'The Equation of Time in Hindu Astronomy'. ^ Das, Sukumar Ranjan (December 1928).'A Lost Monument of Indian Astronomy:Das heliozentrische System in der griechischen, persischen und indischen Astronomie B. 'The Computation of the Length of Daylight in Hindu Astronomy'. History of measurement systems in India.India uses UTC+5:30, referred to as Asia/Kolkata in the IANA time zone database. During the Sino-Indian War of 1962 and the Indo–Pakistani Wars of 19, daylight saving was briefly used to reduce civilian energy consumption. India and the Indian subcontinent observed 'daylight saving (DST)' during the Second World War, from 1942 to 1945. Older time zones, not in use any more since introduction of standardised same time zone across India, were: In 2014 Assamese politicians proposed following a daylight-saving schedule that would be ahead of IST by an hour, but as of March 2020 it has not been approved by the central government. After independence Īfter independence in 1947, the Indian government established IST as the official time for the whole country, although Mumbai and Kolkata retained their own local time for a few more years. This provision lasted from September 1, 1942, to October 15, 1945. Briefly during World War II, clocks under Indian Standard Time were advanced by one hour, referred to as War Time. This continued until the 1940s, when time signals began to be broadcast using the radio by the government.

In 1925, time synchronisation began to be relayed through omnibus telephone systems and control circuits to organisations that needed to know the precise time. However, Calcutta Time was officially maintained as a separate time zone until 1948 and Bombay Time until 1955. Indian Standard Time came into force on 1 January 1906, and also applied to Sri Lanka (then Ceylon). Though British India did not officially adopt the standard time zones until 1905, when the meridian passing east of Allahabad at 82.5° E longitude was picked as the central meridian for India, corresponding to a single time zone for the country ( UTC+05:30). Local time zones were also set up in the important cities of Bombay and Calcutta and as Madras time was intermediate to these, it was one of the early contenders for an Indian standard time zone. In 1802 Madras Time was set up by John Goldingham and this was later used widely by the railways in India. For example, the Jantar Mantar observatory built by Maharaja Sawai Jai Singh in Jaipur in 1733 contains large sundials, up to 90 ft (27 m) high, which were used to accurately determine the local time. For most of India's history, ruling kingdoms kept their own local time, typically using the Hindu calendar in both lunar and solar units.

Despite these early advances, standard time was not widely used outside astronomy. The Surya Siddhanta also described a method of converting local time to the standard time of Ujjain. Taking a day to be 24 hours, the smallest time unit, prāṇa, or one respiratory cycle, equals 4 seconds, a value consistent with the normal breathing frequency of 15 breaths/min used in modern medical research. A nakṣatra māsa, or astronomical month, consists of 30 days. The ghalikā is 60 palas, and the nakṣatra ahórātra, or astronomical day, contains 60 ghalikās. Time that is measurable is that which is in common use, beginning with the prāṇa (or, the time span of one breath). The day used by ancient Indian astronomers began at sunrise at the prime meridian of Ujjain, and was divided into smaller time units in the following manner: The book described the thousands years old customs of the prime meridian, or zero longitude, as passing through Avanti, the ancient name for the historic city of Ujjain, and Rohitaka, the ancient name for Rohtak ( 28★4′N 76☃8′E / 28.900°N 76.633☎ / 28.900 76.633 ( Rohitaka (Rohtak))), a city near the Kurukshetra. The 4th century CE astronomical treatise Surya Siddhanta postulated a spherical earth. See also: Hindu units of time and History of measurement systems in India

India time zones