What Is a Time Zone?
The term time zone can be used to describe several different things.
Time Difference from UTC
The local time within a time zone is defined by its offset (difference) from Coordinated Universal Time (UTC), the world's time standard.
This offset is expressed as either UTC- or UTC+ and the number of hours and minutes.
More Than 24 Time Zones
If each time zone were one hour apart, each one should theoretically be 15 degrees wide on a map, resulting in a total of 24 worldwide. However, there are also several time zones with offsets that are only 30 and 45-minutes apart, making the total number of time zones much higher.
Not the Same as Local Time
The term time zone is often used instead of local time. For instance, during Daylight Saving Time (DST), it is common to say "California and Arizona are now in same time zone". However, the correct thing to say would be: "California and Arizona now have the same local time".
The reason is that California's local time during DST is UTC-7, but the standard time in California is minus one more hour: UTC-8. However, Arizona's local time is always UTC-7, because there's no DST in Arizona, and they remain on standard time all year.
Local Time Zone Names
To confuse matters more, each time zone has many different local time zone names, usually linked to the geographical name of the country or region.
The time zone names in the US are consistent: On the mainland, there are five time zones with five names and corresponding DST names. However, in other places in the world, the time zone names may be completely different, even though the UTC offset is the same.
For instance in Miami, Florida, the standard time zone is UTC-5, and it's called Eastern Standard Time – EST. In Havana, Cuba, the standard time zone is also UTC-5, but it's called Cuba Standard Time – CST.
Military Time Zones
There are also 25 military time zones which follow the 15 degrees longitudes. These are named according to the NATO phonetic alphabet: Alpha, Bravo, Charlie, etc. and are used in aviation, at sea and in telecommunications.
Another point that can cause confusion is that some time zone's names in totally different places have exactly the same abbreviation. For example:
In many parts of the world, especially in countries with only one time zone, time zone names are not commonly used at all.
Hot Political Potato
In most countries the political decision to make adjustments regarding time zones or DST, is made for practical reasons, like saving energy, facilitating trade with neighboring areas, or boosting tourism.
When and Why?
The expansion of the railway, and other transport and communications, as well as trade globalization, during the 19th century, created a need for a more unified time-keeping system, and time zones were introduced.