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What is a Time Zone?

A 'Time Zone' refers to any of 24 regions loosely divided by longitude, where the same standard time is kept.

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The Greenwich Meridian, also known as the prime meridian or International Meridian, is the "starting point" for dividing the Earth's surface into time zones. Each time zone is 15 degrees of longitude wide (with local variations) and the local time is one hour earlier than the zone immediately to the east on the map.

The number of standard time zones is debatable and discussed among various sources, particularly with regards to the International Date Line.

Not always 1 hour's difference

Some countries have non-standard time zones, usually with a 30-minute offset (a few have a 45-minute offset). For example, the time zone in India is five hours and 30 minutes (UTC+5:30) ahead of UTC.

Time zones’ boundaries are irregular mainly because of political factors, and so this has been a subject of criticism. Time zones can be determined by how countries’ and states’ borders are positioned. Individual zone boundaries are not straight because they are adjusted for the convenience and desires of local populations. Moreover, some geographically large countries, such as India and China, use only one time zone but other large countries, such as Russia and the United States, have more than one time zone.  

Why Use Time Zones?

Many towns and cities around the world used to set clocks based on observing the sun and the stars. This occurred prior to the late 19th century. Dawn and dusk occur at different times at different places because of the Earth’s rotation. However, time differences between distant locations were barely noticeable because of long travel times and the lack of long-distance communications. The expansion of transport and communications, as well as trade globalization, during the 19th century created a need for a more unified time-keeping system.

Moreover, various meridians were also used for longitudinal reference among different countries. The Greenwich Meridian was adopted in 1884 as the initial or prime meridian for longitude and timekeeping. Read timeanddate.com’s article on the history of time zones for more detailed information on why we have time zones today.

How Time Zones Work

Each time zone is then theoretically 15 degrees wide, corresponding to a one-hour difference in mean solar time. The shape of time zones is changed, in practice, to match internal and international borders. Civil time changes by one hour forward and backward respectively for every 15 degrees east or west of the Greenwich Meridian. One would need to divide the longitude, in degrees, by 15 to find the appropriate time zone, in hours. For example:

  • At 150 degrees west (or 150° W) longitude, the time should be 150 degrees divided by 15 degrees = 10 hours behind Coordinated Universal Time (UTC/GMT) (or UTC-10).
  • At 75 degrees east (or 75° E) longitude, the time would be 75 degrees divided by 15 degrees = 5 hours ahead of UTC/GMT (or UTC+5).

Time Zones & DST

Time can be measured in different ways. Various time systems have been developed over the years to measure time. Time’s passage can be measured via the orbital motion of Earth and other planets in the solar system (Dynamical Time), or through the oscillations of atoms (International Atomic Time). Solar time is based on the solar day, which measures the time between successive transits of the sun across the meridian. Time is also measured by the Earth’s rotation on its axis with respect to the stars (Universal Time). As mentioned earlier, UTC is the measure of time used as the basis for civil time-keeping. UTC is based on atomic time.

The world is divided into different time zones that are usually an integral number of hours different from UTC. They correspond to local time in countries and states within that zone. However, many countries adopt Daylight Saving Time (DST) in advance of local time during their summer period. One hour must be added, as a general rule, to the standard time if DST is in effect in the time zone.

For example, countries like Spain and France, are one hour ahead of UTC (or UTC+1) during the fall and winter months. However, they add one hour when they use DST (UTC+2).

The same applies for countries such as the United Kingdom, which moves from GMT/UTC to British Summer Time (BST) when it observes DST.

DST is taken into account for all cities in our World Clock.

Today’s Time Zones

The definition for time zones can be written in short form as UTC±n (or GMT±n), where n is the offset in hours. There are also some places that have UTC+13 and UTC+14 but these are not standard time zones, from a point of view that sees the number of standard time zones as 24. However, UTC+13 and UTC+14 are still referred to as integer time zones. There have been adjustments and alterations over the years on the original meaning of having all time zones in the UTC-12 to UTC+12 ranges.

Many sources claim that there are 24 standard time zones (eg. when ignoring the International Date Line) but some sources state that there are 25 time zones. The perspective of the number of time zones depends on the definition of a time zone versus the International Date Line. There are also non-standard time zones that follow a UTC offset of a certain number hours plus 30 or 45 minutes.

Please note that one may travel from a positive UTC offset (for example, UTC+12) to a negative UTC offset (for example, UTC–12) when crossing over the International Date Line. The International Date Line generally covers islands or offshore areas. timeanddate.com provides more details about standard and non-standard time zones.

Note: timeanddate.com wishes to acknowledge the UK’s National Maritime Museum for being a major source for time zone information supplied in some sections of this article. 

Topics: Time Zone, Timekeeping

Coordinated Universal Time (UTC)


Friday, October 9, 2015

UTC is the common time standard across the world

More about UTC

In this Article



  1. Time Zone – what is it and why have them?
  2. Daylight Saving Time
  3. What is Local Mean Time (LMT)?

Time zones worldwide

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