The International Date Line (IDL) explained
The International Date Line
The International Date Line (IDL) is an imaginary line of longitude on the Earth’s surface located at about 180 degrees east (or west) of the Greenwich Meridian.
The date line is shown as an uneven black vertical line in the Time Zone Map above and marks the divide where the date changes by one day. It makes some deviations from the 180-degree meridian to avoid dividing countries in two, especially in the Polynesia region.
The time difference between either side of the International Date Line is not always exactly 24 hours because of local time zone variations.
If you travel around the world, changing standard time by one hour each time you enter a new time zone, then a complete circuit would mean that you adjusted your clock or watch time by 24 hours. This would lead to a difference of one day between the date on your clock and the real calendar date. To avoid this, countries are on either side of the International Date Line which runs down the middle of the Pacific Ocean. If you cross the date line moving east, you subtract a day, whereas if you are moving west you add a day.
GMT vs. UTC
The Greenwich Meridian is a north-south line selected as the zero-reference line for astronomical observations. The line in Greenwich in London, UK represents the world’s prime meridian – longitude zero degrees. Every place on Earth is measured in terms of its distance east or west from this line.
The line divides the Earth’s eastern and western hemispheres just as the equator divides the northern and southern hemispheres.
The Earth’s crust moves very slightly on an ongoing basis so the prime meridian’s exact position is also moving very slightly. However, the prime meridian’s original reference remains to be the Airy Transit Circle in the Royal Observatory in Greenwich, even if the exact location of the line may move to either side of the transit circle’s meridian. The Royal Observatory in Greenwich is the home to the time zone named Greenwich Mean Time – GMT. This time zone was first adopted as the world’s time standard at the Washington Meridian Conference in 1884. However, GMT is now loosely interchanged with UTC to refer to time kept on the Greenwich meridian (longitude zero).