What Is a Time Zone?
The term time zone can be used to describe several different things, but mostly it refers to the local time of a region or a country.
At timeanddate.com we define a time zone as a region where the same standard time is used.
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.
UTC time changes 1 hour forward and backward corresponding to a 1-hour difference in mean solar time for every 15 degrees east or west of the prime meridian (0° longitude) in Greenwich, London, United Kingdom. The 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 1 hour apart, there would be 24 in the world. However, the actual borders on the time zone map have been drawn to match up with both internal and international borders, and rarely match up exactly with the 15-degree longitudes. Also, the International Date Line (IDL), creates 3 time zones and several time zones are only 30 and 45 minutes apart. This makes the total number of time zones worldwide much higher.
Daylight Saving Time Zones
Regions that use Daylight Saving Time (DST) change the time zone name and time during the DST period. The words “daylight” or “summer” are then usually included in the time zone name. The areas that don't use DST remain on standard time zone all year.
Not the Same as Local Time
The term time zone is often used instead of local time. For instance, during DST, it is common to say “California and Arizona are now in the 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 can have different local time zone names, usually linked to the geographical name of the country or region. The time zone names may be completely different, even though the UTC offset is the same.
For instance in Miami, Florida, is 5 hours behind UTC (UTC-5) and the standard time zone is 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 rule of 1 hour per 15 degrees longitude. These are named according to the NATO phonetic alphabet: Alpha, Bravo, Charlie, etc. and are used in aviation, at sea, and in telecommunications.
The reason there are 25 military time zones instead of 24 is that Mike Time Zone (M) and Yankee Time Zone (Y) are the same time, but on either side of the International Date Line. J (Juliet Time Zone) is occasionally used to refer to the observer's local time.
Another point that can cause confusion is that some time zone's names in totally different places have exactly the same abbreviation. For example, India Standard Time (IST) and Israel Standard Time (IST) have the same abbreviation, but completely different UTC offsets of UTC+5:30 and UTC+2:00.
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 boost tourism.