Indian Standard Time Upsc

Indian Standard Time

  • IST that is Indian Standard Time is calculated from 82°30’E meridian passing through Mirzapur.
  • Thereby Indian Standard time is +5.30 Hours from the GMT.

Longitude and Time

  • The time of any region or country is calculated based on the longitude of the earth.
  • The earth rotates from west to east, which makes Sun appears to rise in the east and set in the west.
  • The rotation of the earth along its axis takes 24 hours, that is to complete one circle or 360° of longitudes.
  • 180° of longitudes fall both east and west of the Prime Meridian and thereby the Sun takes 12 hours time to traverse the eastern and western Hemisphere.
  • Accordingly, the sun traverses 15° of longitudes per hour. Otherwise one degree of longitude in every four minutes of time.
  • The time decrease as we move from west to east and increase with our westward movement.

Local Time

  • Local time is determined by the rate of time at which sun traverses over certain degrees longitudes.
  • It is calculate with respect Prime Meridian (0°Longitude) and longitude of particular area.

Example: Find the local time of location X which is located at 90°
east longitude when the time at Greenwich (0°) is 12.00 noon?

Difference between Greenwich and Location X = 90° of longitude

Time difference = 90 x 4 = 360 minutes

Which are 360/60 Hours = 6 Hours. Then local time 6 PM. PM is because it is located east longitude. If it is west it is AM.

  • This method is used to determine the time of any region on the earth.
  • In order to maintain uniformity in time at least within 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.
  • The Standard Meridian is taken in such a way that it is divisible by 150° or 7° 30’ so that the difference between its Standard time and the Greenwich Mean Time is expressed as multiples of an hour or half an hour.


The nations with huge east to west boundaries may choose more than one standard meridian.

For Example Russia has 11 times zones, USA has 6 time zones, Canada has 6 time zones.

Also, Huge countries like India and China avoided multiple time zones. Maybe to avoid confusion?

In total the world is divided into 24 major time zones.

Also Check International Date Line


* * All the Notes in this blog, are referred from Tamil Nadu State Board Books and Samacheer Kalvi Books. Kindly check with the original Tamil Nadu state board books and Ncert Books.