Local Time: Parallels of longitude determine the time at a place. Local time varies, from GMT (London) at the rate of four minutes/degree of longitude.

Standard Time: It is the uniform time fixed by each country. As the local time is found to vary constantly from one plain to another, there is an arrangement by which all places in a certain region agree to use the same time.

Greenwich Mean Time (GMT): It is the UK Standard time; it is based on the local lime of the meridian passing through Greenwich near London.

World Time Zones

International Date Line: The 180o east meridian and 180o west meridian are one and the same line, running over the Pacific Ocean., deviating at Fiji, Samoa, and the Gilbert Islands. The International Date Line roughly corresponds to 180°east west meridian of longitude, which falls on the opposite side of the Greenwich meridian. When one crosses the Dateline from east to west, the date is to be advanced by one day. Similarly, when one crosses the Date Line from west to east, the date is to be set back by one day.