Convert between UTC and IST
Most countries have time zones offset from Coordinated Universal Time (UTC) by whole hours (e.g., UTC+1, UTC-4). India is one of the rare countries (along with Sri Lanka, Afghanistan, Iran, and parts of Australia) that uses a 30-minute offset: UTC+5:30.
The Earth rotates 360 degrees in 24 hours. This means every 15 degrees of longitude equals exactly 1 hour of time difference (360 / 24 = 15).
India is a massive country geographically. Its westernmost point (Gujarat) is at ~68°E longitude, and its easternmost point (Arunachal Pradesh) is at ~97°E.
The difference in solar time between the east coast and west coast of India is almost exactly 2 hours!
Before Independence, India actually had two time zones: Bombay Time and Calcutta Time. In 1906, the British government decided to unify the country under a single time zone for railway and administrative efficiency.
To make it as fair as possible, they picked the exact geographical center of the country: a longitude of 82.5°E, which passes through Mirzapur in Uttar Pradesh.
If you divide 82.5 by 15 degrees-per-hour, you get exactly 5.5 hours. Thus, Indian Standard Time was born as UTC+5:30.
Why is Indian Standard Time offset by an unusual 5 hours and 30 minutes (UTC+5:30) instead of a round hour like UTC+5 or UTC+6?
