GMT – Greenwich Mean Time


Like most people, you’ve probably heard of GMT, or the Greenwhich Mean Time, and you know it’s used to refer to a time zone. However, you’ve probably also heard of UTC, and often, GMT and UTC are confused. Why? While both terms are used to define the “zero” hour on which the time zone calculations around the world are based, there is one big difference between GMT and UTC – and that is, one is a time zone and the other is a time standard. Curiously enough, the history of using GMT and UTC is intertwined, and it’s no wonder it’s still causing confusion. To clear it up, let’s talk about time zones, UTC, UTC offset, and the GMT time zone specifically: after all, time is precious, and understanding time zones can save you a lot of stress and hassle if you work or communicate with people around the world. And if you’re here for some fascinating time zone trivia, be sure to read the entire article – we’ve listed some fun time zone facts at the bottom of the page.

Ready to get your time right? Let’s dig right in:

What Are Time Zones Exactly?

You know that most countries around the world have their own time zone, but you’re not sure why? There’s a simple explanation: Earth rotates by 15 degrees every hour, and that means that noon hits different places of the planet at different times. Midday in Hawaii will occur at a different time than in Moscow precisely because of the rotation of the Earth, so to make sense of time around the world and to calculate what time is it where, time zones were introduced. This happened in the late nineteenth century when a Scottish-Canadian engineer and inventor Sandford Fleming proposed dividing the Earth into 24 slices of 15 degrees each. Each of these slices is a time zone, and each is coordinated using the Coordinated Universal Time (UTC) standard. The UTC standard monitors atomic time and is always at the “zero” hour; anything West of UTC will have a minus offset, and anything East of it will have a positive offset (that’s why you hear time being “behind” or “ahead” when you’re looking at different time zones around the world). As an example, Eastern Standard Time (New York Time) is UTC-5, meaning it’s five hours behind UTC, whereas time in Central Asia will be at UTC+5 – that is, five hours ahead. However, you’ve probably also noticed the same time zones referred to as GMT -5 or GMT+5, and it’s equally correct… or is it? If UTC is a time standard, and GMT is a time zone, why the confusion, and why is UTC and GMT used interchangeably? Let’s take a closer look at GMT to figure it out.

What Is Greenwich Mean Time?

Greenwich Mean Time, abbreviated GMT, is a time zone that covers parts of Europe, Africa and Antarctica. At the center of GMT is the Royal Observatory in Greenwich, London, which sits on the Greenwich Meridian, or longitude zero. For several centuries, GMT was used as both the time standard and a time zone, but it’s not the same as UTC: nowadays, GMT is firmly considered a time zone, and UTC is a time standard. In essence, GMT is basically UTC+0:00. In other words, Greenwich Mean Time has no UTC offset. 

However, this does not mean that all countries in the GMT time zone have the same hour. Some countries are observing Daylight Saving time, so they are in the GMT+1 time zone during the warm months of the year. In fact, even the United Kingdom is observing Daylight Saving Time during the summer months. In other words, the country that invented GMT is not in GMT time all year round. During summer, the UK is in the BST (British Standard Time) time zone.

It’s sometimes confusing to think that several countries around the world can share the time zone, but a simple way of understanding it is this: time zones are defined by degrees East or West rather than degrees South or North. That is, a time zone will cover the same countries vertically, which is why the UK and South Africa observe the same time. Once again, this has to do with the rotation of the Earth: noon is noon from North to South, because the poles do not move; West to East, however, sees a difference as the Earth turns around its own axis. Funnily enough, while time zones are technically strictly defined by geography, it can sometimes be a purely political decision. For example, India spans a vast territory with several time zones in the geographical sense, but ever since the colonial era, the country observes only one single time zone from border to border. It’s the same with China: although its territory is massive and easily hosts at least three or four time zones, the entire country observes only one – Beijing – time. It’s weird to think that the sun is only rising in Mumbai and is already at noon in Kolkata, but both cities agree it’s 9am as per India’s one time zone policy – but then, perhaps even time keeping can be an agreement rather than objective reality.

