Delhi Time Zone — Asia/Kolkata

Why Is the IANA Identifier "Asia/Kolkata" — Not "Asia/India" or "Asia/Delhi"?

The IANA timezone database (the Olson/tz database) follows one strict rule: each timezone identifier is anchored to the most populous city in the timezone at the time the identifier was created, never the national capital. When the tz database was first compiled in the 1980s, Kolkata (then Calcutta) was India's largest city by population. Delhi and Mumbai have since grown larger — but IANA only renames identifiers for compelling reasons, and population shifts don't qualify.

There is no "Asia/India" because timezone IDs must refer to cities, not countries. Country-named identifiers would collide when a nation has multiple timezones (the USA has six) and would invite political disputes. The identifier was Asia/Calcutta until 1993, when it was renamed to Asia/Kolkata following India's official city renaming. The old name remains as a deprecated backward-compatibility alias and still works on all modern systems.

Asia/Kolkata vs IST — Machine Identifier vs Human Abbreviation

Both refer to UTC+5:30, but they serve different audiences. Use Asia/Kolkata in code; use "IST" only in human-facing text where context is unambiguous.

⚠️ Never use IST in timezone config files, databases, or API payloads. Always use Asia/Kolkata.

UTC+5:30 History — How India's Offset Evolved Since 1880

Before British colonial standardisation, every Indian city observed its own local mean solar time based on the sun's position overhead. The journey to today's fixed UTC+5:30 passed through several distinct phases:

Asia/Calcutta → Asia/Kolkata: The 1993 Deprecation

In 1993, the IANA tz database renamed Asia/Calcutta to Asia/Kolkata following India's official city renaming. The original identifier remains in the database as a deprecated backward-compatibility link — it silently maps to Asia/Kolkata on all modern operating systems so that legacy code does not break.

What Countries Use UTC+5:30?

Only two sovereign countries permanently observe UTC+5:30:

• India — all 28 states and 8 union territories, 1.4 billion people. Official name: Indian Standard Time (IST). Fixed year-round since 1945.
• Sri Lanka — ~22 million people. Official name: Sri Lanka Standard Time (SLST). Sri Lanka briefly used UTC+6:30 and UTC+6 between 1996 and 2006 but has been stably at UTC+5:30 since May 2006.

Close neighbours — but not UTC+5:30:

• Nepal — UTC+5:45 (Nepal Standard Time). The extra 15 minutes are Nepal's deliberate differentiation from India.
• Pakistan — UTC+5 (Pakistan Standard Time, PKT). 30 minutes behind IST.
• Bangladesh — UTC+6 (Bangladesh Standard Time, BST). 30 minutes ahead of IST.
• Maldives — UTC+5 (Maldives Time, MVT).

Developer Guide — Asia/Kolkata is DST-Free

Asia/Kolkata has zero DST transitions since 1945. Unlike America/New_York or Europe/London, which shift UTC offset twice per year, Asia/Kolkata is permanently fixed. This dramatically simplifies date arithmetic — but one mistake is common:

Hardcoding +05:30 is fragile

The offset +05:30 happens to be correct today, but hardcoding an offset string instead of the IANA identifier is bad practice. It breaks pre-1945 historical queries, is not a valid timeZone argument in most engines, and fails to communicate intent to future readers of your code.

// ❌ Fragile — raw offset not valid in Intl API
new Date().toLocaleString('en-IN', { timeZone: '+05:30' });

// ✅ Correct — IANA identifier
new Date().toLocaleString('en-IN', { timeZone: 'Asia/Kolkata' });

What zdump shows

On Linux/macOS, zdump -v Asia/Kolkata | tail -5 shows the last recorded transition was 6 September 1945. After that: no transitions. The timezone data is a flat line from 1945 to the end of the IANA database.

Code Snippets: Asia/Kolkata in 7 Languages

const fmt = new Intl.DateTimeFormat('en-IN', {
  timeZone: 'Asia/Kolkata',
  dateStyle: 'medium',
  timeStyle: 'long'
});
console.log(fmt.format(new Date()));

import pytz
from datetime import datetime

ist = pytz.timezone('Asia/Kolkata')
now = datetime.now(ist)
print(now.strftime('%Y-%m-%d %H:%M:%S %Z'))

from zoneinfo import ZoneInfo
from datetime import datetime

now = datetime.now(ZoneInfo('Asia/Kolkata'))
print(now)  # 2026-03-20 14:30:00+05:30

import java.time.*;

ZoneId kolkata = ZoneId.of("Asia/Kolkata");
ZonedDateTime now = ZonedDateTime.now(kolkata);
// 2026-03-20T14:30:00+05:30[Asia/Kolkata]

loc, _ := time.LoadLocation("Asia/Kolkata")
now := time.Now().In(loc)
fmt.Println(now.Format(time.RFC3339))
// 2026-03-20T14:30:00+05:30

-- Convert stored UTC timestamp to IST
SELECT created_at AT TIME ZONE 'Asia/Kolkata'
FROM orders;

-- Current IST time
SELECT NOW() AT TIME ZONE 'Asia/Kolkata';

$tz  = new DateTimeZone('Asia/Kolkata');
$now = new DateTime('now', $tz);
echo $now->format('Y-m-d H:i:s P');
// 2026-03-20 14:30:00 +05:30

TZ='Asia/Kolkata' date
# Fri Mar 20 14:30:00 IST 2026

# Permanent for session:
export TZ='Asia/Kolkata'

The 82°30′E Meridian — Why India's Timezone Is UTC+5:30, Not UTC+5 or UTC+6

Indian Standard Time is anchored to the 82°30′E meridian. The math is direct: longitude ÷ 15 = UTC offset in hours. 82.5°E ÷ 15 = 5.5 hours = UTC+5:30. The +5:30 offset was not a political compromise — it was the straightforward solar-time calculation for the chosen meridian.

The 82.5°E line was chosen in 1906 as the mathematical midpoint between India's western extreme (~68°E, Kutch, Gujarat) and eastern extreme (~97°E, Arunachal Pradesh–Myanmar border). This meridian physically passes through Mirzapur, Uttar Pradesh — a mid-sized city on the Ganges with no special significance beyond lying on this line. You may see IST described as "Mirzapur time" in older documents for this reason.

India spans 30 degrees of longitude — about 2 hours of solar time. The sun rises roughly 1 hour earlier in Agartala (Tripura, 91°E) than in Ahmedabad (Gujarat, 72°E), yet both cities use the same clock. In parts of Arunachal Pradesh the sun rises before 4 AM by clock time in summer — a regularly cited argument for a northeast India timezone (UTC+6:30). As of 2026, no second Indian timezone has been enacted.