dateline runs through the Pacific Ocean.
International date line marks the spot on the earth’s surface where each new calendarday begins. Because the earthrotates on its axis, the sun is always rising and setting someplace on earth, and the day is just beginning or ending.
To help keep dates straight in all parts of the world, it is necessary to have one place where we can note that the calendar day begins. The new date begins on the western side of the date line. As the earth turns, this new date sweeps westward around the earth.
What happens when you cross the date line? The Date Line or (IDL) is an imaginary — and arbitrary — line on Earth’s surface that runs from the North Pole to the South Pole. When you cross the IDL, the day and date change. If you cross it traveling westward, the day goes forward by one, and the date increases by one.
Why is IDL drawn in a zigzag manner? IDL passes through the Pacific Ocean. It is an imaginary line, like longitudes and latitudes. The time difference on either side of this line is 24 hours. So, the date changes as soon as one crosses this line. To avoid any confusion of date, this line is drawn through where the sea lies and not land. Hence, the IDL is drawn in a zig-zag manner.
Content for this question contributed by Joseph Aguila, resident of Albany, New York, USA