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UTC vs GMT: What's the Difference, and Which Should You Use?
By Daniel Reyes · Engineering contributor5 min read
UTC and GMT look interchangeable but are defined differently. Here's what each one means, when the distinction matters, and the right one to use for schedules, code and contracts.
The short answer
For everyday scheduling they are interchangeable — they differ by less than a second. For software, legal contracts and scientific work, use UTC.
What each one is
GMT (Greenwich Mean Time) is a time zone, historically defined by the position of the Sun at the Royal Observatory in Greenwich. The UK uses it in winter.
UTC (Coordinated Universal Time) is a time standard, defined by atomic clocks and kept aligned with Earth's rotation through occasional leap seconds. It is not a time zone — it's the reference every time zone is offset from.
When the distinction matters
- Code: always store and transmit timestamps in UTC.
- Contracts: "by 17:00 UTC" is unambiguous; "by 17:00 GMT" can be misread as London local (which is BST in summer).
- Casual chat: either is fine.
Authoritative references
- BIPM: Coordinated Universal Time (UTC) — the body that defines UTC.
- Royal Observatory Greenwich: What is GMT?
- RFC 3339 — the timestamp format you should be using in APIs.
Frequently asked
- Is UTC the same as Zulu time?
- Yes — 'Zulu' is military/aviation shorthand for UTC. '14:00Z' means 14:00 UTC.
- Does UTC observe daylight saving?
- No. UTC never shifts. Only local zones offset from it (like BST or EDT) do.
- Why use UTC in databases?
- Storing all timestamps in UTC means a single instant has one canonical representation, regardless of where the user or server is. Convert to local time only at the display layer.