India Standard Time, Irish Standard Time, and Israel Standard Time: one abbreviation, three offsets
The letters "IST" identify three distinct time zones. The abbreviation alone gives you no offset.
| Abbreviation | Full name | UTC offset | When it applies |
|---|---|---|---|
| IST | India Standard Time | UTC+5:30 | Year-round |
| IST | Irish Standard Time | UTC+1 | Summer only (in Ireland) |
| IST | Israel Standard Time | UTC+2 | Winter only (in Israel) |
One is fixed. Two shift. All three share the same three letters.
India Standard Time (UTC+5:30)
India Standard Time covers the whole of India. It is set at UTC+5:30, based on the 82.5°E meridian passing through Mirzapur, Uttar Pradesh. India does not observe daylight saving time. The offset stays UTC+5:30 every day of the year.
- IANA zone: Asia/Kolkata
- Population covered: over 1.4 billion
- Major cities on IST: Mumbai, Delhi, Bengaluru, Hyderabad, Ahmedabad, Chennai, Kolkata, Pune, Jaipur, Lucknow
- Sri Lanka uses the same offset (UTC+5:30) under the name Sri Lanka Standard Time. Its IANA zone is Asia/Colombo. The CLDR metazone "India" covers both Asia/Kolkata and Asia/Colombo.
On 15 January 2026, relative to India Standard Time: New York is 10.5 hours behind, London 5.5 hours behind, Dubai 1.5 hours behind, Singapore 2.5 hours ahead, Tokyo 3.5 hours ahead, Sydney 5.5 hours ahead.
On 15 July 2026, relative to India Standard Time: New York is 9.5 hours behind, London 4.5 hours behind, Dubai 1.5 hours behind, Singapore 2.5 hours ahead, Tokyo 3.5 hours ahead, Sydney 4.5 hours ahead.
The gap between January and July figures for New York, London, Sydney and Chicago comes from daylight saving time in those locations. India does not shift.
Irish Standard Time (UTC+1, summer only)
Irish Standard Time is the summer zone for Ireland. It runs from the last Sunday in March to the last Sunday in October.
- UTC offset: UTC+1
- IANA zone: Europe/Dublin
- Used: during Irish summer (March, October)
- Winter name: Greenwich Mean Time (GMT), UTC+0
When Ireland is on IST, it matches British Summer Time (BST). When Ireland is on GMT, it matches London.
Israel Standard Time (UTC+2, winter only)
Israel Standard Time is the winter zone for Israel. It runs from the last Sunday in October to the Friday before the last Sunday in March.
- UTC offset: UTC+2
- IANA zone: Asia/Jerusalem
- Used: during Israeli winter (October, March)
- Summer name: Israel Daylight Time (IDT), UTC+3
When Israel is on IST, it matches Eastern European Time (EET). On IDT, it matches Eastern European Summer Time (EEST).
How to tell which IST is meant
The abbreviation alone cannot tell you. Context does.
- A meeting with a colleague in Mumbai: India Standard Time.
- A flight arriving into Dublin in August: Irish Standard Time.
- A conference with a Tel Aviv office in December: Israel Standard Time.
- A time listed on a website without a country: assume nothing. Ask "which IST?" or look for the UTC offset.
If the offset is given, use it. If only the abbreviation appears, check the location of the person or organisation who wrote it.
IST in IANA and CLDR databases
The IANA time zone database assigns separate zones to each IST:
| Meaning | IANA zone | CLDR metazone |
|---|---|---|
| India Standard Time | Asia/Kolkata | India |
| Irish Standard Time | Europe/Dublin | Europe:Eastern (or Dublin) |
| Israel Standard Time | Asia/Jerusalem | Israel |
The CLDR metazone "India" covers both Asia/Kolkata and Asia/Colombo. The metazone for Ireland is "Dublin" and for Israel is "Israel".
Common scheduling mistakes caused by IST ambiguity
The three ISTs produce scheduling errors that are easy to make and hard to catch.
Mistake 1: Assuming IST always means UTC+5:30.
A person in Ireland setting a meeting for 14:00 IST in August means 14:00 UTC+1. A person in India reading that as India Standard Time would arrive at 14:00 UTC+5:30. That is 4.5 hours late.
Mistake 2: Rounding the half-hour offset.
India Standard Time is UTC+5:30. People rounding to UTC+5 or UTC+6 miss the meeting by 30 minutes.
Mistake 3: Confusing IST with US time zones.
"IST" is sometimes misread as a typo for CST or EST in US contexts. If you see "IST" on a US-based calendar invite and the host is in Chicago, check again.
Mistake 4: Assuming Ireland uses IST year-round.
Ireland switches to GMT (UTC+0) in winter. A meeting scheduled for February in "IST" is likely a mistake. The correct winter abbreviation for Ireland is GMT.
Other less common IST meanings
- IST can also stand for Indian Standard Time in a computing context (the POSIX time zone abbreviation for Asia/Kolkata).
- IST is used informally in some aviation and military contexts for India Standard Time.
- IST is not a standard abbreviation for any other time zone in the IANA database. The three listed above are the only official uses.