Time ZonesGMT, UTC and IST explained
GMT: Greenwich Mean Time
GMT is the time at the Royal Observatory in Greenwich, London. For a long time it was the world's reference. Today it is the name of the time zone used by the United Kingdom in winter and by several West African countries all year.
UTC: Coordinated Universal Time
UTC replaced GMT as the scientific standard in the 1960s and 1970s. It is kept by atomic clocks around the world. Now and then a "leap second" has been added to keep it in step with the slightly uneven turning of the Earth. For daily life, UTC and GMT show the same time.
IST: Indian Standard Time
IST is UTC+5:30. It is set by the longitude 82.5° east, which passes near Prayagraj. Since each 15 degrees of longitude is one hour, 82.5 degrees gives exactly five and a half hours. IST applies to the whole country, from Gujarat to Arunachal Pradesh, although the Sun rises almost two hours earlier in the far east than in the far west.
Converting in your head
- UTC or GMT to IST: add 5 hours 30 minutes. 10:00 UTC is 3:30 pm in India.
- IST to UTC: subtract 5 hours 30 minutes. 9:00 am in India is 3:30 am UTC.
- A trick: add 6 hours, then take off 30 minutes.
Where you meet UTC
Flight plans, weather reports, server logs, and many international events and examinations give times in UTC so that no one has to ask "whose time?". If a webinar is at 14:00 UTC, it starts at 7:30 pm in India.
Three common mistakes
- Treating GMT as "UK time" all year. In summer the United Kingdom is on British Summer Time, one hour ahead of GMT. A meeting fixed for "3 pm GMT" in July is at 4 pm on a London clock.
- Assuming IST means India. The same three letters are used for Irish Standard Time and Israel Standard Time. Outside India, write "India time" or UTC+5:30.
- Forgetting the date. At 2:00 am IST it is still 8:30 pm UTC on the previous day. Whenever you subtract and pass midnight, the date goes back by one.
See time in India, the India time tables, or the abbreviations list.