GMT to IST Converter

Greenwich Mean Time was formalised at the International Meridian Conference in Washington DC in October 1884, where 22 nations agreed to use the Greenwich meridian (0 degrees longitude, passing through the Royal Observatory in Greenwich, London) as the global prime meridian. The agreement gave the world a single reference for civil time and replaced dozens of competing national meridians, including Paris (which France held onto for several more decades). GMT is the direct ancestor of Coordinated Universal Time (UTC), which since 1972 has been the global atomic-time-based standard.

The Royal Observatory at Greenwich, founded by Charles II in 1675, was originally tasked with solving the longitude problem at sea. John Harrison's marine chronometer in 1761 made longitude measurable; the 1884 conference made it universal. The famous Shepherd Gate Clock outside the observatory still displays GMT to passers-by, and the prime meridian is marked by a brass strip in the courtyard that has become a tourist photo opportunity.

British Summer Time (BST), or UTC+1, was first introduced in the UK on 21 May 1916, following German DST adoption a few weeks earlier to save coal during the First World War. The idea is older - the New Zealand entomologist George Hudson proposed it in 1895, and the English builder William Willett campaigned for it in 1907 with a pamphlet titled ‘The Waste of Daylight’. Willett died in 1915, the year before his proposal became law. The Summer Time Act 1972 enshrined the modern rules: BST runs from the last Sunday of March to the last Sunday of October.

Indian Standard Time, by contrast, has been a single year-round offset of UTC+5:30 since 1906. It was set at the Madras Observatory using the 82.5 degree east meridian, which runs through Mirzapur in Uttar Pradesh. India does not observe DST; the question was revisited briefly during the 1971 Bangladesh war but rejected on the grounds that India's near-equatorial latitude provides relatively uniform daylight year-round and that synchronising the country's railways and schools twice a year would be more disruptive than beneficial.

The UK-India relationship has been shaped by timezone math for more than a century. The British India Office in Whitehall coordinated with viceregal offices in Calcutta, Bombay, and later Delhi across what was then a 5-hour 53-minute gap (Calcutta Time was UTC+5:53). The 1906 standardisation made the math easier; the 1947 independence kept the offset because it served Indian commercial and administrative needs. Today the gap is 5:30 hours in winter (GMT) and 4:30 hours during BST - one of the shortest US/Europe-to-India gaps, and the reason London is a natural overlap partner for Bengaluru, Mumbai, and Hyderabad.

Modern UK-India business spans finance (Standard Chartered, HSBC, Barclays), consulting (Accenture, Deloitte, PwC, KPMG, EY all run large Indian delivery centres), and government (Indian high commissions in London and British high commissions in Delhi). The London Stock Exchange opens at 08:00 GMT (13:30 IST) and closes at 16:30 GMT (22:00 IST), giving Mumbai trading desks a 2-hour overlap with their final trading window. Indian IT firms (TCS, Infosys, Wipro, HCL) maintain large UK delivery centres in London, Edinburgh, and Birmingham.

Modern timezone software, including the dual clocks on this page, draws on the IANA Time Zone Database (TZDB), founded by Arthur David Olson at NIH in 1986 and maintained collaboratively at iana.org/time-zones. The identifiers Europe/London and Asia/Kolkata encode every DST rule change going back to 1880, including the 1916 BST introduction, the 1968-71 British Standard Time experiment when the UK stayed on BST year-round, and the modern 1972 schedule. Every Linux, macOS, iOS, Android and modern browser ships a copy.