A time zone is a geographical area in which clocks are synchronised to show a uniform time, known as a ‘standard time’.
Due to the size of some countries and the location of places that come under their jurisdiction, one country can be divided into a number of time zones. France, for example, covers twelve time zones.
Why do we have time zones?
Using time zones makes it easier for us to organise everything from commercial operations to travel. People living within the same time zone know that they will be working to the same times, whilst someone in one time zone can work out the time in another time zone by doing a simple calculation.
The history of time zones
Time has traditionally been measured according to the position of the sun in the sky, but this 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 but, as time calculations based on the sun’s position can vary by four minutes for each degree of longitude, a better system became required so that railways and new telecommunications systems could operate efficiently.
In 1847, British railway companies began to standardise the time that they were using by equipping staff with portable chronometers, resulting in what became known as ‘Railway Time’. However, in 1855, the Royal Greenwich Observatory began to transmit time signals and in 1880, Greenwich Mean Time (GMT) became the country’s official standard time.
Other countries established their own standard times and, in the late 1880s, the International Meridian Conference proposed a standardised 24-hour day beginning at midnight GMT. Eventually the rest of the world began to use this system, creating the time zones we know today.
UTC (Universal Time Coordinated)
UTC, also known as Universal Coordinated Time or Coordinated Universal Time, replaced Greenwich Mean Time (GMT) as the time calculation by which all time zones calculate their times in 1961. It’s based on International Atomic Time (TAI), a high-precision time calculation which is arrived at by taking an average time from more than 200 atomic clocks located around the world, although UTC is slightly behind International Atomic Time.
The word’s time zones are usually shown as either as a positive offset from UTC (i.e. ‘+’ a specified number of hours UTC, meaning ahead of UTC) or a negative offset (i.e. ‘-’ a specified number of hours UTC, meaning behind UTC).
DST (Daylight Saving Time)
Daylight Saving Time is used by many countries, including the United Kingdom, Germany, Holland, Spain and France, and in most American States during the summer, so that it is lighter for longer during the evenings and darker for longer in the mornings. Clocks in regions that use Daylight Saving Time are typically moved forward by one hour at the beginning of the spring and moved back again by an hour again in autumn.
The system, the idea for which is usually attributed to George Vernon Hudson, an English-born astronomer and entomologist, was first introduced in Germany during World War I, partly in order to try to save coal by reducing the need for lighting in the evenings. Other countries, including the UK, soon followed suit.
Standard time is the name used for the uniform time obtained by synchronising the clocks within a time zone. In the UK, standard time was introduced in the mid-nineteenth century, when clocks began to be set according to Greenwich Mean Time (GMT), the time 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 offsets from GMT until 1961 when Coordinated Universal Time (UTC) was introduced.
When countries use Daylight Savings Time in summer, the standard time is usually shown without taking this into account.
Time differences between different time zones are simple to work out as a result of Universal Time Coordinated (UTC). As standard times are shown as positive or negative offsets of UTC, time differences are calculated by establishing UTC, and calculating the times in each zone from there. Differences to standard times as a result of Daylight Savings Time also need to be taken into account, if applicable.
The simplest way to discover the time in another part of the world is to consult this website. It will help you to establish which cities are in which time zones and tell you the standard times within each time zone. It will also perform time difference calculations for you.