India runs on a single IANA time zone: Asia/Kolkata. It covers every state, union territory, and island group. The offset is fixed at UTC+5:30, year-round. No daylight saving time, no seasonal shifts.
The zone is named after Kolkata, a major port and former colonial capital. The name does not signal any special status for the city. All of India uses the same clock.
Asia/Kolkata: India's Official IANA Zone
Asia/Kolkata uses India Standard Time (IST). The offset never changes. India experimented with daylight saving during wartime (1942-1945 and again in 1962) but abandoned it both times. No national DST has been observed since.
The zone covers every Indian state and union territory. Some of the largest cities using Asia/Kolkata include:
| City | Population |
|---|---|
| Mumbai | 12,691,836 |
| Delhi | 11,034,555 |
| Bengaluru | 8,495,492 |
| Hyderabad | 6,993,262 |
| Ahmedabad | 6,357,693 |
| Chennai | 4,681,087 |
| Kolkata | 4,631,392 |
| Surat | 4,591,246 |
| Pune | 3,124,458 |
Asia/Colombo: Sri Lanka's Zone (Same Offset)
Sri Lanka uses a separate IANA zone: Asia/Colombo. It shares the same UTC+5:30 offset as India. The two zones are not interchangeable in software. They have different historical records and legal definitions.
The CLDR metazone "India" covers both Asia/Kolkata and Asia/Colombo. If you are building an application that groups India and Sri Lanka under one label, "India" is the correct metazone name.
Using Asia/Kolkata for a server in Colombo will produce wrong results for dates before 2006, when Sri Lanka changed its offset. Use the zone that matches the location.
Using Asia/Kolkata in Programming and Systems
When you need to set a time zone in code, use the string "Asia/Kolkata". Here is how it appears in common environments:
- Python:
zoneinfo.ZoneInfo("Asia/Kolkata") - JavaScript (Intl):
Intl.DateTimeFormat("en-IN", { timeZone: "Asia/Kolkata" }) - Linux:
TZ='Asia/Kolkata'or linked from/usr/share/zoneinfo/ - Java:
ZoneId.of("Asia/Kolkata")
Do not use the abbreviation IST in code. IST can mean India Standard Time, Irish Standard Time (UTC+1), or Israel Standard Time (UTC+2). Always use the full IANA identifier.
IST in Windows and Linux Time Zone Settings
Windows uses a different naming system. The Windows equivalent of Asia/Kolkata is "India Standard Time". You can find it in the time zone picker under the heading "UTC+05:30" or by searching for "Kolkata".
On Linux, the file path is /usr/share/zoneinfo/Asia/Kolkata. Most distributions install the IANA database by default. If you manage servers, ensure the tzdata package is up to date. Outdated databases can cause incorrect conversions, especially after a country changes its rules.
Never hardcode the offset +05:30. Always use the zone name. Offsets change over time for political reasons. The zone name guarantees you follow whatever rules the government sets.
Why is the zone named after Kolkata and not New Delhi?
The IANA database uses the largest or most historically significant city in a region as the zone name. Kolkata was a major port and colonial capital. The name does not affect the time. All cities in India use the same offset.
Can I use Asia/Calcutta instead of Asia/Kolkata?
No. Asia/Calcutta is a legacy entry that was replaced by Asia/Kolkata in the IANA database. Use the current name.
Does Asia/Kolkata include the Andaman and Nicobar Islands?
Yes. All Indian territories use Asia/Kolkata, including the island territories.
What happens if India adopts a second time zone?
If India introduces a second time zone, the IANA database will create a new zone for that region. Asia/Kolkata would likely stay as the zone for the western part of the country. No such change has been adopted as of 2026. Northeastern states and some economists have made periodic calls for a second zone, but the one-nation-one-time policy remains dominant.
Who should use Asia/Colombo instead of Asia/Kolkata?
Anyone configuring a system inside Sri Lanka. The two zones share the same offset today, but their histories differ. Using the wrong zone can cause errors when converting historical dates before 2006, when Sri Lanka changed its offset.