UTC vs GMT: What's the Difference?
If you look at a clock set to UTC and one set to GMT, they will show the same time. For planning a call or reading a flight schedule you can treat them as equal. But they are different things, and mixing them up can cause real mistakes in summer.
GMT: a time zone
Greenwich Mean Time is the mean solar time at the Royal Observatory in Greenwich, London, which sits on the prime meridian (0° longitude). In the 1800s it became the reference for navigation and, from 1884, the basis of the world's time zones.
Today GMT is used as the name of a time zone: the one used in winter by the UK, Ireland, Portugal and Iceland, and all year round by several West African countries such as Ghana and Senegal.
UTC: a time standard
Coordinated Universal Time is the world's reference clock. It is kept by a network of extremely accurate atomic clocks around the world rather than by the position of the sun, and every time zone is defined as an offset from it, such as UTC−5 for New York in winter or UTC+9 for Tokyo.
The Earth's rotation isn't perfectly steady, so atomic time and solar time slowly drift apart. Over the decades, a "leap second" has occasionally been added to UTC to keep it within a second of solar time. Leap seconds have been rare in recent years, and the world's timekeeping bodies have agreed to stop adding them by 2035.
In short, GMT is a time zone; UTC is the standard that time zones are measured against.
Why the abbreviation UTC?
The English name would suggest "CUT" and the French name (temps universel coordonné) would suggest "TUC". The compromise was UTC, which matches neither language exactly.
Where people get caught out: British Summer Time
UTC never changes for daylight saving time. The UK does. From late March to late October, the UK uses British Summer Time (BST), which is UTC+1. So in summer, London is not on GMT, even though many people still say "GMT" when they mean "UK time".
If someone sends you a time "in GMT" in July, it is worth asking whether they mean UTC or the time on a London clock, because the two are an hour apart.
Zulu time
Pilots, the military and weather services often write UTC as "Z" or "Zulu time", as in 1400Z. Zulu is simply the radio alphabet word for Z, the letter used for the zero-offset zone. It is the same as UTC.
Which should you use?
- For anything technical, such as servers, logs, flight plans and scientific data, use UTC. It never jumps for daylight saving time.
- For talking to people in the UK, say "UK time" or "London time" so that summer time is accounted for.
- To convert either one into your own time, use the UTC to EST or GMT to EST converters, or the Unix timestamp tool for computer times.
Quick answers
Is UTC the same as GMT?
They show the same time, but GMT is a time zone used by the UK in winter and some other countries, while UTC is the atomic-clock standard all time zones are measured from.
Is London on GMT in summer?
No. From late March to late October the UK is on British Summer Time, which is UTC+1.
What is Zulu time?
Zulu time is another name for UTC, used by pilots and the military. Times are written with a Z, such as 1400Z.
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