Nearly two solar hours from Gujarat to Arunachal Pradesh
At 68°E in Gujarat and 97°E in Arunachal Pradesh, the country covers 29° of longitude. Every 15° equals one solar hour. The sun rises and sets nearly two hours earlier in the east than in the west.
The same clock shows very different solar times at the two ends. When the sun is directly overhead in Arunachal Pradesh, around 11:00 local solar time, the IST clock reads about 05:30. Overhead in Gujarat, the clock reads about 07:30. The gap is persistent, not a seasonal quirk.
At 04:00 IST in June, the sky over Itanagar is bright. Over Bhuj, it’s still pitch black. At 19:30 IST in July, people in Surat are still outdoors; in Dibrugarh, they’ve already had dinner. The clock doesn’t shift, but daily life does.
The 1906 decision: unifying a subcontinent under one time
Before 1906, India didn’t have one time zone. It had several.
- Madras Time (UTC+5:21:14) was used by railways and observatories.
- Calcutta Time (UTC+5:53:20) was used in Bengal.
- Bombay Time (UTC+4:51) was used in the west.
Railway timetables were a mess. A single standard was needed for a unified railway network. On 1 January 1906, British India adopted a single Indian Standard Time based on the 82.5°E meridian, which passes through Mirzapur, near Prayagraj in Uttar Pradesh. The meridian was chosen because it sits roughly centrally among the major population centres of undivided India.
The offset was set at UTC+5:30, a half-hour offset, not a full hour. (The reason for the half-hour is covered in the article on why India is 30 minutes off.)
Post-independence continuity: why Nehru's government kept it
After independence in 1947, the Dominion of India and later the Republic of India retained IST unchanged. The question of splitting into two zones came up repeatedly: in a 2006 Planning Commission report, a 2018 CSIR-NPL proposal, and a 2021 parliamentary question. None were adopted.
Why keep one zone?
- National unity. A single time zone reinforces the idea of one country. Changing it would mean different states in the same country follow different clocks.
- Railway and logistics complexity. Indian Railways runs on IST nationwide. A second zone would require timetable splits, signalling changes, and confusion at zone boundaries.
- Economic integration. Financial markets (NSE, BSE) trade on IST. A second zone would fragment the trading day.
No IANA entry exists for any other Indian time zone. The entire country is in Asia/Kolkata.
How daily life adapts: Chaibagan Time and social jet lag
People in the northeast don’t wait for the clock to change. They adapt.
Chaibagan Time. In Assam’s tea gardens, workers often operate on an unofficial time called Chaibagan Time (tea garden time), set one hour ahead of IST at UTC+6:30. This is not a legal time zone. It’s a local practice to make better use of daylight. Clocks in the garden may show a different time than clocks in the nearest town. The practice is not universal and has no statutory backing. Check with the garden management for current practice.
Social jet lag. When the clock says 19:00 in Gujarat, the sun is still up, and people are active. When the clock says 19:00 in Assam, the sun has been down for two hours, and the body feels it should be later. This mismatch between clock time and solar time is called social jet lag. A 19:00 dinner in Gujarat feels like a 21:00 dinner in Assam because, by solar time, it is.
The case for and against a single zone
Arguments for keeping one zone:
- Simplicity. One country, one clock. No confusion for travellers, no split timetables, no border disputes.
- National identity. A single time zone is part of the idea of India.
- Business continuity. Markets, banks, and government offices operate on one schedule.
Arguments against:
- Lost productivity in the northeast. Workers start their day in darkness or end it in early sunset. Studies have estimated energy savings and productivity gains from a second zone.
- Health impact. Social jet lag is linked to sleep disruption, especially in the east.
- Chaibagan Time is a hack. The fact that tea gardens unofficially change their clocks shows the system doesn't fit everyone.
The debate continues, but no change is pending. In 2026 and 2027, IST remains the only legal time in India.
India's single time zone at a glance
| Item | Detail |
|---|---|
| Official time zone name | India Standard Time (IST) |
| IANA zone identifier | Asia/Kolkata |
| UTC offset | UTC+5:30 (fixed year-round) |
| Daylight saving time | Not observed |
| Reference meridian | 82.5°E (Mirzapur, Uttar Pradesh) |
| Adopted | 1 January 1906 |
| Longitudinal span | ~29° (68°E to 97°E) |
| Solar time difference | ~1 hour 56 minutes (Gujarat to Arunachal Pradesh) |
| Number of IANA zones | 1 |
| Unofficial local time | Chaibagan Time (UTC+6:30) in some Assam tea gardens |
| Timekeeper | CSIR, National Physical Laboratory (NPL), New Delhi |
Why does India use a half-hour offset instead of a whole hour?
India's offset of UTC+5:30 comes from the 82.5°E meridian, which is 5.5 hours ahead of UTC. The half-hour was chosen because the meridian falls between two full-hour zones. For a full explanation, see why India is 30 minutes off.
Does any other country use the same time zone as India?
Sri Lanka uses the same offset (UTC+5:30) for Sri Lanka Standard Time, but it is a separate legal standard. Nepal uses UTC+5:45, 15 minutes ahead of IST. Bangladesh uses UTC+6:00.
Has India ever used daylight saving time?
Only during World War II (1942 to 1945) under British rule. Independent India has never observed daylight saving time.
What is Chaibagan Time and is it official?
Chaibagan Time is an unofficial practice in some Assam tea gardens where clocks are set one hour ahead of IST (UTC+6:30). It is not a legal time zone and has no statutory backing. Check with the garden management for current practice.
Why doesn't India adopt two time zones for the east and west?
The proposal has been discussed multiple times but not adopted. Arguments against include national unity, railway complexity, economic fragmentation, and the cost of changing infrastructure. See the India two time zones proposal article for details.
What is social jet lag and how does it affect people in India?
Social jet lag is the mismatch between clock time and solar time. In northeast India, the sun rises and sets much earlier than the clock suggests, so people's daily schedules feel misaligned. A 19:00 dinner in Gujarat feels like 21:00 in Assam by solar time.