IST to GMT time converter

How far apart are Kolkata and London?

It depends on the date. London is 4 hours 30 minutes behind Kolkata while Kolkata is on IST and London on BST. London is 5 hours 30 minutes behind Kolkata while Kolkata is on IST and London on GMT. The exact dates for 2026 and 2027 are below, worked out from the same time zone database your browser uses — not remembered.

2026

2027

IST to GMT hour table

Every hour of a Kolkata day, read across to London — one column per gap, so pick the column for today's date from the table above. A row marked +1 day is already tomorrow there; −1 day is still yesterday.

Each column is one value of the gap; the clock names under it are the ones in force when that gap applies. For a half-hour time or a third city, use the timeline above: every clock moves to the instant you pick.

Working across Kolkata and London

India and the United Kingdom are the pair with the most usable overlap of any on these pages: five and a half hours apart in the British winter and four and a half in summer, which leaves most of London's morning inside India's afternoon. A 9am start in London is 2:30pm in Kolkata under GMT and 1:30pm under British Summer Time; India's 9am is 3:30am or 4:30am in London, so the overlap runs one way — Indian afternoons against British mornings — and a team that keeps its shared meetings between about 9am and 1pm UK time inconveniences nobody.

The word GMT needs care on this page more than any other. Greenwich Mean Time is UTC+0, and London keeps it from the last Sunday in October to the last Sunday in March. For the other seven months the UK is on British Summer Time, UTC+1, and a Londoner's clock is not showing GMT at all. So "IST to GMT" is really two questions: the conversion to Greenwich Mean Time, which is always five and a half hours and never changes, and the conversion to whatever London's clocks currently show, which is what people almost always mean and which is four and a half hours in summer. The table above gives both; the tool above uses the zone Europe/London and switches on the right day by itself.

India's offset, UTC+5:30, was defined against Greenwich in the first place — the half hour comes from a meridian roughly 82.5° east of London — and India has not moved it since. Every change in the gap between the two countries comes from the British side, on dates set by UK law and matching the rest of Europe: clocks go forward at 1am GMT on the last Sunday in March and back at 2am BST on the last Sunday in October. The dates printed above are computed from your browser's time zone data for this year and next, not remembered.

The abbreviation on the Indian side is the ambiguous one. IST is India Standard Time here, but the same letters mean Irish Standard Time — Ireland's summer time, UTC+1, and therefore the same clock as London for seven months of the year — and Israel Standard Time at UTC+2. A Dublin colleague writing "IST" and a Delhi colleague writing "IST" are four and a half hours apart. This page and the tool work in IANA zone names, where Asia/Kolkata and Europe/Dublin cannot be confused.

Because the difference is under six hours, the calendar date rarely changes between the two — only between about 6:30pm and midnight London time is it already tomorrow in India. That still catches Friday-evening deadlines: "end of day Friday, UK" is Saturday morning in Mumbai. The hour table marks the rows where it happens.

Sources

The offsets, dates and hour tables above are computed when this page is built, from the time zone rules your browser itself ships. The rules come from:

• IANA Time Zone Database — the rules every browser ships
• GOV.UK — When do the clocks change?
• ECMA-402 — the Intl.DateTimeFormat standard these figures are computed with