Difference Between GMT and UTC

Edited by Diffzy | Updated on: October 07, 2023


Difference Between GMT and UTC

Why read @ Diffzy

Our articles are well-researched

We make unbiased comparisons

Our content is free to access

We are a one-stop platform for finding differences and comparisons

We compare similar terms in both tabular forms as well as in points


Greenwich Mean Time and Coordinated Universal Time show the same time. No wonder many people scratch their heads trying to understand how they differ. Some people respond that both are the same when asked how they differ. Though this may seem hilarious, it is understandable why this notion persists. Similar to the former function of GMT, time zones are expressed in relation to UTC.

Most time zones differ from the Coordinated Universal Time by some hours; this difference is termed offset. However, no time difference exists between the Greenwich Mean Time and the Coordinated Universal Time. This is the reason why most people use the words interchangeably. UTC is a time-keeping system the world adopted to regulate time. Though considered a more accurate time standard than GMT, this system, too, has its flaws.


Greenwich Mean Time (GMT) is a time zone, whereas Coordinated Universal Time (UTC) is a time standard (a specification to measure time).

Difference Between GMT And UTC In Tabular Form

Parameters of ComparisonGMTUTC
Local TimeGMT is the official time zone of European and African countries.No country, territory, or region used UTC as the local time.
Abbreviation conformityGreenwich Mean Time was shortened to GMT in accordance with the abbreviation rule (combining the initial letter of each word).Coordinated Universal Time was shortened as UTC, which does not follow the abbreviation patterns. This non-conformity resulted from the desire to use the same abbreviation in all languages. (The English proposed CUT, whereas the French proposed TUC. Thank god people settled on UTC; using different abbreviations would have been confusing.)
UsesGreenwich mean time is the UK’s civil time according to the Interpretation Act 1978. Moreover, the Royal Navy, BBC, and Middle East Broadcasting Centers use GMT.All time zones are expressed as UTC offsets (positive or negative time difference between local solar time and UTC). Air traffic control, Internet standards such as Network Time Protocol, and International Space Station use UTC.
Time measurement methodGMT uses the Earth’s rotation to measure time.UTC uses International Atomic Time (a coordinate time scale) with added leap seconds to measure time.
Zero HoursIn GMT, zero hours refers to noon under the Astronomical Convention and midnight under the Civil Convention.Zero hours always refer to midnight under this time standard.

What Is GMT?

Greenwich Mean Time was the original Universal Time (though it was named as such only in 1928), which the UTC and UT1 later succeeded. In the International Meridian Conference of 1884, the Greenwich meridian was chosen as the prime meridian (0-degree longitude), and thus, Greenwich Mean Time became the international standard time. The other nations calculated time in relation to this standard.

Most time zones were expressed as a number of hours (sometimes half or quarter hours) ahead or behind GMT. Coordinated Universal Time superseded GMT on 1 January 1972 as the time standard. At present, GMT is only a time zone. The following countries use it as the standard time and advance their clocks one hour in summer:

  • United Kingdom (in summer BST, that is GMT+1 is used).
  • Ireland
  • Portugal
  • Canary Islands
  • Faroe Islands

Greenwich Mean Time for the whole year in countries such as Iceland, Guinea, Mauritania, Liberia, Ivory Coast, etc. The International Space Station uses GMT to synchronize the sleeping cycles of its space exploring crews. This practice of using Earth-based time is essential because a spacecraft encounters several sunrises and sunsets in 24 hours, making it impossible to measure time using the sun.

Everyone knows people experience jet lag due to trans-meridian travel. So, what exactly is this jet lag? It is the alterations to the circadian rhythms. People traveling from London to New York (from the GMT zone to the Eastern Time zone) feel as if the time is five hours earlier than their local time. This phase delay in the circadian rhythm is easier to adjust to.

However, when people travel east to west (for example from New York to London), they experience phase-advance in the circadian rhythm, making it much harder to adjust to the time zone. The pilots suffer more compared than the passengers, as they have to travel to and fro. Fortunately, airlines have regulations to combat pilot fatigue.

What Is UTC?

The Coordinated Universal Time came as a more precise replacement for Greenwich Mean Time. UTC is the preferable time standard, as it is based on over 450 atomic clocks’ measurements of time. An atomic clock monitors the resonant frequency of atoms to measure time. The results are more accurate and stable than using the earth’s rotation to measure time (as in the case of GMT).

As most people know, time units such as seconds, minutes, hours, days, and so on make up time. That remains the same in UTC; however, though seconds and smaller time units have constant duration, larger time units have variable duration. Makes one want to scream out in frustration, doesn’t it? Can’t people explain a concept in a non-complex way?

Well, the simple explanation is that a second is made up of 1000 milliseconds, and a thousand microseconds make up a millisecond; that remains constant (no changes whatsoever). However, a minute may be made of 59, 60, or 61 seconds. Why? Because leap seconds are added (rarely) to compensate for the difference between TAI and the time measured using earth’s rotation. Consequently, some UTC days have more than 86,400 seconds in them. Leap seconds are necessary to keep UTC within ± 0.9 seconds of UT1. UT1/Universal Time 1 is based on Earth’s rotation.

Operating systems use UTC as the time-keeping standard and provide additional services to convert it to local times and back again. International websites show UTC and may or may not show an additional arbitrary time zone. Google Calendars (time management software with calendars) stores time according to the UTC but displays the events according to the local time. The Windows-based systems used the local time until the emergence of Windows 2000. Every operating system Microsoft released after Windows NT4.0 came with the option to use UTC as the basic time system. Linus and Mac OS X use UTC by default.

