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How do time zones work and why do we have them?

UTC, Greenwich meant time, Pacific standard? Time zones are confusing. How do time zones work, what are they, and why they are important?

By Annie André ⦿ updated January 10, 2024  
how do time zones work
how do time zones work

You’re probably familiar with the term “time zone,” but how many of us really understand how the concept of time zones work?

With names like UTC, Zulu time, Pacific Standard Time, Greenwich Mean Time, and Western European Time, it’s easy to get lost because time division across the world can be confusing. 

Here’s how time zones work. 

What is a time zone?

Time zones are like imaginary lines that divide the world into 24-hour sections based on time.

Knowing a country’s time zone helps us keep track of what time it is in different parts of the world, which is useful when travelling or communicating with people who live far away.

Time Zone Names:

map of world time zones

Most time zones are one hour ahead of or behind their neighbouring time zone.

To avoid confusion caused by different time zones and daylight saving time changes, countries around the world use an international standard for synchronizing clocks and timekeeping.

This standard is called UTC, short for Coordinated Universal Time, which replaced Greenwich Mean Time (GMT) in 1972.

UTC is not a physical location; it’s a reference point represented by how far a time zone is located from UTC+00:00, which is the time at the Prime Meridian in Greenwich, England

What’s the difference between (GMT) Greenwich Mean Time and UTC?

UTC always uses a 24-hour clock system, so there’s no need for AM or PM. It’s based on atomic clocks.

GMT is based on solar mean time (the position of the sun in the sky) at the Royal Observatory in Greenwich, London.

Although they are technically different, the terms UTC and GMT are often used interchangeably and refer to the same time standard.

Here is the current time at UTC±00:00

UTC 0, Greenwich Mean time map

Any country and city that is within the time zone UTC±00:00 will have the same local time.

For example, the following cities and countries are located in time zone UTC±00:00.

Cities Countries
Greenwich Scotland (UK)
London United Kingdom (UK)
Edinburgh Northern Ireland (UK)
Belfast Iceland
Dublin Portugal
Cardiff Ireland
Morocco Liberia
Monrovia Senegal

 (UTC±00:00) Time zone and Greenwich Mean Time 

UTC has many synonyms or equivalents that are all located at UTC+00:00.

These synonyms are used in different contexts and have different meanings, but they all refer to the same standard time. They include the following: 

  • GMT (Greenwich Mean Time) – Time zone that was once used as the world’s time standard. It was replaced by UTC in 1972 but is still sometimes used as a synonym for UTC.
  • WET (Western European Time) – Time zone used during the winter months. One hour behind CET.
  • Zulu Time or Z Time (Military Time) – the military term for UTC for timekeeping often used in aviation and military operations.

Regional time zone names:

North America Canada Mexico Time zones

In addition to UTC, each time zone can have different corresponding regional names. 

For example, if you’re from North America, you may already be familiar with the regional time zone names, Pacific Standard Time, central standard time, and Eastern Standard Time.

  • (UTC-05:00) = EST (Eastern Standard Time)
  • (UTC-06:00) = CST (Central Standard Time) 
  • (UTC-07:00) = MST (Mountain Standard Time)
  • (UTC-08:00) = PST (Pacific Standard Time)
  • (UTC909:00) = AKST (Alaska Standard Time)
  • (UTC-10:00) = HST (Hawaii Standard Time) 

Every country has at least one time zone.

Every country has at least one time zone, but many countries have more either because of the country’s size or due to overseas territories. 

For example, Metropolitan France, the part that is in Europe, has one time zone, but France has 12 more time zones due to its overseas territories. 

Here is a list of all the countries with three or more time zones. 

  • France time zones :13 
  • Russia time zones: 12
  • United States time zones: 11
  • Australia time zones: 9
  • United Kingdom time zones: 9
  • Canada time zones: 6 
  • New Zealand time zones: 5
  • Brazil time zones:4
  • Mexico time zones: 4
  • Greenland time zones: 4
  • Chile time zones: 3
  • Indonesia time zones: 3
  • Kiribati time zones: 3 

How are time zone borders determined?

France time zone is Central European Time (CET) zone, which is UTC+1

Theoretically, time zone borders are established based on the straight longitude lines on the Earth’s surface and run from pole to pole.

In reality, most are not straight lines.

If they were, a city located on a longitude line could have neighbors in two different time zones.

Instead, countries choose to create their own time zones based on their geographical features, borders, political boundaries, or other factors. As a result, most time zones are curved or jagged.

