Often, we hear the term "Time Zone" when it's used to describe the current time for different areas of the world.
What it actually refers to is one of the specific regions or areas -- out of the 24 total regions in the world -- that are divided up by longitude. Within each one of those regions, a standard version of time is maintained.
The different time zones are calculated based on their relation to the coordinated universal time or UTC.
Related: What is UTC?
How Are Time Zones Determined?
What you may know as the prime meridian -- or more formally the Greenwich Meridian -- is the initial division line for all time zones. It's where dividing the Earth up into regions effectively starts.
The prime meridian is an imaginary line -- invisible by all rights -- that runs from north to south along the 0 degree longitude line of Earth. It is more commonly associated with Greenwich, England where the line passes through.
Each of Earth's 24 zones is one hour earlier then the one east of it, which equates to about 15 degrees longitude for each. Although this is the general rule, there's not always a one hour difference between each time zone.
Some countries have adopted a non-standard time, which may include a 30 or 45 minute offset. For instance, one of the more popular forms of this is in India. The India time zone is actually five hours and 30 minutes earlier than UTC time (UTC +5:30).
There are many reasons why this might be true. Sometimes it has to do with political factors, like in India, but other times it might have to do with the location of a countries' borders. Since time zones are defined by their longitude coordinates, some countries even have multiple time zones within them. Russia and the United States have multiple time zones within their borders, whereas other countries like India and China only use one.
Why Do Time Zones Exist?
Before the late 19th century, the standard for clocks was usually discerned by the sun and star patterns. It's just one of the reasons why astronomy was so important back then. This makes a lot of sense too considering the sun rises (dawn) and sets (dusk) at different times all over the world.
At the same time, travel back then -- which was primarily by sea -- took long periods of time. That meant that when people crossed over between time zones, they didn't experience much of a difference in terms of the local time.
However, when transportation and global communications became much more prominent in the 19th century, a need for a more unifed and universal time standard arose.
Naturally, in 1884 the Greenwich Meridian was designated as the central or "prime" meridian for the measurement of longitude and timekeeping.
How Is Standard Time Decided?
Each time zone is 15 degrees wide and there's a one hour difference between each one. Depending on the distance -- east or west -- from the Greenwich Meridian you must either add or subtract the appropriate time for every 15 degree interval.
To find the time zone in hours of a particular location, you can take the longitude -- in degrees -- and divide it by 15. So, for example, 75° E would be 75/15 which equals 5. That translates to the time zone being 5 hours ahead of UTC or GMT time, which can also be labeled as UTC+5.
Let's do another one. 30° E would be 30/15 which equals 2. When you match that with the correct time zone description it's UTC+2.
Obviously, if the latitude is west then you would subtract the appropriate hours. 150° W is UTC-10, because 150/15 equals 10.