GMT and UTC are often used as if they're the same thing — and in everyday life they line up exactly. But technically they're different, and knowing which to use matters for software, aviation and global scheduling. Here's the difference, explained simply.
To see live time anywhere relative to UTC, use our world clock or time zone converter.
The quick answer
- UTC (Coordinated Universal Time) is the world's time standard — the reference all clocks and time zones are measured against. It is not a time zone; it never changes for daylight saving.
- GMT (Greenwich Mean Time) is a time zone — the local time at the Greenwich meridian (0° longitude) in London during winter.
In practice, UTC and GMT show the same clock time (UTC+0). The difference is what they are: UTC is the standard; GMT is one zone that happens to sit at UTC+0.
The key differences
| UTC | GMT | |
|---|---|---|
| What it is | Global time standard | A time zone |
| Based on | Atomic clocks (+ leap seconds) | Earth's rotation / the Sun at Greenwich |
| Used by | Aviation, computing, science, international standards | The UK (winter), West Africa, everyday speech |
| Daylight saving? | Never changes | The UK shifts to BST (UTC+1) in summer |
Why was UTC created?
GMT is based on the Sun's position over Greenwich — but Earth's rotation isn't perfectly steady. In the 20th century, scientists built atomic clocks far more precise than the Earth's wobble. UTC was introduced to combine that atomic precision with the astronomical day, occasionally adding a leap second to stay in step with the planet's rotation. So UTC is the modern, precise successor to GMT.
Which should you use?
- For software, servers and APIs: always UTC — it's unambiguous and never shifts for daylight saving.
- For everyday UK time: people say GMT in winter and BST in summer.
- For scheduling across countries: anchor to UTC, then convert — our time zone converter and meeting planner do this for you.
Quick answers
Is GMT the same as UTC? They show the same time (both UTC+0), but UTC is a global standard based on atomic clocks, while GMT is a time zone based on the Sun at Greenwich.
Does UTC change for daylight saving? No. UTC never changes. The UK's clock changes to BST (UTC+1) in summer, but UTC itself stays fixed.
Why do pilots and computers use UTC? Because it's a single, unambiguous global reference that never shifts for daylight saving — avoiding confusion across time zones.
What is a leap second? An occasional one-second adjustment added to UTC to keep it aligned with Earth's slightly irregular rotation.