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General Information on Time Zones

Have you ever wondered how time zones work and why we have them? timeanddate.com’s general article on time zones explores the concept of time zones, why they exist and how they operate. This article also gives a brief overview of the International Date Line, the Greenwich Meridian, and other time-related matters regarding time zones.

What is a Time Zone?

A standard time zone generally refers to any of the 24 regions of the earth’s surface (loosely divided by longitude) in which standard time is kept. However, number of standard time zones is debatable and discussed among various sources, particularly with regard to the International Date Line. More information about this issue is briefly examined later in this article. It is important to note that some countries have non-standard time zones, usually with a 30-minute offset (some have a 45-minute offset). For example, India maintains a time zone of five hours and 30 minutes ahead of Coordinated Universal Time (UTC+5:30).

The Greenwich Meridian, also known as the prime meridian or International Meridian, bisects the primary division of time zones. Each time zone is 15 degrees of longitude in width, with local variations, and observes a clock time one hour earlier than the zone immediately to the east.

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 We Have 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:

If you travel around the world, changing standard time by one hour each time you enter a new time zone, then a complete circuit would mean that you adjusted your clock or watch time by 24 hours. This would lead to a difference of one day between the date on your clock and the real calendar date. To avoid this, countries are on either side of the International Date Line which runs down the middle of the Pacific Ocean. If you cross the date line moving east, you subtract a day, whereas if you are moving west you add on a day.

The International Date Line

The International Date Line is an imaginary line of longitude on the earth’s surface located at about 180 degrees east (or west) of the Greenwich Meridian. This is the line across which the date changes by one day. It makes some deviations from the 180-degree meridian to avoid dividing countries in two, especially in the Polynesia region. Moreover, the time difference between either side of the International Date Line is not always exactly 24 hours because of local time zone variations.

The International Date Line is mentioned in a book by Jules Verne, titled Around the World in Eighty Days, which describes an English adventurer’s challenge to circle the globe within 80 days or less. The central character embarks on this quest and as the story comes to a close, he believes he has lost the bet but discovers that he forgot to adjust his timekeeping for having crossed the International Date Line and wins the bet after all.

The Greenwich Meridian

The Greenwich Meridian, which was mentioned earlier in this article, is a north-south line selected as the zero-reference line for astronomical observations. The line in Greenwich, the United Kingdom (UK), represents the world’s prime meridian – longitude zero degrees. Every place on earth is measured in terms of its distance east or west from this line. The line divides the earth’s eastern and western hemispheres just as the equator divides the northern and southern hemispheres.

The earth’s crust moves very slightly on an ongoing basis so the prime meridian’s exact position is also moving very slightly. However, the prime meridian’s original reference remains to be the Airy Transit Circle in the Royal Observatory in Greenwich, even if the exact location of the line may move to either side of transit circle’s meridian. This location is the home of Greenwich Mean Time (GMT) and the world’s prime meridian, which was chosen in 1884.


GMT was first adopted as the world’s time standard at the Washington Meridian Conference in 1884. GMT is no longer the basis for civil time but is now loosely interchanged with UTC to refer to time kept on the Greenwich meridian (longitude zero). Places such as the United Kingdom observe GMT during the non-daylight saving period. UTC is the basis for civil time in many places worldwide.

Time, Time Zones and Daylight Saving Time

It is important to note that 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, such as Spain and France, are one hour ahead of UTC (or UTC+1) during the non-daylight saving period. However, they add one hour during when they observe DST.  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 also applies to the Royal Observatory in Greenwich, UK. DST is taken into account for places shown on timeanddate.com’s World Clock.

Today’s Time Zones

Many nations worldwide use UTC in the definition of their time zones instead of GMT. 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. 


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