World-Time-Zone

The local time or solar time at any particular location on the world map changes continuously with the longitudinal change. Hence, the local time of a place is determined by the longitude that passes through it.
However, severe practical problems appear while calculating the local time of a place. For instance, timings will be different in different places on the surface of earth. So, to stay in tune with the different timings of the various locations, people have to continuously adjust their watch. Therefore, to maintain uniformity and ease of transport, communication and for other practical purposes, there has to be a common standard of time, at least in one considerable extent of space, such as administrative divisions, regions, or individual countries. The Standard Time Zone of World Map is thus very helpful.
This exigency had led to the adoption of the concept of standard time for a number of locations over a reasonable physical distance or area. In the late nineteenth century, Sir Sandford Fleming, a Canadian railroad engineer put forth a scheme for a worldwide standard time. Subsequently, in 1884, representatives from 27 countries came together in Washington D.C. to agree on a system on which the present-day international world time zone map is largely based.
The present world time zone map uses 24 standard meridians of longitude that run from the North Pole to the South Pole, at right angles to the equator, and are 15? apart, starting from the prime meridian or 0? longitude that passes through Greenwich near London, England. These meridians are therefore, theoretically speaking, the centers of the 24 standard time zones. Each 15? zone to the east or west of the initial 0? zone is therefore represented by one hour. However, in practice these zones are not sacrosanct and are altered or subdivided, for convenience' s sake, by individual countries that fall within them. For instance, in China, there is only one time zone for the whole country, whereas the USA is divided into six time zones. Similarly, the International Date Line is extended eastwards around the Pacific island country of Kiribati to avoid cutting through it so that one uniform date could be maintained for the whole country on one particular day.