Definitions that matter
UTC (Coordinated Universal Time) is the atomic reference used by computers, aviation, satellites, and anybody who cares about precision. GMT (Greenwich Mean Time) is a historical term tied to the mean solar time at the Royal Observatory in London. In practice, the UK government uses GMT during winter and British Summer Time (BST, UTC+01:00) during summer. When a meeting invite says “10:00 GMT” in July, six different people will interpret it six ways. UTC never shifts, so using it removes the guesswork.
When the difference appears
Example: March product launch
A launch stream scheduled for March 24, 2026 includes production staff in London and viewers in Toronto. The UK moves to British Summer Time on March 29, so on the 24th both UTC and GMT align at 0 offset. If the run shifts to April 3, London becomes UTC+01:00 while UTC holds steady. Calling the show “18:00 GMT” would actually refer to 19:00 London after the shift. Basing the invite on UTC keeps the slot stable for crew and broadcast partners.
Example: November maintenance
Many SRE teams freeze deployments during the North American DST rollback because 01:30 repeats. If your runbook says “maintenance window 00:30–02:30 GMT” on November 1, UK-based responders assume the window occurs overnight locally. American responders read it as UTC. Clarify by stating “00:30–02:30 UTC” and linking to the World Clock wallboard that highlights London, New York, and UTC simultaneously.
How to communicate clearly
Use UTC in tooling, APIs, and incident timelines. For humans, present UTC alongside their local time. The TimeyKit Timezone tool makes this trivial: set city A to UTC and city B to the target region, then screenshot the overlap row. For permanent displays such as operations centers or pubs, pin UTC as a row in the World Clock wallboard so everyone can visually compare.
If you must mention GMT because of contract language or a broadcaster request, include the exact date and note whether the UK is on BST (UTC+01:00). Example: “Doors open 17:00 GMT (18:00 London during BST).”
Operational checklist
- Store timestamps in UTC, even if the interface shows GMT/BST wording.
- Write the weekday plus date on every schedule change notice.
- Link to the World Clock view that includes UTC, London, and your home base so stakeholders can confirm at a glance.
Try the tool
Load Timezone with one column set to UTC and the other to your collaborator’s city to preview conversions for any date. Keep a World Clock tab open for 24h rolling awareness.
FAQ
- Is GMT obsolete?
- No, but it is mostly ceremonial. Aviation, networking, and scientific communities rely on UTC.
- Does UTC observe leap seconds?
- Yes. UTC occasionally inserts leap seconds to stay aligned with Earth rotation. Most business scheduling tools smooth over the leap second internally.
- What about Zulu time?
- Zulu (Z) is the NATO phonetic for UTC+00:00. If you see 14:00Z it means 14:00 UTC.