Indian Standard Time

Indian Standard Time (IST) is five hours and 30 minutes ahead of Coordinated Universal Time (UTC), expressed as UTC+05:30. Its reference is India's standard meridian, 82°30′ east longitude, which passes near Mirzapur in Uttar Pradesh. The National Physical Laboratory of India maintains the national time standard and provides an official IST display.

India spans a wide east–west distance, so local solar noon occurs at noticeably different clock times in different parts of the country. A single national standard makes schedules and administration consistent across regions, while also meaning that sunrise and sunset are earlier by the clock in the east than in the west. The standard meridian is near the country's longitudinal midpoint, serving as a practical reference rather than matching local solar time everywhere.

Before a nationwide standard was established, different regions and institutions used local times, and railway operations encouraged the use of a shared reference. The Indian government formally adopted IST on 1 January 1906, using the meridian at 82°30′ east. Local time continued to be used in some places for a period; Calcutta adopted IST in 1948 and Bombay (now Mumbai) in 1955.

The UTC+05:30 offset is a half-hour difference rather than a whole-hour offset. India does not currently observe daylight saving time, so the national offset remains UTC+05:30 throughout the year. IST is a civil time standard; local apparent solar time still varies with longitude and the Sun's seasonal path.