India's Single Zone: UTC+5:30 Coast to Coast
One clock governs 1.35 billion people. India spans roughly 29° of longitude, a distance that would typically contain two solar hours. Yet every railway station, airport departure board and smartphone from Gujarat to the easternmost state reads the same: IST, fixed at UTC+5:30. The IANA zone identifier is Asia/Kolkata for the entire country. There is no daylight saving time.
The reference meridian is 82.5°E, passing through Mirzapur in Uttar Pradesh. British India chose this line in 1906 because it sat roughly centrally among the major population centres of the undivided territory.
Solar Time vs Clock Time: The East-West Gradient
A single clock creates a visible east-west gradient in solar time. When the sun is directly overhead at the 82.5°E meridian, clocks read 12:00 IST. Solar noon in the far west and far east happens at very different clock times.
| Location | Longitude | Approx. solar noon in IST |
|---|---|---|
| Dwarka, Gujarat | 68°E | ~11:10 IST |
| Mirzapur (reference) | 82.5°E | 12:00 IST |
| Dibrugarh, Assam | 95°E | ~12:50 IST |
| Anjaw, Arunachal Pradesh | 97°E | ~13:00 IST |
The solar time difference between Gujarat and the easternmost state is roughly 2 hours. The sun rises and sets nearly two hours later by the clock in the northeast than in the west.
Key Cities and Their Solar Noon in IST
Solar noon is the moment the sun reaches its highest point. In a multi-zone country, this would sit close to 12:00 everywhere. In India, it shifts eastward across the day.
| City | State | Longitude | Solar noon in IST (approx.) |
|---|---|---|---|
| Ahmedabad | Gujarat | 72.6°E | 11:25 |
| Mumbai | Maharashtra | 72.8°E | 11:25 |
| New Delhi | Delhi | 77.2°E | 11:45 |
| Mirzapur | Uttar Pradesh | 82.5°E | 12:00 |
| Kolkata | West Bengal | 88.4°E | 12:30 |
| Guwahati | Assam | 91.7°E | 12:40 |
| Imphal | Manipur | 93.9°E | 12:50 |
| Itanagar | Arunachal Pradesh | 93.7°E | 12:50 |
How India Compares to Multi-Zone Countries of Similar Width
India's longitudinal span is comparable to other large countries. Its time zone policy is not.
| Country | Longitude span | Number of time zones |
|---|---|---|
| India | ~29° | 1 (IST, UTC+5:30) |
| China | ~62° | 1 (China Standard Time, UTC+8) |
| United States (contiguous) | ~45° | 4 (Eastern, Central, Mountain, Pacific) |
| Australia | ~40° | 3 (AWST, ACST, AEST) |
| Brazil | ~49° | 4 (Fernando de Noronha, Brasília, Amazon, Acre) |
China is the only other major country that uses a single time zone across a similar or larger longitudinal span. The US and Australia, both narrower than China, split into multiple zones to keep solar noon closer to 12:00.
Visualising the Two Time Zone Proposal on a Map
A recurring proposal suggests splitting India into two zones: IST (UTC+5:30) for the western and central parts, and a second zone at UTC+6:00 for the northeast. The proposed dividing line often runs near the 89°E meridian, roughly through the border of West Bengal and Assam.
Under such a plan, Guwahati would see solar noon shift from 12:40 IST to approximately 12:10 IST in the new zone. Still not 12:00, but closer to solar time than the current 40-minute lag. The CSIR-National Physical Laboratory has studied the proposal. Parliament has discussed it. No change has been made. The full details of the two time zone proposal belong to its own page.
If you live in or travel to India's northeast, know this: the sun rises and sets roughly two hours earlier than the clock suggests. In the easternmost state, the solar day starts before 04:00 IST in June. Plan mornings and school timings around the sun, not just the clock.