The Indian Standard Time is ahead of Greenwich Mean Time by

Indian Standard Time (IST) is calculated based on the Standard Meridian of India, which is 82°30' E longitude [3]. This meridian passes near Prayagraj (formerly Allahabad) and was chosen because it is a multiple of 7°30' [3]. Since the Earth rotates 360° in 24 hours, every 15° of longitude corresponds to a one-hour time difference [4]. By dividing 82.5° by 15, the resulting time offset is exactly 5.5 hours [4]. Because India is located east of the Prime Meridian (0° longitude) at Greenwich, its time is ahead of Greenwich Mean Time (GMT) [1]. Therefore, IST is consistently 5 hours and 30 minutes ahead of GMT (IST = GMT+5:30) [2]. This standardization ensures a uniform time across India's vast longitudinal extent, which spans approximately 30 degrees [4].

Sources

1. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 18: Latitudes and Longitudes > Indian Standard Time > p. 245
2. [3] INDIA PHYSICAL ENVIRONMENT, Geography Class XI (NCERT 2025 ed.) > Chapter 1: India — Location > INDIA – LOCATION > p. 2
3. [4] https://www.britannica.com/science/Indian-time-zone
4. [1] Exploring Society:India and Beyond. Social Science-Class VI . NCERT(Revised ed 2025) > Chapter 1: Locating Places on the Earth > LET'S EXPLORE > p. 21