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Time Zone
 

A time zone or timezone is a region of the earth that has uniform standard time, usually referred to as the local time. By convention, time zones compute their local time as an offset from the Coordinated Universal Time, UTC ( or Greenwich Mean Time, GMT). Local time is UTC plus the current time zone offset for the considered location.

GMT is a term originally referring to mean solar time at the Royal Observatory in Greenwich, London where sits on the longitude 0°. It is commonly used in practice to refer to UTC when this is viewed as a time zone.

Moreover, Time zones are divided into standard and daylight saving (or summer). Daylight saving time zones (or summer time zones) include an offset (typically +1) for daylight saving time.

If you have the basic knowledge of a SUNDIAL, you will understand why Time Zones exist.

Time Zone Explanation

The black lines are shadows of the sundials.

For each 1° westward, the sun need 4 minutes more to go to the highest position which represents the 12:00 noon.

If the time at longitude 0° is the reference time, for each 1° westward, we have to add 4 minutes to tell the same time; for each 1° eastward, we have to subtract 4 minutes to tell the same time.

For a time zone, it includes 15° or 60 minutes in time scale.

e.g. Hong Kong is in the time zone UTC + 8.

Timezone Iceland

However, the time zone is not completely according to the geography.

For example, Iceland sits between longitude 15° and 25°, but it uses the time zone of United Kingdom which is the reference time UTC.

 

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