What is a Local Time
Parallels of longitude determine the time at a place. Local time is 12 noon when the sun is exactly overhead. Local time varies, from Greenwich time (London) at the rate of four minutes/degree of longitude. The time of a place depends on whether it lies east or west of Greenwich. As the earth rotates from west to east, the places that lie to the east of Greenwich are the first to receive sunlight. The earth rotates through 360° in 24 h or15° in1hor 1° in 4 min. Therefore the local time varies at the rate of 4 min/degree of longitude from the Greenwich time.
For example, Kolkata is approximately 90° east of Greenwich. So, Kolkata will be (90° x4) or 6 h ahead of the time in London.
What is a Standard Time
Standard Time 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. It is the uniform time fixed in relation to the mean time of a certain meridian which passes through it.
For this purpose, the earth is divided into 24 longitudinal zones, each, being 15° or 1 h apart in time. The zero is at Greenwich (London) which gives us the Greenwich Mean Time {GMT). The 12th zone is divided by the 180th meridian, the International Date Line.
The zones to the east of this line are numbered from 1-12 with the prefix minus (-) indicating the number of hours to be subtracted to obtain the Greenwich Time. The zones to the west are also numbered 1-12 with the prefix plus (+) which means the number of hours that must be added to get the Greenwich Time.
What is a Greenwich Mean Time (GMT)
Greenwich Mean Time is the UK standard time. It is based on the local time of the meridian passing through Greenwich near London.
What is an International Date Line
The 180° east meridian and 180″ west meridians 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 or west, meridians of longitude, which fall on the opposite side of the Greenwich meridian. When one crosses the date line from east to west, a date is to be advanced by one day.
Similarly, when one crosses the dateline from west to east, the date is to be set back by one day.