Indian time zone
Indian time zone, time zone in India observing Indian Standard Time (IST)—five and a half hours ahead of Coordinated Universal Time (UTC)—based on the Indian standard meridian (82°30′ E), which passes through Mirzapur, Uttar Pradesh. IST is the sole time zone observed in India across its vast east-west expanse. Using one time zone for the country, which spans more than 1,800 miles (2,900 km) from east to west, ensures standardization between cities that have huge variations in their local solar schedules. This resolves historical timekeeping issues while also creating regional challenges.
Indian Standard Time
India spans about 30 degrees of longitude wide. Since Earth rotates 360° in 24 hours, every 15° of longitude corresponds to a one-hour time difference. This means that, without a standard time, India’s westernmost and easternmost regions would have a natural two-hour difference in local time.
Indian Standard Time is observed as UTC+5:30 to synchronize time in the country by balancing such local offsets in time. This time offset is derived from the country’s standard meridian at 82°30′ E. Since 60′ make up 1° of longitude, 30′ is half a degree, which means the standard meridian can be expressed as 82.5° E. Every 15° of longitude corresponds to a one-hour time difference, so dividing 82.5° by 15 gives a 5.5-hour offset. Since this meridian is east of 0° longitude, IST is 5.5 hours ahead of UTC.
History
Timekeeping practices in India have shifted from traditional astronomical methods to calculate local times for regions in the precolonial period to a single national time zone.
Precolonial timekeeping
Regions in precolonial India kept time through solar and lunar calculations. Indian astronomical traditions codified time calculations, as seen in the 4th-century ce treatise Surya Siddhanta, which mentions a “prime meridian” passing through Avanti (identified as the region near Ujjain in present-day Madhya Pradesh state). The text provides detailed units of time and methods to convert daylight times from various other regions to Ujjain time.
East India Company and railway time
In 1786 British East India Company officer William Petrie created a private observatory in Egmore in present-day Chennai. On his return to England in 1789, Petrie gave the observatory equipment to the Madras government. Michael Topping, the East India Company’s astronomer and surveyor, used Petrie’s equipment and framework to establish the Madras Observatory in Nungambakkam, also now in Chennai, setting up Asia’s first modern observatory. When its equipment and some staff were shifted to an observatory established by the company in Kodaikanal in 1899, the Madras Observatory gradually lost prominence, though it continued to be used for astronomical observation to regulate Indian Standard Time until it was closed in 1931.
Early in India’s colonial period, the Bombay and Bengal presidencies defined time based on local solar conditions. In the Madras Presidency John Goldingham, the first head of Madras Observatory—Asia’s first modern observatory—calculated Madras Time against Greenwich Mean Time (GMT) as GMT+5:21:14 (5 hours 21 minutes 14 seconds ahead of GMT) in 1802. The company used this calculation to record local time in the Madras Presidency.
The growing importance of the railways and the telegraph prompted British authorities to implement a standard time for railway operations from the 1850s onward, as frequent conversions between local times for departures and arrivals caused scheduling confusion. They chose Madras Time for this, because Madras (now Chennai) was roughly midway between the Bombay and Bengal presidencies, and the Madras Observatory, which ran the telegraph service used to synchronize railway station times, helped regulate this time. Initially the difference between railway time and local time would be noted in railway timetables and travel guides, and two time references would thus coexist at railway stations: Madras Time and local time based on solar patterns. British authorities tried to standardize Madras Time with little success. For instance, in 1881, the governor of Bombay tried imposing it on civilians in Bombay. This led to considerable dissent, and local time was restored in 1883. Local times largely continued to coexist with Madras Time wherever the railways had stations.
International developments
In 1884 the International Meridian Conference was held in Washington, D.C., to attempt to standardize time globally. It proposed to set the Greenwich meridian as the prime meridian and calculate times in zones of one-hour increments based on distance from the prime meridian. It posited the use of the 75th meridian for Bombay (now Mumbai) and the 90th for Calcutta (now Kolkata), which would have set the standard times in these zones to GMT+5 and GMT+6, respectively. But the conference failed to vote on the use of zones with one-hour increments because of practical difficulties based on political borders, among other reasons. It did succeed in establishing the Greenwich meridian at 0° longitude. This meant that the local times that were observed in Calcutta and Bombay prior to the conference gained global validity as British India’s two official time zones. Defined in terms of offsets to GMT, Bombay’s local time was GMT+4:51:00, whereas that at Calcutta in the Bengal Presidency was GMT+5:53:20. But the use of Madras Time continued, as the new time zones did not offer a single alternative standard for the railways to use.
Introduction of Indian Standard Time
In 1905 Viceroy of India Lord Curzon announced that the 82°30′ E longitude passing through Mirzapur, east of Allahabad (now Prayagraj), would be the basis for the new Indian Standard Time (IST). Shortly after this, he also announced the partition of Bengal. These decisions intensified regional dissent against Britain’s imposition of a standard time on civilian schedules. The opposition to nonlocal time in Bombay developed overtones of colonial resistance. Class-based resistance also added to the opposition to the new standard as the already overworked and underpaid Bombay textile mill workers would now have to start their work before dawn. This long-running debate over time came to be called the Battle of the Clocks in local news, and it would be resolved only after independence.
The Royal Society in London proposed two time zones for India in 1904–05 at one-hour offsets from GMT: GMT+5 and GMT+6. But the colonial government rejected the proposal for two time zones in favor of one falling in the middle of both of the proposed zones, leading to the selection of GMT+5:30 as the offset.
Meanwhile, British authorities formally adopted the new standard on January 1, 1906, with concessions to local times in dissenting regions. Bombay and Calcutta continued to use their own local time until long afterward. Calcutta would adopt IST in 1948, and Bombay would follow suit only in 1955.
IST today
IST was officially adopted by independent India on September 1, 1947, to unify time across the country, simplifying business, government, and transport operations. The Council of Scientific and Industrial Research’s National Physical Laboratory (CSIR-NPL) at Delhi maintains IST. Though there is no official legislation or constitutional provision that defines IST, the government of India reinforced the drive to standardize IST implementation with the Draft Legal Metrology (Indian Standard Time) Rules in 2025. These rules make provisions for accurately maintaining IST through dedicated metrology centers and mandate that government offices and public institutions display IST on all time-keeping devices. They aim to ensure standardization and accuracy in various sectors such as navigation and banking.
Given India’s large east-west extent, a single time zone also presents challenges. In India’s northeastern states—Arunachal Pradesh, Assam, Manipur, Meghalaya, Mizoram, Nagaland, and Tripura—the Sun rises almost two hours earlier than in western Indian trade centers such as Ahmedabad. A 2018 study by researchers at the CSIR-NPL and other collaborators posited that starting the day significantly after sunrise in the region leads to lower productivity, as office-goers and students begin their day out of synchronization with circadian rhythms. It also increases electricity consumption, as offices continue to work until after sunset. Conceding the difficulty of implementing one time zone over a large east-west expanse, the Plantations Labour Act of 1951 allows state governments to set working hours for certain industries as per local daylight conditions. For instance, tea plantations in Assam follow an informal time zone one hour ahead of IST called chai bagan (“tea garden”) time.
The Planning Commission proposed using two time zones for the country in 2006, and similar proposals have emerged through public interest litigation and demands from political figures in India over time. The CSIR-NPL proposed a second time zone for states east of West Bengal, IST-II, with the time offset UTC+6:30 based on the longitude 89°52′ E. However, all proposals for a second time zone have been rejected because of logistical challenges.