A Short History of GMT

After all, this is precisely what GMT is: an agreement. Introduced to help with naval navigation when travelling around the world, GMT was once the international civil time standard. GMT was established in the seventeenth century by the Royal Observatory in Greenwich, and it was used by sailors and naval officers as a time reference at different locations around the world. Sail fifteen degrees West of the Greenwich Meridian, and you offset the time by an hour; sail another fifteen degrees West, and it’s an offset by two hours, and so on.

However, we now know what the seventeenth century sailors did not: that GMT isn’t as precise as we once thought. Remember that whole Earth rotation thing? Well, as scientists have observed, there sometimes are slight irregularities in the rotation rate, and those irregularities sometimes cause what we know as “leap seconds”, or moments when zero hour isn’t exactly zero but is a few seconds off. To account for those missing seconds, the time standard needs to be periodically adjusted to follow the real, atomic time – and this is where UTC comes in.

UTC is a time standard that sets the base for time keeping and time zones around the world. However, in the United Kingdom, GMT is still being used for civil purposes, even though not formally. The Greenwich Mean Time was created in England and adopted at the International Meridian Conference of 1884; then, the Earth was split into 24 time zones, with each zone being based on the Greenwich Meridian Time. Each time zone had an offset of a number of hours ahead of GMT or behind GMT, as we’ve explained before.

However, because Earth’s rotation is irregular and slows down every year, GMT was replaced as the world’s standard by the Coordinated Universal Time standard. UTC time is kept using extremely precise atomic clocks that are placed in various locations around the world. UTC also has a leap second system in place. Because the Earth’s rotation is slowing down, there are minor differences between the UTC time and the atomic clock time. Periodically, a second is added to UTC to make up for the difference. Up to date, 27 additions have been made, so while GMT may sometimes be used to refer to a time standard, it would already be 27 seconds late – whereas UTC is not, thanks to those atomic clocks.

So can you still use GMT and UTC together? Sort of: whether you say UTC-5 or GMT-5, it’ll still define New York Time. For the sake of clarity, though, just remember that GMT now defines a time zone along the zero longitude, while UTC is a precise time standard based on atomic clocks.

How Does GMT Work?

The way Greenwich Mean Time (GMT) works is a little bit different from standard time zones such as EST or PST or daylight time zones such as CDT or EDT. In fact, it’s not really what people call typical time zones. The main difference is that Greenwich Mean Time doesn’t have an offset from Coordinated Universal Time (UTC). In fact, it’s used alongside standard time on some continents, including Europe, Africa, North America, and Antarctica. 

Unlike usual time zones, Greenwich Mean Time is calculated based on the sun. Specifically, it’s 12:00 noon at Greenwich when the sun is at its highest point. At noon, the sun is exactly above the Prime Meridian, the hypothetical line of 0° longitude that splits Earth into two equal parts. 

Greenwich Mean Time is the “mean” of the everyday time in the year when the Sun crosses the Prime Meridian at the Royal Observatory Greenwich. Therefore, GMT is more of a clock time and not an astronomical time that varies during the year. Instead, GMT is measured by a clock every day. The main purpose of Greenwich Mean Time is to standardize time regardless of the location. Consequently, GMT watches are widely used by travelers.

How to Convert GMT to Other Time Zones?

Since GMT is not a standard time that has an offset from UTC, converting it to other time zones can be a little bit more complicated. Mostly, people calculate time based on the Coordinated Universal Time (UTC) which is considered a worldwide standard. GMT is no longer used within the community of scientists but sometimes people still want to find out how GMT works and how to convert it to other time zones. 

If you know the principles of converting UTC to different time zones, then you can convert GMT as well very easily. The reason is very simple. There is no time difference between Coordinated Universal Time and Greenwich Mean Time. However, it doesn’t mean that they are exactly the same. In fact, GMT is a time zone while UTC is just standard. GMT is officially used in some European and African countries. But no country uses UTC as local time. Interestingly, neither GMT nor UTC change for Daylight Saving Time but some countries that observe GMT switch to different time zones during DST

In order to convert GMT to other time zones, use the following guide:

  • Greenwich Mean Time is 4 hours ahead of Eastern Time (ET).
  • Greenwich Mean Time is 5 hours ahead of Central Time (CT).
  • Greenwich Mean Time is 6 hours ahead of Mountain Time (MT).
  • Greenwich Mean Time is 7 hours ahead of Pacific Time (PT). 