BBC radio stations use the Greenwich Time Signal (based on UTC and formerly based on GMT) to mark the start of the hour. The GTS is nothing but a series of six short pips. The first five pips are broadcast on each of the 5 seconds leading to the hour, and the sixth pip sounds on the exact second the hour begins. Thus, the time interval between each pip is a second. The iconic six-pip format did not prevent the BBC radios from accommodating leap seconds. A seventh (long) pip is broadcast at 00:00:00 to indicate a leap second.

Adding or subtracting hours according to the UTC offsets helps determine the civil time of any time zone. UTC is not adjusted for daylight saving time. As such, a time zone seven hours behind the UTC will be only six hours behind when the country or region practices daylight saving. Time zones are mostly an hour apart, but sometimes they are only a half or quarter hour apart. Most countries use several time zones, whereas some countries use a single time zone. The time zones to the west of the prime meridian are hours behind the UTC and are expressed as negative UTC offsets. The ones to the east are ahead of UTC and are expressed as positive UTC offsets.

It is not unusual to be confused reading about how to express time zones. That is why the International Organization for Standardization established the ISO 8601 standard. A ‘Z’ is added after the time written down without space if the time is in UTC. Z is the zone designator for zero UTC offset. Therefore, 10:30 UTC is represented in textual form as 10:30Z or 1030Z. UTC offsets are written in the format ±hh:mm, ±hhmm, or ±hh. For example, Algeria’s time is expressed as +01:00, +0100, or +01. UTC is also known as Zulu Time since Zulu is the phonetic alphabet (code word) of the letter Z.

Though UTC is unaffected by daylight saving time, UTC offsets change with it. For example, the North American Central Time Zone is expressed as -06:00 in the winter (Central Standard Time is used in winter) and -05:00 in the summer (Central Daylight Time is used in summer). Time zones are often abbreviated; however, the ISO 8601 standard does not set forward any specification for it. Consequently, these abbreviations/designations can be ambiguous. CST may be the abbreviation of Central Standard Time, Cuba Standard Time (-05:00), or China Standard Time (+08:00).

Converting Time Zones Using UTC Offsets

UTC offsets make time zone conversions a breeze. Some people feel dealing with the conversion equation is like wrestling with math equations. However, this is a misconception. The conversion equation is straightforward and is as follows:

Time in zone A – UTC offset for zone A = Time in zone B – UTC offset for zone B (each side yields the UTC).

This equation can be rearranged as follows:

Time in zone B = Time in zone A – UTC offset for zone A + UTC offset for zone B.

Makes calculating time zones a cinch, right? The answer is yes until daylight saving time enters the equation. People should run for the hills if they come across a person who can calculate various time zones within seconds with DST playing a role in the equation. Who knows what else that person is capable of (shudder! Just kidding.). In all seriousness though, calculating time zones taking into account daylight saving time is tedious if done manually. On the other hand, using Google to solve the equation saves people a lot of time and frustration.

Alternatively, people can purchase a world clock or world-time watches that display the time for all the major time zones in the world (yay! No need to calculate time zones). Some manufacturers add the world-time label to clocks and watches displaying the time of a few time zones. However, these timepieces are erroneously labeled. Therefore, people have to double-check whether the timepiece indicates the time of all major time zones before they buy it.

Main Difference Between GMT And UTC (In Points)

  • GMT uses the 24 and 12-hour format to display time. Though UTC uses both time formats, the 24-hour format is often referred to as Zulu time (the military name for UTC).
  • The UTC measures time more precisely than GMT.
  • Initially, GMT was the time standard used; however, UTC succeeded it as the time standard in 1972.
  • United Kingdom’s civil time is advanced one hour forward of GMT during the British Summer Time (the period from one o’clock on the last Sunday in March to one o’clock on the last Sunday in October). This daylight saving practice has the effect of changing the time zone. No such changes occur in the UTC.
  • In UTC, minutes and the larger time units such as hour, day, week, and so on are of variable duration (due to the addition of leap seconds). This variability is not present in the time units of GMT.


People will get the same result whether they look at the time zone (GMT) or the time standard (UTC) if they want to know the universal time. Therefore, the world will not end if people cannot wrap their heads around the difference between GMT and UTC. Still, it is better to know the basic/primary difference between them to avoid looking like a fish out of water when people start conversing about time.

Besides, once people understand the various time-related concepts (time standard, time zones, and how to calculate and express them), they may find it worth exploring more. People never know what they may find interesting enough to obsess over until they try something. Moreover, people can show off the next time they accidentally end up at the nerds' table instead of trying to glare a hole into their brains for talking about something that sounds like Greek and Latin.


  • https://en.wikipedia.org/wiki/Coordinated_Universal_Time
  • https://en.wikipedia.org/wiki/Time_zone
  • https://en.wikipedia.org/wiki/Universal_Time
  • https://en.wikipedia.org/wiki/Greenwich_Mean_Time
  • https://www.worldtimeserver.com/learn/utc-vs-gmt/
  • https://24timezones.com/gmt-vs-utc
  • https://en.wikipedia.org/wiki/Daylight_saving_time
  • https://en.wikipedia.org/wiki/Greenwich_Time_Signal
  • https://en.wikipedia.org/wiki/Jet_lag
  • https://en.wikipedia.org/wiki/World_clock
  • https://mperdikeas.github.io/utc-vs-ut1-time.html


Cite this article

Use the citation below to add this article to your bibliography:



MLA Style Citation

"Difference Between GMT and UTC." Diffzy.com, 2024. Mon. 26 Feb. 2024. <https://www.diffzy.com/article/difference-between-gmt-and-utc>.

Edited by

Share this article