For example, France and the UK are geographically located along the prime meridian, which is the line of longitude for the UTC 0 time zone. ** See the image above.

However, France and several other European countries decided to adopt Central European Time (CET) as its standard time zone, which is UTC+1 instead, putting France’s time one hour ahead of the UK. 

France’s decision to adopt UTC+1 was made because it’s easier and more practical to have one standard time with other European countries for communication and transportation within the continent.

You can read more about the history of time zones at BBC. It involves trains and is pretty interesting. 

How are time zones calculated using UTC (Universal Coordinated Time)?

UTC times and time zones showing prime meridian and UTC +-00:00 in a world map

If you look at a map of the world’s time zones, UTC±00:00 is the center or the first time zone.

All other time zones are calculated in relation to how far ahead or behind they are to UTC±00:00, aka Greenwich, England.

How to calculate time based on two different UTC time zones. 

  • When you move East (to the right), one hour is added for each time zone you cross: you lose time. 
    • UTC+01:00
    • UTC+02:00
    • etc.
  • When you move West (to the left), one hour is subtracted for each time zone you cross: you gain time.
    • UTC-01:00
    • UTC-02:00
    • etc

Example calculation using Paris time zone

You can calculate the time difference using UTC times for two places by subtracting the UTC time of the first location from the UTC time of the second location.

For example, here’s how you would calculate the local time in San Francisco, if you were in the Paris time zone using their UTC times. 

  • Paris time zone= UTC+01:00 (CET) Central European Timezone.
  • San fanciso time zone: UTC-8:00 (PST) pacific standard time.

UTC+1 – (-8) = +1 + 8   =  9

So if it is 2 pm local time in San Francisco, 14:00 hours, you add 9, which means it’s 23:00 hours or 11 pm in Paris.

Let’s do the opposite:

UTC -8-(+1) = -8 – 1 = -9

So, if it’s 11 pm / 23:00 hours in Paris, you subtract -9, which means it’s 14:00 hours or 2 pm in San Francisco.

Paris Time San Francisco Time
00:00 (12:00 AM) 16:00 (4:00 PM)
01:00 (1:00 AM) 17:00 (5:00 PM)
02:00 (2:00 AM) 18:00 (6:00 PM)
03:00 (3:00 AM) 19:00 (7:00 PM)
04:00 (4:00 AM) 20:00 (8:00 PM)
05:00 (5:00 AM) 21:00 (9:00 PM)
06:00 (6:00 AM) 22:00 (10:00 PM)
07:00 (7:00 AM) 23:00 (11:00 PM)
08:00 (8:00 AM) 00:00 (12:00 AM)
09:00 (9:00 AM) 01:00 (1:00 AM)
10:00 (10:00 AM) 02:00 (2:00 AM)
11:00 (11:00 AM) 03:00 (3:00 AM)
12:00 (12:00 PM) 04:00 (4:00 AM)
13:00 (1:00 PM) 05:00 (5:00 AM)
14:00 (2:00 PM) 06:00 (6:00 AM)
15:00 (3:00 PM) 07:00 (7:00 AM)
16:00 (4:00 PM) 08:00 (8:00 AM)
17:00 (5:00 PM) 09:00 (9:00 AM)
18:00 (6:00 PM) 10:00 (10:00 AM)
19:00 (7:00 PM) 11:00 (11:00 AM)
20:00 (8:00 PM) 12:00 (12:00 PM)
21:00 (9:00 PM) 13:00 (1:00 PM)
22:00 (10:00 PM) 14:00 (2:00 PM)
23:00 (11:00 PM) 15:00 (3:00 PM)

Wrapping up world time zones

You may never had to manually calculate time zones, but it’s useful to understand how they work because they help us tell these different time periods apart and synchronize our activities across different parts of the world for business, communication, travel and for planning our activities more efficiently.

Disclosure: This post may contain affiliate links, meaning I get a 'petite commission' at no extra cost to you if you make a purchase through my links. It helps me buy more wine and cheese. Please read my disclosure for more info.

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Annie André

Annie André

About the author

I'm Annie André, a bilingual North American with Thai and French Canadian roots. I've lived in France since 2011. When I'm not eating cheese, drinking wine or hanging out with my husband and children, I write articles on my personal blog annieandre.com for intellectually curious people interested in all things France: Life in France, travel to France, French culture, French language, travel and more.

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