So, in order to convert GMT to ET you need to subtract 4 hours; In order to convert GMT to CT you need to subtract 5 hours; In order to convert GMT to MT you need to subtract 6 hours, and in order to convert GMT to PT you need to subtract 7 hours.

Major Territories in GMT

Greenwich Mean Time is being used as the standard time the entire year (there is no Daylight Saving time adjustment) in the following countries:

  • Burkina Faso
  • Mauritania
  • Guinea-Bissau
  • Ivory Coast
  • Guinea
  • Sierra Leone
  • Liberia
  • Iceland
  • Mali
  • Gambia
  • Senegal
  • Ghana
  • Ascension
  • Togo
  • Saint Helena
  • Tristan da Cunha

There are also 5 countries that are in the GMT time zone, but that observe Daylight Saving time (GMT+1):

  • United Kingdom
  • Portugal
  • Western Sahara
  • Republic of Ireland
  • Morocco

Other territories that are in the GMT time zone include the Faroe Islands and the Canary Islands.

Largest Cities in Greenwich Mean Time

There are many major cities in the GMT time zone and in the GMT+1 times zone (Daylight Saving Time). Notable examples include London, Belfast, Cardiff, Dublin, Madrid, Lisbon, Edinburgh, Cork, Galway, Port, Aveiro, Reykjavik, and Danmarkshavn, so fi you’re calling a friend or a colleague in any of those cities, Google what the GMT is at the moment – otherwise, you’re risking interrupting them at an inconvenient time.

The exact number of people residing in the Greenwich Mean Time Zone is not known. However, it is estimated that there are fewer people in GMT than there are in the EST time zone in the United States (around 142 million people live in the EST time zone). A little-known fact is that Greenland is in the same time zone as Britain. Weird, right?

What is a GMT watch?

Did you know that GMT is the only time zone that has a special watch calculating GMT time? GMT watch is a 24-hour format watch that measures a second-time zone. Surprisingly, GMT watches aren’t made with the usual 24-hour scale but it’s accompanied by several benefits. 

GMT watches allow customers to set an additional time zone. Therefore, you can choose one, two, or three additional time zones on your watch. The watch has the ability to rotate once every 24 hours. It’s a perfect way to observe time together at various time zones without calculating time or using online time converters. 

Another benefit of GMT watches is that if you own one, there’s no way that you misinterpret a.m. with p.m. or vice versa. It’s an especially useful function for those who travel during the night. This function is caused by the fact that all 24 hours are accounted for on a watch and you just can’t confuse a.m. and p.m. As a result, GMT watches are frequently used by pilots as they ease the process of communication and navigation and help them avoid confusion. 

The First Public GMT Watch in the History

The Shepherd gate clock is considered as the first GMT watch ever that showed the public Greenwich Mean Time. The clock is connected to the Shepherd master clock and was installed at the Royal Observatory Greenwich in 1852. Since it’s masted from far away, the Shepherd gate clock is also called the “slave” clock. 

This network of “master” and “slave” clocks was created and installed by Charles Shepherd. Today it’s rather unusual in using the 24-hour dial but originally, it showed astronomical time starting at 12 noon instead of midnight. The most obvious thing about the Shepherd gate clock is that it has 24 hours instead of the usual 12 hours. Therefore, the 12 o’clock hour hand is pointing down rather than upwards. 

From 1850 to 1893, the Shepherd master clock led the time system of Great Britain. Using the telegraph wires, the clock sent the time to the major cities such as London, Edinburgh, Glasgow, Dublin, Belfast, and more. Even more, from 1866 time was also sent to Harvard University in Cambridge and Massachusetts. 

