Indian Standard Time (IST) is a national time standard, a concept that defines the single time zone observed across the Republic of India. It is calculated with a fixed offset of UTC+05:30, meaning it is five and a half hours ahead of Coordinated Universal Time.
The need for a unified time arose because India spans approximately 30 degrees of longitude, which would naturally result in a two-hour difference between the local solar times of the western and eastern regions. To solve this problem and standardize operations, particularly for the railways, the British colonial government officially adopted IST on January 1, 1906, phasing out earlier local standards like Madras Time. After Independence, the Union government established IST as the official time for the entire country on September 1, 1947.
The mechanism of IST is based on the local mean time at the 82°30' E longitude, which is the country's standard meridian. This meridian passes near Mirzapur in Uttar Pradesh and was chosen as it is centrally located between the country's longitudinal extremes. The official time-keeping and dissemination are entrusted to the National Physical Laboratory (NPL) in New Delhi, which is part of the Council of Scientific and Industrial Research (CSIR). India does not observe Daylight Saving Time (DST), though it was briefly used during the wars of 1962, 1965, and 1971.
While the time offset has remained constant, a significant institutional change is underway to formalize the concept. The government has drafted the Legal Metrology (Indian Standard Time) Rules, 2025, which aim to make IST the mandatory time reference for all legal, commercial, digital, and administrative operations. This move, known as 'One Nation, One Time,' is intended to strengthen digital security and reduce reliance on foreign time sources like GPS.