UTC, GMT and time zones: what is the difference and how offsets work

UTC is the world's time standard. A time zone is a rule that says how far local clocks are from it.

UTC

UTC (Coordinated Universal Time) is the time scale that the whole world's civil time is based on. Time zones are described as offsets from UTC: India is UTC+05:30 (5 hours 30 minutes ahead), Japan UTC+09:00, New York UTC-05:00 in winter and UTC-04:00 in summer. UTC itself has no daylight saving.

GMT

GMT (Greenwich Mean Time) is the time zone name used in the United Kingdom in winter, and an older time standard. In everyday use "GMT" and "UTC" show the same clock reading, but UTC is the precise standard, and UTC is the right word in technical writing. Note that the United Kingdom is on GMT only in winter: in summer it is on British Summer Time (UTC+01:00), which is not GMT.

Offsets and zone names

An offset (+05:30) is a fixed number. A time zone (Asia/Kolkata) is a set of rules that includes the offsets over time, including daylight saving and historical changes. Use zone names to schedule things in the future in a place, and offsets to record what actually happened. The IANA time zone database holds the zone names that operating systems and programming languages use.

Odd offsets

Most offsets are whole hours, but several are not: India UTC+05:30, Nepal UTC+05:45, parts of Australia UTC+09:30 and UTC+10:30 (in summer), Newfoundland UTC-03:30 (UTC-02:30 in summer), and the Chatham Islands UTC+12:45. The converter on this site handles them correctly; we tested it in some of these zones against Python.

Daylight saving time

Many places move their clocks by an hour in summer. In the spring, an hour of local time does not exist; in the autumn, an hour happens twice. This affects scheduled jobs, meetings and any calculation of durations in local time. India has no daylight saving.

Writing times safely

1. Include the date and the zone or offset.
2. Prefer the ISO 8601 form: 2026-11-13T10:20:00+05:30 or 2026-11-13T04:50:00Z.
3. Store times in UTC, and convert for display.
4. For the future in one place, store the local time and the zone name.
5. Do not use abbreviations as identifiers: IST can mean India, Israel or Ireland.

Common mistakes

1. Adding or subtracting 5.5 by hand and forgetting the date change.
2. Assuming that every zone changes its clocks on the same day.
3. Using GMT for a UK summer time.

This is an independent guide. It is not connected with the IERS, BIPM, IANA, NIST or any company named here. Facts were read on 14 November 2026 from the sources named on each page; calculated values come from Python.