Back then, the Shepherd gate clock played the most important role in distributing accurate time to people in various places of the world. As a result, today it’s considered one of the most important clocks that were ever made. 

The first thing you notice about the clock is that it has 24 hours on its face rather than the usual 12. That means at 12 noon the hour hand is pointing straight down rather than straight up.

Initially, the Shepherd gate clock showed astronomical time according to which the counting of the 24 hours of a day starts at noon. However, in the 20th century. The clock was changed to GMT and from this time the counting of 24 hours started from midnight. The clock hasn’t been adapted to British Summer Time and today the clock still shows the GMT.

Interesting Facts About Time Zones

Now that you know everything there is to know about Greenwich Mean Time, UTC, and the UTC offset, let’s discover a few fascinating facts about time zones. Some of these are seriously mind-blowing:

  • Want to know what’s the time in space? It turns out, the time on the International Space Station follows Greenwich Mean Time. There is also an atomic clock placed on the space station to calculate the precise time for the UTC standard, but as a rule, astronauts adhere to the good old GMT.
  • GMT was the first standard to refer to the noon as zero hours. All other standards referred to midnight as zero hours (this practice is dating back to the Romans) – until the Brits intervened.
  • When GMT was established at the Royal Observatory in Greenwich, the site was declared to be “by international decree, the official starting point for every new day, year, and millennium.” No pressure, Greenwich!
  • The United Kingdom is not always on GMT because it observes Daylight Saving time during the summer. In other words, the British are on GMT+1 during the summer months, even though they created the GMT time standard… it seems the Brits are in the habit of getting ahead of themselves!
  • GMT is also called Western European time and sometimes Greenwich Mean UT (an abbreviation of Universal Time). It’s convenient but inaccurate as several Western European countries are in fact in GMT+1 or GMT+2, but then, it’s merely an hour’s difference.
  • Several countries such as Belgium and Canada use Greenwich Mean Time to define their local time. Hey, if it works, it works.
  • China is huge, yet it only has one single time zone: GMT+8:00, referred to internally as Beijing time. On the other hand, France, although small in comparison, has as many as 12 different time zones because it still holds territories around the world; bizarrely enough, this means that the sun never sets over France – we bet Louis XIV, or the Sun King, would be amused by this fact.
  • There is a three-hour time difference between China and Pakistan, and they share a border. If you were to travel between the two, you would need to adjust your clock by three hours as soon as you crossed the border, even if it took you five minutes to complete the actual crossing… Go figure!
  • Speaking of borders: the tiny island of Märket in the Baltic Sea is the size of three football fields, but it observes two different time zones because it belongs to both Sweden and Finland. Although you can literally walk to either side within a minute, you’ll need to adjust your watch to Swedish or Finnish time accordingly.
  • Russia spans over 12 time zones (yes, it’s huge). However, the Russians use just 9 time zones. Why? Because Vladimir Putin, the president, decided to get rid of 3 of the times zones. They are no longer being used in Russia – indefinitely. Even time has nothing on Mr. Putin, it would seem.
  • Not all countries use whole hours in their UTC offset. For example, India uses an offset of 5:30 hours (UTC+05:30). And Nepal time is UTC+05:45. So there are fractions of an hour being used for the UTC offset as well. Talk about attention to detail!
  • Feel like having multiple birthday parties? Thanks to the time zones, you can: celebrate your birthday at home, hop on a plane, and fly to a country that’s several time zones behind. Lo and behold: it’s your birthday again!
  • In fact, you can also fly into…yesterday or tomorrow. Because of the time zones across the world, you can leave Paris at 8am on Monday morning and find yourself back on Sunday in LA. Daylight savings suddenly start to seem cute, doesn’t it?

Now that you know what’s a time standard and what’s a time zone, what’s the history behind the Greenwich Mean Time, and how this whole world timekeeping works, you’ll never miscalculate time again – even in space. However, if you work or communicate with people scattered across the world, it’s usually more convenient to simply use a time zone converter rather than trying to calculate who’s behind and who’s ahead – after all, time is precious.