The short answer
UTC is the international time standard. GMT is commonly used as the name of the zero-offset time zone. For ordinary schedules, 15:00 UTC and 15:00 GMT refer to the same clock reading. They are not identical definitions, so UTC is the clearer term for software, science and global coordination.
What UTC means
Coordinated Universal Time is the internationally agreed reference for world time. NIST explains that UTC is based on International Atomic Time and is adjusted with leap seconds to remain close to UT1, a measure connected to Earth's rotation.
What GMT means today
Greenwich Mean Time is now commonly used as the time-zone name at the prime meridian and as a local representation of UTC. In the United Kingdom, GMT is the winter clock; British Summer Time is one hour ahead.
UTC and GMT compared
| Question | UTC | GMT |
|---|---|---|
| What is it? | International time standard | Time-zone name and historical mean-solar-time term |
| Zero-offset clock reading | 15:00 UTC | 15:00 GMT |
| Seasonal clock change? | No | GMT itself does not change; the UK switches between GMT and BST |
| Best use | APIs, logs, databases, aviation and international schedules | UK winter time, historical or regional wording |
| ISO/RFC offset form | Z or +00:00 | Prefer an explicit offset or named zone in machine data |
Which should you use?
- Use UTC for a timestamp that must be understood globally: server logs, APIs, databases and technical incident records.
- Use a named IANA zone such as
Europe/Londonfor a future local appointment that must follow regional clock changes. - Use GMT when you specifically mean UK winter time or are explaining the historical/regional term.
A fixed label such as “3 PM GMT” does not automatically follow British Summer Time. For an exact future comparison, use the date-aware meeting planner.
Frequently asked questions
Is UTC the same as GMT?
Their zero-offset clocks agree for ordinary civil use, but the terms describe different concepts: UTC is the international standard, while GMT is commonly a time-zone name.
Should software use UTC or GMT?
Use UTC for timestamps, APIs, databases and technical coordination. Use GMT when the time-zone name itself matters, such as UK winter time.
Is London always on GMT?
No. The UK uses GMT after the autumn clock change and BST, one hour ahead, after the spring change.
Sources
- NIST: how UTC relates to GMT, TAI and UT1
- UK Government: GMT and British Summer Time changes
- RFC 3339: date and time on the Internet
Last reviewed: 10 September 2026.