The Prime Meridian or the 0° longitude passes through Greenwich near London and the local time calculated at the British Royal Observatory is considered as the accurate time for the Prime Meridian, and is called Greenwich Mean Time or GMT.
Since the earth rotates from west to east, the places east of Greenwich are ahead of GMT and those to the west are behind it and all the places located on a particular meridian or a particular longitude have the same local time.
Indian Standard Time or IST
In order to maintain uniformity of time as far as possible within the territorial limits of a country, the time at the central meridian of the country is taken as the Standard Meridian and its local time is taken as the standard time for the whole country.
The Standard Meridian is selected in a manner that it is divisible by 15° or 7° 30’ so that the difference between its standard time and the Greenwich Mean Time may be expressed as multiples of an hour or half an hour.
The Indian Standard Time (IST) is calculated from 82°30’E meridian passing through Mirzapur.
Therefore, IST is plus 5 hours 30 minutes from the GMT ((82°30’ x 4) (60 minutes=5 hours 30 minutes).
Similarly, all countries of the world choose the standard meridian within their territory to determine the time within their administrative boundaries.