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Had each time region been an hours apart, there'd be 24 in the whole wide globe. The real boundaries on the time zoning maps, however, have been set to coincide with both domestic and external boundaries and seldom exactly coincide with 15 degree lengths. In addition, the IDL provides 3 time domains and multiple time domains are only 30 and 45 mins apart.
As a result, the overall number of time zones is significantly higher around the world. Areas that use DST (Daylight Saving Time) modify the name and time of the time area during the DST season. In this case, the words "daylight" or "summer" are usually contained in the name of the time area. Those areas that do not use DST stay in the default time band throughout the year.
California, for example, uses Pacific Daylight Time (PDT) during summer time, but Pacific Standard Time (PST) during the year. This is because California's daylight saving time is UTC-7, but California's default time is minus another hour: Arizona time is always UTC-7 because there is no summer time in Arizona and they stay at normal time all year round.
What do we have time for? In order to further muddle the matter, each time domain may have different time domain designations, usually associated with the geographic name of the land or area. Timezone name can be totally different, even if the equivalent value is set for the timezone counter. In Miami, Florida, for example, 5 hrs past unTC (UTC-5) and the default time is Eastern Time ( EST ).
Also in Havana, Cuba, the default time is UTC-5, but it is referred to as Cuba Default Time (CST). Also there are 25 armies of time, which usually run from 1 hr per 15º long. This is because Mike Time Zone (M) and Yankee Time Zone (Y) are the same time, but on both sides of the International Date Line.
The J (Juliet Time Zone) is sometimes used to relate to the observer's time. A further point that can cause disorientation is that the name of some time zones has exactly the same shortcut in different places. India Standard Time (IST) and Israel Standard Time (IST), for example, have the same abbreviations, but very different offset UTCs of UTC+5:30 and UTC+2:00.
Timezone designations are not used at all in many parts of the globe, especially in single time domain states. The policy decisions in most jurisdictions to make changes to time zoning or summer time are taken for operational purposes, such as conserving electricity, easing trading with neighbouring areas or promoting tourist activities.
Timezone boundaries and summer time can be a policy instrument in some cases, most recently in Russia, Ukraine and North Korea.