GMT – Greenwich Mean Time

We all hear the term GMT, or Greenwich Mean Time. In general, we hear it when people refer to their time zone. And many people say that they are in UTC time when what they want to say is that they are in GMT time. There is a difference between GMT and UTC – and a pretty big on to say the least. The problem is that most people don’t know exactly what a time zone is and what it is being used for. They also don’t know what the UTC standard is and what the UTC offset is used for. So let’s talk about time zones, UTC, UTC offset, and the GMT time zone specifically. You can also find a list of interesting facts about time zones at the end of this article.

What Are Time Zones Exactly?

Without time zones, it would be impossible for all countries on Earth to have the sun at the highest point in the sky at noon. Why? Because Earth rotates by 15 degrees every hour. This is exactly why time zones were created. Basically, the planet was split into 24 slices of 15 degrees each. Each slice is a time zone. And all time zones are coordinated using the Coordinated Universal Time (UTC) standard. This is why Eastern Standard Time is UTC-5 and Pacific Standard Time is UTC-7. The difference between a given time zone and UTC time is called the UTC offset. In other words, Eastern Standard Time has a UTC offset of minus 5 hours (UTC-5). But remember, UTC is a time standard. GMT, on the other hand, is a time zone. So it is incorrect to say that you are in the UTC time zone. Yes, UTC and GMT are being used interchangeably at the time of writing. But this does not mean it is correct.

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. Again, it is important to note that UTC and Greenwich Mean Time (GMT) are not the same. GMT is a time zone and UTC is a time standard. 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.

A Short History of GMT

Introduced to help with naval navigation when travelling around the world, GMT was once the international civil time standard. At present time, this function is being fulfilled by the Coordinated Universal Time standard. However, in the UK 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. 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. 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 places 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.

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
  • The 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):

  • The 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. 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.

Interesting Facts About Time Zones

Now that you know everything there is to know about Greenwich Mean Time and about UTC and the UTC offset, let’s present a few very interesting things about time zones:

  • 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.
  • 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).
  • 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.”
  • 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.
  • GMT is also called Western European time and sometimes Greenwich Mean UT (an abbreviation of Universal Time).
  • Several countries, such as Belgium and Canada use Greenwich Mean Time to define their local time.
  • China is huge, yet it only has one single time zone: GMT+8:00. On the other hand, France has 12 different time zones.
  • 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.
  • 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. 
  • 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.
  • Speaking of India, this country – even though large – uses a single time zone (Indian Standard Time). China has a single time zone as well, and it’s called Beijing Time. This poses a problem because China is wide, and it should be in several times zones, not just in one. 

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