What is the location of measurement of the Indian Standard Time ?

Indian Standard Time (IST) is calculated based on the longitude of 82°30' E, which is equivalent to 82.5° East of the Prime Meridian [2]. This specific meridian was selected as the 'standard meridian' of India because it is a multiple of 7°30' of longitude, a common international practice for defining time zones [1]. The meridian passes through Mirzapur in Uttar Pradesh (near Prayagraj) and serves as the reference point to ensure time uniformity across the country, which spans nearly 30 degrees of longitude [4]. Consequently, IST is exactly 5 hours and 30 minutes ahead of Greenwich Mean Time (GMT) or Universal Coordinated Time (UTC) [3]. This standardization resolves the approximately two-hour time lag between India's easternmost point in Arunachal Pradesh and its westernmost point in Gujarat [3][t3].

Sources

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