Delhi runs 5 hours 30 minutes ahead of London for half the year. For the other half, the gap shrinks to 4 hours 30 minutes. India never moves. The UK does.
IST to UTC: The Fixed Conversion
Indian Standard Time is UTC+5:30, year-round. India does not observe daylight saving time.
To convert IST to UTC, subtract 5 hours and 30 minutes.
| IST (India) | UTC |
|---|---|
| 00:00 (midnight) | 18:30 (previous day) |
| 06:00 | 00:30 |
| 12:00 (noon) | 06:30 |
| 18:00 | 12:30 |
This relationship never changes. The offset, maintained by the CSIR, National Physical Laboratory in New Delhi, is permanent.
IST to GMT: The Seasonal Variation
Greenwich Mean Time (GMT) is the United Kingdom's winter time: UTC+0. When London runs on GMT (roughly late October to late March), the difference from IST is exactly 5 hours 30 minutes.
From the last Sunday in March to the last Sunday in October, the UK switches to British Summer Time (BST), which is UTC+1. During BST, India is 4 hours 30 minutes ahead of London.
| Location | Time | IST |
|---|---|---|
| London (January, GMT) | 06:30 | 12:00 |
| London (July, BST) | 07:30 | 12:00 |
The mental shortcut: subtract 6 hours then add 30 minutes back. For 14:15 IST: 14:15 minus 6 hours is 08:15, plus 30 minutes is 08:45 GMT. During BST, subtract 5 hours then add 30 minutes: 14:15 minus 5 hours is 09:15, plus 30 minutes is 09:45 BST.
GMT vs UTC: What's the Difference?
For every practical purpose, scheduling a call, booking a flight, converting a time, they are the same. GMT and UTC agree to within a fraction of a second.
The difference is technical. GMT is a time zone: the mean solar time at the Greenwich meridian, still used as the UK's winter time. UTC is a measurement standard kept by atomic clocks worldwide.
Use UTC in technical documentation and log files. Use GMT when you mean "UK winter time" in everyday conversation. India's standard time is officially UTC+5:30, not GMT+5:30.
IST to BST (British Summer Time)
BST is UTC+1, used in the UK from the last Sunday in March to the last Sunday in October.
| IST (India) | BST (London) |
|---|---|
| 00:00 | 19:30 (previous day) |
| 06:00 | 01:30 |
| 12:00 | 07:30 |
| 18:00 | 13:30 |
A standing London meeting shifts its Indian time twice a year. A 09:00 London meeting is 14:30 IST in winter (GMT) and 13:30 IST in summer (BST). That hour shift can push the meeting into lunch for half the year.
Quick Reference Conversion Table
| IST | UTC | GMT (winter) | BST (summer) |
|---|---|---|---|
| 00:00 | 18:30 (prev) | 18:30 (prev) | 19:30 (prev) |
| 03:00 | 21:30 (prev) | 21:30 (prev) | 22:30 (prev) |
| 06:00 | 00:30 | 00:30 | 01:30 |
| 09:00 | 03:30 | 03:30 | 04:30 |
| 12:00 | 06:30 | 06:30 | 07:30 |
| 15:00 | 09:30 | 09:30 | 10:30 |
| 18:00 | 12:30 | 12:30 | 13:30 |
| 21:00 | 15:30 | 15:30 | 16:30 |
Why "GMT+5:30" Is Technically Incorrect
India's offset is UTC+5:30. GMT is a time zone, not a universal standard. Saying "GMT+5:30" is common in casual speech, but it implies India's time is tied to the UK's civil time, which changes twice a year. India's time does not change.
Use "UTC+5:30" for clarity. Software and scheduling tools sometimes treat "GMT" as the UK's winter time, which can cause errors in July.
Common Conversion Mistakes
Mistake 1: Using +5:30 to London in July. London is on BST (UTC+1) from late March to late October. The correct offset is +4:30. Calling a London number at 12:00 IST thinking it is 06:30 there? In July, it is 07:30.
Mistake 2: Assuming India has multiple time zones. India has one: UTC+5:30, applied from Kutch to Kohima. Solar time varies by about 1 hour 56 minutes across the country, but the clock does not.
Mistake 3: Confusing IST with other ISTs. IST also stands for Israel Standard Time (UTC+2) and Irish Standard Time (UTC+1). In scheduling software, always write "India Standard Time" or use the IANA zone "Asia/Kolkata".
Mistake 4: Forgetting the date roll. Subtracting 5 hours 30 minutes from a time before 05:30 IST flips the date. 03:00 IST on 15 January is 21:30 UTC on 14 January.
The next clock change in the UK happens on the last Sunday of October 2026, when BST ends and the gap with India returns to 5 hours 30 minutes. Mark that date if you schedule calls with London.