The Indian Standard Meridian is the line of longitude 82° 30 minutes east, whose local time is taken as the standard time for the whole country (Indian Standard Time, IST), to avoid the confusion of many local times across India's east-west spread.
- The standard meridian passes through Mirzapur (near Naini, Prayagraj) in Uttar Pradesh; IST is 5 hours and 30 minutes ahead of Greenwich Mean Time (GMT or UTC plus 5:30).
- India's longitudinal extent is from about 68° 7 minutes east to 97° 25 minutes east, a span of nearly 30°, which would otherwise mean about two hours difference in local time between the western (Gujarat) and eastern (Arunachal) extremes.
- Each 15° of longitude equals a one-hour difference, and each 1 degree equals 4 minutes; the sun rises in the east, so the northeast sees daybreak about two hours before the western border on clock-free local time.
- A single standard meridian gives one uniform clock time across the country, but the far northeast effectively loses early daylight, a recurring argument for a separate eastern time zone.
- The standard meridian is chosen as a whole multiple of 7.5° so that IST differs from GMT by an exact half hour.
The value 82° 30 minutes east, that it passes through Mirzapur (Uttar Pradesh), IST as GMT plus 5:30, and the longitude-to-time conversion (15° per hour, 1 degree per 4 minutes) are recurring map and time facts.
The standard meridian (82.5° east, time) is different from the Tropic of Cancer (23.5° north, a latitude); IST is 5:30 ahead of GMT, not 5 hours; the meridian passes near Prayagraj-Mirzapur, not through Delhi.
India's standard meridian is 82° 30 minutes east through Mirzapur (UP), giving Indian Standard Time as GMT plus 5 hours 30 minutes.