What is a Time Zone?

A ‘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.

The Time Zone Map shows time zones as vertical bars on the map.

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

• 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, 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.