1. TIME ZONES AND INTERNATIONAL DATE LINE (IDL)

2. Contents Time Zone World Time Zones International data line (IDL)
Introduction
History
World Time Zones
International data line (IDL)
Location
How IDL Works

3. Time Zone
Any region of the globe throughout which the same standard time is used, is called time zone.

4. Introduction
Before 1972, all time zones were specified as an offset from Greenwich Mean Time (GMT), which was the mean solar time at the meridian passing through the Royal Observatory in Greenwich, London.
In January 1972, however, the length of the second in both Greenwich Mean Time and atomic time was equalized.
Today, many countries operate on variations of the time zones proposed by Sir Fleming.
All of China (which should span five time zones) uses a single time zone.
Australia uses three time zones - its central time zone is a half-hour ahead of its designated time zone.

5. History
Before the invention of clocks, people marked the time of day with apparent solar time.
For Example, the time on a sundial – which was typically different for every settlement.
The concept of standard time was adopted in the late 19th century .
Sir Sandford Fleming outlined a plan for worldwide standard time in the late 1870s. 
He recommended that the world be divided into twenty-four time zones, each spaced 15 degrees of longitude apart.
The present system employs 24 standard meridians of longitude (lines running from the North Pole to the South, at right angles to the Equator) 15º apart, starting with the prime meridian through Greenwich, England.

6. World Time Zones
There are 25 integer World Time Zones from -12 through 0 (GMT) to +12. Each one is 15° of Longitude as measured East and West from the Prime Meridian of the World at Greenwich, England.  Some countries have adopted non-standard time zones, usually 30 minutes offset which have a * designation.

8. GMT (Greenwich Mean Time)
GMT is an absolute time reference and does not change with the seasons.
London time is the same as Greenwich Mean Time less than half of the year.  
GMT was established in 1884 at the International Meridian Conference, when it was decided to the place the Prime Meridian at Greenwich, England.

9. International data line (IDL)
The International Date Line is an imaginary line which runs from the North Pole to the South Pole and is 180° away from the Greenwich Meridian.

10. Introduction While the world is divided into 24 time zones
There has to be a place where there is a difference in days
There should be a place where the day truly "starts" on the planet.
Thus, the 180° line of longitude, exactly one-half way around the planet from Greenwich, England and 0° longitude is approximately where the International Date Line is located.

11. History
The International Conference in 1884 deemed that there would be a single Universal Day and that this would begin at mean midnight at Greenwich.
Twenty five time zones were established to the east and west of Greenwich with the International Date Line lying along the 180° line of longitude. The line deviates in places to avoid crossing any land. Along this line the calendar moves into a new day but only in local time, which is measured relative to Greenwich Mean Time.
The earliest recommendations issued referring to the date line appear to date from 1899 and 1900
Two adjustments of the date line took place in 1910 near the island chain of Hawaii and between Samoa and the Chatham Islands

12. Historical alterations
Philippines
1844
Alaska
18 October 1867
Samoan Islands and Tokelau
4 July 1892

13. Location
Over the years, the position of the International Date Line has changed several times. Until 1845, the Philippines were on the eastern side of it. When the United States bought Alaska in 1867 the line was moved to the west of it.
The most recent change in the line was in 1995 when Kiribati moved a large segment of it to the east, so that the entire nation would be on the same side of the International Date Line.

14. How IDL Works
Let's say you fly from the United States to Japan. Let's suppose you leave the United States on Tuesday morning. Since you're traveling west the time advances slowly thanks to time zones and the speed at which your airplane flies, but once you cross the International Date Line, it's suddenly Wednesday.
On the reverse trip home you fly from Japan to the United States. You leave Japan on Monday morning but as you cross the Pacific Ocean, the day gets later quickly as you cross time zones moving eastward in an airplace. However, once you cross the International Date Line, the day changes to Sunday.

15. References