UTC — Coordinated Universal Time
The reference time the world’s clocks are defined against. UTC never observes daylight saving.
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UTC +00:00
How it works
UTC is the reference every other zone is expressed against. It does not observe daylight saving and never shifts, which is why logs, APIs, aviation and international agreements are written in it: two systems that both record UTC agree without having to know anything about each other.
The clock here is derived from your device, converted to a zero offset. That makes it as accurate as your own computer, which is usually within a second or two of true UTC but is not a substitute for a synchronised time source. The clock check tool measures how far out your device actually is.
UTC is kept within 0.9 seconds of the Earth's rotation by inserting an occasional leap second. Those are announced only six months ahead, and the browser clock this page reads does not expose them, so a leap second passes here as an ordinary repeated second.
Examples
Reading a server log. Logs are stamped in UTC precisely so machines in different regions agree. A line at 2026-08-17T22:15:00Z is 18:15 the same evening in New York and 03:45 the following morning in New Delhi. The date changes for one of them, and that is the detail that trips people up when correlating events across regions.
Announcing a deployment window. "02:00–04:00 UTC" means one thing everywhere. "2am our time" does not, and it quietly means something different once one office changes its clocks and another has not yet.
Meeting a stated deadline. Where a cut-off is published in UTC, converting once and writing the local equivalent into your calendar is safer than doing the arithmetic later under pressure.
Practical notes
UTC never observes daylight saving, so a UTC timestamp means exactly one instant. That is why it is the right format for logs, APIs and anything two systems must agree on, and the wrong format for telling a colleague when to join a call — they will have to convert, and that is where errors enter.
The offset shown against your own zone is the one in force today. If you are scheduling something months out, check the offset on that date rather than assuming today's.
Note that this clock is your device's idea of the time expressed at zero offset. It inherits whatever error your device has. For anything where sub-second accuracy matters, use a synchronised source rather than a browser page.
Frequently asked questions
What is the difference between UTC and GMT?
In everyday use they name the same clock reading. UTC is the modern atomic standard; GMT is a time zone that the United Kingdom uses in winter, switching to UTC+1 in summer.
Does UTC change for daylight saving?
No. UTC is fixed all year, which is why it is used for logs, aviation, and any system that must not shift.
What is Zulu time?
Zulu time is the aviation and military name for UTC, from the letter Z used to mark a UTC timestamp.
Is UTC the same thing as GMT?
Day to day they name the same clock reading and share an offset. They are defined differently: GMT is a time zone tied to the Greenwich meridian, while UTC is an atomic time standard steered to stay within a second of the Earth's rotation.
What does the Z on the end of a timestamp mean?
Zero offset — the timestamp is already UTC, with no conversion pending. It is read aloud as "Zulu", from the NATO phonetic alphabet, which is why UTC is sometimes called Zulu time.
Why do systems log in UTC?
Because it never shifts, so two machines in different countries record the same instant the same way. Converting to local time is a display concern, not a storage one.
What is a leap second?
An extra second occasionally inserted to keep UTC within 0.9 seconds of the Earth's rotation. They are announced six months ahead and browser clocks do not expose them.
Is UTC a time zone?
Strictly it is a time standard that zones are defined against. In practice it behaves like a zone at zero offset that never observes daylight saving.
Does UTC ever change?
The offset never does. Leap seconds adjust it against the Earth's rotation without altering the zone.
Guides on this topic
- What Unix time is, and what it is not What a Unix timestamp counts, why it is the same in every time zone, how leap seconds and milliseconds fit in, and where 32-bit systems run out.
- ISO 8601, the date format that sorts itself How to read and write ISO 8601 dates, times, offsets, week dates, ordinal dates and durations, what the Z means, and how RFC 3339 differs.
- UTC and GMT: the same time, for different reasons Why UTC and GMT are usually treated as the same, where they genuinely differ, what leap seconds have to do with it, and which one to write in software.
- How to store dates and times correctly When to store UTC, when to store local time with a zone name, why offsets alone are not enough, which database types to use, and the parsing traps in JavaScript.