Civil time & global coordination
UTC vs GMT: What Is the Difference?
Understand UTC, the Greenwich GMT reference and date-aware London time. Everyday zero-offset arithmetic is simple; a city’s seasonal clock can still change a meeting result.
Short answer: UTC is the modern international reference used to coordinate civil time. GMT is the historical Greenwich time reference and is still used as a practical zero-offset label. India Standard Time is UTC+5:30.
UTC, GMT and IST in plain English
GMT is associated with the historic time at Greenwich, London. UTC is a modern, highly precise international time standard. For ordinary clock conversion, GMT and UTC are often treated as the same zero-hour reference, but they are not the same technical system.
India uses IST, UTC+5:30 throughout the country. India does not currently change clocks seasonally. A meeting listed as 10:00 UTC is 3:30 PM IST.
Why Greenwich appears in time lessons
The international prime meridian at Greenwich became the zero-longitude reference used for global longitude. That historical decision is separate from India's 82.5°E standard meridian and from Ujjain's historical astronomical importance.
When the distinction matters
Use UTC for technical systems, aviation, computing and international coordination. Use GMT when a schedule or institution specifically uses that label. For a city meeting, use the named city and date because daylight-saving rules can change the local result.
Read about Greenwich, Ujjain and India's meridian · Convert IST to another city · See sources and calculation limits
Choose the right label
| Label | Meaning in this guide | Scheduling example |
|---|---|---|
| UTC | Coordinated reference for civil timezone offsets. | 10:00 UTC = 15:30 IST. |
| GMT | Historic Greenwich mean-time reference; often a zero-offset schedule label. | 10:00 GMT normally uses the same zero-hour arithmetic in an everyday conversion. |
| London time | Date-aware local clock; may be GMT or BST. | 10:00 London in summer is not 10:00 UTC. |
| IST / Asia/Kolkata | Indian Standard Time, UTC+05:30. | Use the named timezone in recurring invitations. |
A London meeting is a dated city-time question
At 10:00 AM in London on 15 January 2026, the corresponding India time is 3:30 PM IST. At 10:00 AM in London on 15 July, it is 2:30 PM IST because London is on British Summer Time.
By contrast, 10:00 UTC converts to 3:30 PM IST on both dates. A schedule labelled “London” and one labelled “UTC” can therefore differ by an hour in summer. Try the reverse converter to see both local dates.