India has a longitudinal extent of 30°. Due to this there is a time lag of two hours between the sunrise on the easternmost and the westernmost horizons of India. Due to rotation, the earth takes 4 minutes to rotate through 1° of longitude. The difference in time is one hour for 15° of longitude. Therefore, for a longitudinal extent of 30° of India, there is a time lag of 2 hours. When it is 6 a.m. in Arunachal Pradesh, it is still 4 a.m. in Gujarat. But the watches in all parts of India run according to standard time measured from 82/4° E longitude. So the watches in Arunachal Pradesh and Gujarat show the same time despite the different sunrise.
