The Indian Standard Time (IST) is based on

Indian Standard Time (IST) is based on the 82°30' E meridian, which is often expressed as 82.5° E longitude [3]. This specific meridian was selected as the 'standard meridian' of India because it passes roughly through the center of the country's longitudinal extent, specifically near Mirzapur in Uttar Pradesh [5]. By international convention, countries typically choose standard meridians in multiples of 7°30' or 15° to ensure time offsets are in half-hour or one-hour increments; 82°30' E is a multiple of 7°30' [3]. This results in IST being exactly 5 hours and 30 minutes ahead of Greenwich Mean Time (GMT), which is based on the 0° Prime Meridian [5]. Adopting this single standard time helps eliminate confusion caused by the two-hour time lag between India's easternmost and westernmost points [4].

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 18: Latitudes and Longitudes > Indian Standard Time > p. 245
2. [3] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 18: Latitudes and Longitudes > Standard Time and Time Zones > p. 243
3. [2] INDIA PHYSICAL ENVIRONMENT, Geography Class XI (NCERT 2025 ed.) > Chapter 1: India — Location > INDIA – LOCATION > p. 2
4. [5] https://www.britannica.com/science/Indian-time-zone
5. [4] CONTEMPORARY INDIA-I ,Geography, Class IX . NCERT(Revised ed 2025) > Chapter 1: India Size and Location > SIZE > p. 2