A time zone is a geographical area that has the same state-regulated time, known as "standard time".
Due to the size of some countries and the spread of other places belonging under their jurisdiction, one country can be divided into a number of time zones. The United States, for example, contain twenty-nine time zones.
Why do we have time zones?
The use of time zones makes it easier for us to organise our lives, from commercial operations up to traveling. People living within the same time zone know that they will be working at the same time, while someone in another time zone will work based on the local time of his or her own time zone.
The history of time zones
Time has traditionally been measured according to the position of the sun in the sky, which is different depending on where you are in the world. In the nineteenth century, when mechanical clocks began to become popular, time was calculated locally. Since the time calculations were based on the position of the sun, they could vary by four minutes for each degree of longitude. A better system was required to enable an efficient operation of railways and new telecommunication systems.
In 1847, British railway companies began to standardize the time they were using by providing their staff with portable chronometers, resulting in what became known as ‘Railway Time’. However, in 1855, the Royal Greenwich Observatory started transmitting time signals and in 1880, the Greenwich Mean Time (GMT) became the country’s official standard time.
Other countries created their own standard times and, in the late 1880s, the International Meridian Conference proposed a standardised 24-hour day, starting off at midnight GMT. Eventually the rest of the world began to use this system, shaping the time zones we know today.
UTC (Universal Time Coordinated)
UTC, also known as Universal Time Coordinated, replaced the Greenwich Mean Time (GMT) in 1961 as the time calculation system by which all zones calculated their time. It’s based on the International Atomic Time (TAI), a high-precision time calculation which is composed of an average time of more than 200 atomic clocks around the world, although UTC is slightly behind the International Atomic Time.
The world’s time zones are usually shown as either a positive (i.e. ‘+’ a specified number of hours UTC, meaning ahead of UTC) or a negative deviation of UTC (i.e. ‘-’ a specified number of hours UTC, meaning behind UTC).
DST (Daylight Saving Time)
The Daylight Saving Time is used by many countries during the summer, including the United Kingdom, Germany, Holland, Spain and France and in most American States, so that it is longer daylight in the evenings and longer dark in the mornings. In regions that are using Daylight Saving Time the clocks are typically moved forward by one hour at the beginning of the spring and moved back again by an hour in autumn.
The system, which is usually attributed the English-born astronomer and entomologist George Vernon Hudson, was first introduced in Germany during World War I to save coal by requiring less light in the evening. Other countries, including the UK, soon followed Germany.
Standard time is the name used for the uniform time created by the synchronization of all clocks within a time zone. In the UK, the standard time was introduced in the mid-nineteenth century, when clocks were set according to the Greenwich Mean Time (GMT), shown on the clock on the Prime Meridian at the Royal Observatory in Greenwich, London.
Standard times in other time zones around the world were shown as deviations from GMT until 1961 when the Universal Time Coordinated (UTC) was introduced.
The standard time of countries using the Daylight Savings Time is usually given without taking the DST into account.
Time differences between time zones are simple to work out due to the Universal Time Coordinated (UTC). Standard times are shown as positive or negative deviations of UTC, that's why time differences are calculated by determining UTC and calculating each time zone from there. If applicable, differences to standard times as a result of DST also need to be taken into account.
The simplest way to figure out the time in another part of the world is to make use of this website. It will help you to discover what cities are in which time zones and it will give you the standard times within each time zone. Furthermore, it will do time difference calculations for you.