Before getting into the specifics of time zones we should probably start off by pointing out what a time zone is. A time zone is a region that registers a uniform standard time at any given time in its entirety for social, legal, or commercial purposes. As a rule, time zones follow the boundaries of all countries and their subdivisions to ease communication with the world around them at all times.
There are 24 time zones, each referring to 24 regions divided by longitude in which a standard time is kept. The prime meridian is called the ‘Greenwich Meridian’ and is considered to be the starting point for dividing our planet into time zones. Each of these time zones is 15 degrees of longitude wide and they are mostly one hour later than the nearest western time zone. While the number of standard time zones is debatable, the international community tends to agree on the specifics of time zones.
The purpose of time zones
It used to be that each city around the world had its own particular time, as they would set their clocks by observing the stars. Because dusk and dawn occurred at different times around the world, it was extremely hard for regular people to establish trading and social relations with people from different, sometime far away towns and cities. Actually, due to the fact that cities aren’t positioned within specific distances from one another, the further the cities war, the harder it was to determine what the local time was.
Following the expansion of trade, transport, and international communications during the 19th century, the creation of an international, unified time-keeping system became imperative. As we already pointed out, the first adopted Meridian was the Greenwich Meridian in 1884 and it remained the de facto ‘starting point’ for the succession of all 24 time zones. In theory, each time zone is 15 degrees wide, corresponding to a one-hour difference from the time zone in its immediate proximity.
One hour difference between time zones
Given the theoretical 15 degrees difference between time zones, most time zones are one hour away from each others, but not always. Some countries, like India for example, have non-standard time zones, suffering from a 30 or even 45 minute offset between time zones, meaning that India’s time zone is five and a half hours ahead of UTC. This is mainly due to political factors and has been openly criticized by the international community for quite some time now.
Most of the time, however, time zones are determined by how countries are positioned and are mostly established considering the convenience of local populations. Large countries like Russia, China, and the United States on the other hand, have more than one time zone.
We should point out that each time zone is 15 degrees wide from one another, corresponding to a one hour difference in solar time. Therefore, civil time changes by one hour either forward or backward for every 15 degrees east or west if the Greenwich Meridian.
The many time zones
We nowadays use UTC (Coordinated Universal Time) as the primary time standard by which every country in the world regulates their time. We also use DST (Dynamical Time) as a way to measure time’s passage via the orbital motion of our planet and other planets in the solar system.
You see, solar time is based on the solar day – the time it takes our sun to transit across the meridian. This is why sometimes people also use Earth’s own rotation on its axis to quantify the passage of time while taking its alignment with the stars under consideration (Universal Time).
Depending on who you ask, people will tell you that there are either 24 or 25 different time zones. This varies because of the very definition of a time zone which refers to the Greenwich Meridian as the standard, whereas some consider the International Date Line (IDL) to be the standard people should use to measure time zones. Therefore, there are 24 time zones if we are to take standard time zones in account, but 25 when considering non-standard time zones.