The mistake most people make with IST
A calendar invite lands with “15:00 IST” and the sender in Tel Aviv. Your phone, set to Asia/Kolkata, shows it as 18:30. You dial in late. The abbreviation IST maps to three different UTC offsets. You cannot know which one a sender means without context. The safe choice is to write the full name and offset every time.
Three time zones called IST
| Abbreviation | Full name | Offset | Used when |
|---|---|---|---|
| IST | Indian Standard Time | UTC+5:30 | Year-round, no daylight saving |
| IST | Israel Standard Time | UTC+2:00 | Winter; switches to IDT (UTC+3:00) in summer |
| IST | Irish Standard Time | UTC+1:00 | Summer only; Ireland uses GMT (UTC+0:00) in winter |
Indian Standard Time: UTC+5:30
India uses a single time zone across all 28 states and 8 union territories. The offset is UTC+5:30, based on the 82.5°E meridian near Mirzapur, Uttar Pradesh. India does not observe daylight saving time. The offset stays UTC+5:30 in January and July.
New Delhi, the capital, sits at this offset year-round. On 21 June 2026, sunrise in New Delhi is at 05:23 and sunset at 19:21, giving 13 hours 58 minutes of daylight. On 21 December 2026, sunrise is at 07:09 and sunset at 17:28, giving 10 hours 19 minutes of daylight.
When someone in India says “IST”, they mean Indian Standard Time. That is the only meaning in domestic use.
Israel Standard Time: UTC+2:00
Israel Standard Time is UTC+2:00 in winter. During summer, Israel switches to Israel Daylight Time (IDT) at UTC+3:00. The change happens on a Friday before the last Sunday of March and ends on the last Sunday of October.
A meeting scheduled for “15:00 IST” in December means 15:00 UTC+2:00 in Tel Aviv. The same string in July means 15:00 UTC+3:00. The abbreviation does not tell you whether daylight saving is in effect.
Irish Standard Time: UTC+1:00 (summer)
Ireland uses Greenwich Mean Time (UTC+0:00) in winter and Irish Standard Time (UTC+1:00) in summer. The switch occurs on the last Sunday of March and the last Sunday of October, matching the rest of the European Union.
“IST” in an Irish context refers only to the summer offset. From late October to late March, Ireland uses GMT, and the abbreviation disappears entirely. If a colleague in Dublin sends an invite for “10:00 IST” in August, they mean 10:00 UTC+1:00. In January, they would write “10:00 GMT”.
How software handles IST (often wrongly)
Many scheduling tools, calendar apps, and time zone converters treat “IST” as Indian Standard Time by default. Some let you pick from a dropdown that lists all three. Others apply a single mapping based on the user’s locale or the app’s data source.
The IANA time zone database, which powers most operating systems, uses these identifiers:
- Asia/Kolkata for India
- Asia/Jerusalem for Israel
- Europe/Dublin for Ireland
When a calendar app receives “IST” without a IANA zone, it guesses. Guesses are wrong often enough to cause missed calls, late arrivals, and double-booked slots.
How to avoid IST confusion in scheduling
Write the offset or the full country name. Do these three things:
- Use UTC offsets in written communication. “The meeting is at 14:00 UTC+5:30” cannot be misinterpreted.
- Specify the country name in calendar titles. “Standup (India)” or “Sync (Israel)” makes the intent clear.
- Check your tool’s time zone list. If your scheduling app lists “IST (UTC+5:30)”, it means India. If it lists “IST (UTC+2:00)”, it means Israel. If it lists both, pick the one with the IANA zone attached.
For recurring meetings with participants in multiple countries, the safest format is a UTC time with a note: “15:00 UTC (which is 20:30 IST in India, 17:00 IST in Israel, 16:00 IST in Ireland)”.
Best practice: use “India Standard Time” or UTC+5:30
When you schedule with teams in India, write “India Standard Time” or “UTC+5:30” instead of “IST”. The same applies for Israel and Ireland: write the full name.