How Time Zones Work

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Have you ever woken a friend or relative with a call at 3:00 AM because you forgot what time zone they’re in? You wouldn’t be the first to make this common mistake, and you most certainly won’t be the last. In an increasingly intertwined global community, understanding time zones is as important a skill as ever. Once you learn how time zones work, you won’t need to recall time differences from memory or have to repeatedly look them up: you’ll be able to work them out.

How does the day/night cycle on Earth work?

The Earth rotates about its own axis. This means that a particular point on the Earth will complete a full rotation every day. This particular point will sometimes be facing towards the Sun and sometimes be facing away from the Sun.

As a quick analogy, let’s use a fidget spinner to depict the day/night cycle experienced by China and the USA, which are on opposite sides of the globe.


As the fidget spinner rotates in the clockwise direction, China experiences sunrise and then daylight while during the same period of time, the USA experiences sunset and then nightfall. China is around 13 hours ahead of the USA, meaning that the USA experiences daily phenomena 13 hours after China does.

The same logic can be applied to the Earth. As the Earth rotates, it causes countries to periodically face towards and away from the Sun in a specific order that corresponds to their positions on Earth.


How can I estimate the time difference between two countries?

Time differences are measured relative to Greenwich, United Kingdom, which is in the centre of a conventional Eurocentric world map.


If you have a rough idea of where place is located on a map of world, you can estimate the time zone it’s in. As an example, let’s imagine you live in Finland and want to call a friend who’s in Alaska, USA.

Question: Would it be suitable to call your Alaskan friend at a local Finland time of 2:00 PM?


With the knowledge that Alaska is to the far left of the conventional map, you could guess that it’s around 9 or 10 hours behind Greenwich time, and therefore is in the time zone UTC -9.

Similarly, Finland is a bit to the right of the map, so you could guess that is around 2 or 3 hours ahead of Greenwich time and is therefore in the time zone UTC+2.

Then you combine these results to conclude that Alaska is 11 hours behind Finland, so when it is 2:00 PM in Finland its 3:00 AM in Alaska, which is not a good time to call your friend.

Why do we need time zones?

If time zones didn’t exist, the conventional day/night cycle would occur at different times around the world, so in some places sunrise could be a 6:00 AM UTC (coordinated universal time) and in other places it could be at 9:00 PM UTC.

Time zones ensure that a conventional day is similar wherever you are on Earth. Sunrise is typically before 6AM and sunset is typically after 6PM in almost all geographical locations. This avoids confusion when travelling.

In fact, China has decided to have one time zone for the whole country (UTC+8), even though it spans five different UTC time zones. Western China feels the brunt of this decision, where in Winter months the Sun doesn’t rise until 10:00 AM.


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