The easternmost longitude of India is 97° 25′ E. It is located in the state of Arunachal Pradesh. The westernmost longitude is 68o7’E. Due to the great longitudinal extent of east-west ends about 29°, there could be wide differences in local time of places located at two extreme points of India. As such, the difference between these two points would be of about two hours.
The local time of places differ from one place to another. In India, we find a difference of about 1 hour and 45 minutes in the local times of Dwarka in Gujarat and Dibrugarh in Assam. Hence, it is necessary to adopt the local time of some central meridian of a country as the standard time for the country. The longitude of 82½° E (82° 30’E) is considered as the standard meridian. The local time stated or mentioned at this meridian is considered as the standard time for the whole country. This is called as the Indian Standard Time (IST).