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GMT vs. UTC: What's the Difference?

Edited by Harlon Moss || By Janet White || Updated on October 7, 2023
GMT (Greenwich Mean Time) is a time zone; UTC (Coordinated Universal Time) is a time standard unaffected by Daylight Saving.

Key Differences

GMT and UTC are terms that are often used interchangeably, but they have distinct meanings and origins. GMT stands for Greenwich Mean Time and refers to the time zone that covers zero degrees longitude, based in Greenwich, London. In contrast, UTC, which means Coordinated Universal Time, is a coordinated time standard used globally.
GMT traces its roots to the Royal Observatory in Greenwich, London. The sun's position at noon at this location was once the basis for GMT. Meanwhile, UTC doesn't rely on the sun. Instead, it's based on atomic clocks and is occasionally adjusted with leap seconds to remain in sync with the Earth's rotation.
Over the years, GMT has been largely replaced by UTC in defining civil time. While GMT is affected by the minute shifts in the Earth's rotation, UTC ensures a more consistent, precise measurement, making it crucial for computing, navigation, weather forecasting, and international events.
Both GMT and UTC serve as reference points for the world's other time zones. However, it's important to note that while GMT can be affected by Daylight Saving Time changes, UTC remains consistent year-round, without adjustments for seasonal time changes.

Comparison Chart


Greenwich Mean Time
Coordinated Universal Time


Sun's position at the Royal Observatory
Atomic clocks

Affected by DST?

Yes (in regions that observe it)


Less precise due to Earth's rotation
More precise, adjusted with leap seconds


Historical time reference
Modern time standard worldwide

GMT and UTC Definitions


Based on the mean solar time at the Royal Observatory
GMT was historically determined by sundials in Greenwich.


The time standard used in aviation, computing, and space exploration
GPS systems synchronize to UTC for accuracy.


A predecessor to the modern UTC standard
GMT was widely used before the adoption of UTC.


Remains consistent and isn't adjusted seasonally
While local times may change with the seasons, UTC remains constant.


The time zone centered on zero degrees longitude
London operates on GMT during the winter months.


Based on highly precise atomic clocks
UTC ensures accuracy with atomic timekeeping.


Unaffected by atomic clock precision
GMT relies more on the Earth's rotation than precise atomic measurements.


Not a time zone but a time standard
While Paris is UTC+1, UTC itself doesn't shift with daylight saving.


Subject to changes in Daylight Saving Time in certain regions
When London observes Daylight Saving, it's no longer on GMT.


Adjusted with leap seconds
To account for Earth's irregular rotation, leap seconds are added to UTC.


The local time at the 0 meridian passing through Greenwich, England; it is the same everywhere


Greenwich Mean Time updated with leap seconds


Are GMT and UTC the same?

They're often used interchangeably, but GMT is a time zone and UTC is a time standard.

Is GMT still in use?

While UTC has largely replaced it, GMT is still used in certain contexts.

Why was UTC created?

For a more precise and consistent global timekeeping standard.

Does GMT rely on atomic clocks?

No, it's based on the mean solar time at Greenwich.

How often are leap seconds added to UTC?

They're added as needed, not on a regular schedule.

Which is used in aviation?

UTC is the standard in aviation.

Does UTC change for Daylight Saving Time?

No, UTC remains consistent year-round.

When was UTC first implemented?

UTC was formalized in 1963.

Which countries reference UTC for their standard time?

Most countries now reference UTC when setting their standard time.

Which is more precise, GMT or UTC?

UTC is more precise as it's based on atomic clocks.

Why don't we use GMT for everything?

UTC offers greater precision and consistency for modern needs.

Can I say London is always on GMT?

Not always; during Daylight Saving Time, it's on BST, not GMT.

Was GMT ever based on atomic clocks?

No, it's traditionally based on the sun's position at Greenwich.

Where is GMT based?

It's based on the Royal Observatory in Greenwich, London.

Do all countries adjust for Daylight Saving Time?

No, only certain countries observe DST.

Is UTC affected by Earth's rotation?

Indirectly; leap seconds are added to UTC to account for changes in Earth's rotation.

Are there other time standards besides GMT and UTC?

Yes, but UTC is the primary global standard.

Is GMT the same as UTC+0?

Technically, yes, but remember GMT is a time zone and UTC is a standard.

How do I convert from UTC to local time?

Add or subtract the local time zone offset from UTC.

Why is precise timekeeping like UTC important?

For accuracy in areas like computing, navigation, and international communication.
About Author
Written by
Janet White
Janet White has been an esteemed writer and blogger for Difference Wiki. Holding a Master's degree in Science and Medical Journalism from the prestigious Boston University, she has consistently demonstrated her expertise and passion for her field. When she's not immersed in her work, Janet relishes her time exercising, delving into a good book, and cherishing moments with friends and family.
Edited by
Harlon Moss
Harlon is a seasoned quality moderator and accomplished content writer for Difference Wiki. An alumnus of the prestigious University of California, he earned his degree in Computer Science. Leveraging his academic background, Harlon brings a meticulous and informed perspective to his work, ensuring content accuracy and excellence.

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