A time zone 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 UTC (see also Greenwich Mean Time). Local time is UTC plus the current time zone offset for the considered location.
Introduction
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.
Standard time zones can be defined by geometrically subdividing the Earth's spheroid into 24 lunes (wedge-shaped sections), bordered by meridians each 15° of longitude apart. The local time in neighboring zones would differ by one hour. However, political boundaries, geographical practicalities, and convenience of inhabitants can result in irregularly-shaped zones. Moreover, in a few regions, half-hour or quarter-hour differences are in effect.
Before the adoption of time zones, people used local solar time. Originally this was apparent or true solar time, as shown by a sundial, and later it became mean solar time, as kept by most mechanical clocks. Mean solar time has days of equal length, but the difference between mean and apparent solar time, called the equation of time, averages to zero over a year.
The use of local solar time became increasingly awkward as railways and telecommunications improved, because clocks differed between places by an amount corresponding to the difference in their geographical longitude, which was usually not a convenient number. This problem could be solved by synchronizing the clocks in all localities, but in many places the local time would then differ markedly from the solar time to which people were accustomed. Time zones are a compromise, relaxing the complex geographic dependence while still allowing local time to approximate the mean solar time. There has been a general trend to set the boundaries of time zones west of their designated meridians in order to create a permanent daylight saving time effect. The increase in worldwide communication has further increased the need for interacting parties to communicate mutually comprehensible time references to one another.
Standard time zones
Until fairly recently, time zones were based on Greenwich Mean Time (GMT, also called UT1), the mean solar time at longitude 0° (the Prime Meridian). But as a mean solar time, GMT is defined by the rotation of the Earth, which is not constant in rate. So, the rate of atomic clocks was annually changed, or steered, to closely match GMT. In January 1972, however, atomic clock rates were fixed and predefined leap seconds replaced rate changes. This new time system is called Coordinated Universal Time (UTC). Leap seconds are inserted to keep UTC within 0.9 seconds of UT1. In this way, local times continue to correspond approximately to mean solar time, while the effects of variations in Earth's rotation rate are confined to simple step changes that can be more easily applied to obtain a uniform time scale (International Atomic Time or TAI). With the implementation of UTC, nations began to use it in the definition of their time zones instead of GMT. As of 2005, most but not all nations had altered the definition of local time in this way (though many media outlets fail to make a distinction between GMT and UTC). Further change to the basis of time zones may occur if proposals to abandon leap seconds succeed.
Due to daylight saving time, UTC is the local time at the Royal Observatory, Greenwich only between 01:00 UTC on the last Sunday in October and 01:00 UTC on the last Sunday in March. For the rest of the year, local time there is UTC+01, known in the United Kingdom as British Summer Time (BST). Similar circumstances apply in many other places.
Notation of time (ISO 8601)
UTC
If the time is in UTC, add a 'Z' directly after the time without a space. 'Z' is the zone designator for the zero UTC offset. "09:30 UTC" is therefore represented as "09:30Z" or "0930Z". "14:45:15 UTC" would be "14:45:15Z" or "144515Z".
UTC time is also known as 'Zulu' time, since 'Zulu' is the NATO phonetic alphabet word for 'Z'.
Time zone as offsets from UTC
Time zone are written as offset from UTC in the format ±:, ±, or ±. So if the time being described is one hour ahead of UTC (such as the time in Berlin during the winter), the zone designator would be "+01:00", "+0100", or simply "+01". This is appended to the time in the same way that 'Z' was above. The offset from UTC changes with daylight saving time, e.g. a time offset in Chicago, would be "−06:00" for the winter (Central Standard Time) and "−05:00" for the summer (Central Daylight Time).
Abbreviations
Time zones are often represented by abbreviations such as "EST, WST, CST" but these are are not part of the international time and date standard ISO 8601 and their uses as sole designator for a time zone is not recommended.
Such designation can be ambiguous as some abbreviations may refer to different timezone in other continents for example EST WST CST can refer to Eastern, Western or Central, Summer Time or Standard Time in both North America and Australia.
Examples
These examples give the local time at various locations around the world at 12:00 UTC when daylight saving time (or summer time, etc.) is not in effect:
Where the adjustment for time zones results in a time at the other side of midnight from UTC, then the date at the location is one day later or earlier.
Some examples when UTC is 23:00 on Monday when daylight saving time is not in effect:
- Cairo, Egypt: UTC+02; 01:00 on Tuesday
- Wellington, New Zealand: UTC+12; 11:00 on Tuesday
Some examples when UTC is 02:00 on Tuesday when daylight saving time is not in effect:
- Honolulu, Hawaii, United States: UTC−10; 16:00 on Monday
- Toronto, Ontario, Canada: UTC−05; 21:00 on Monday
The time-zone adjustment for a specific location may vary because of daylight saving time. For example New Zealand, which is usually UTC+12, observes a one-hour daylight saving time adjustment during the Southern Hemisphere summer, resulting in a local time of UTC+13.
Time zone conversions
Conversion between time zones obeys the relationship
in which each side of the equation is equivalent to UTC. (The more familiar term "UTC offset" is used here rather than the term "zone designator" used by the standard.)
The conversion equation can be rearranged to
For example, what time is it in Los Angeles (UTC offset= −08) when the New York Stock Exchange opens at 09:30 (−05)?
In Delhi (UTC offset= +5:30), the New York Stock Exchange opens at
These calculations become more complicated near a daylight saving boundary (because the UTC offset for zone X is a function of the UTC time).
History
Greenwich Mean Time (GMT) was established in 1675 when the Royal Observatory was built, as an aid to (English) mariners to determine longitude at sea. At the time, each town's local clock in the area was calibrated to its local noon. Therefore, each clock across England had a slightly different time. The first time zone in the world was established by British railway companies on December 1, 1847—with GMT kept by portable chronometers. This quickly became known as Railway Time. About August 23, 1852, time signals were first transmitted by telegraph from the Royal Observatory, Greenwich. Even though 98% of Great Britain's public clocks were using GMT by 1855, it was not made Britain's legal time until August 2, 1880. Some old clocks from this period have two minute hands—one for the local time, one for GMT. This only applied to the island of Great Britain, not to the island of Ireland.
On November 2, 1868, the then-British colony of New Zealand officially adopted a standard time to be observed throughout the colony, and was perhaps the first country to do so. It was based on the longitude 172°30′ East of Greenwich, that is 11 hours 30 minutes ahead of GMT. This standard was known as New Zealand Mean Time.
Timekeeping on the American railroads in the mid 19th century was somewhat confused. Each railroad used its own standard time, usually based on the local time of its headquarters or most important terminus, and the railroad's train schedules were published using its own time. Some major railroad junctions served by several different railroads had a separate clock for each railroad, each showing a different time; the main station in Pittsburgh, Pennsylvania, for example, kept six different times. One can imagine the confusion for travelers making a long journey that involved several changes of train.
Charles F. Dowd proposed a system of one-hour standard time zones for American railroads about 1863, although he published nothing on the matter at that time and did not consult railroad officials until 1869. In 1870, he proposed four ideal time zones (having north–south borders), the first centered on Washington, D.C., bu
The World Clock (extended version)
The World Clock – Time Zones ... Related time zone tools. Search for a city and find local time; Personal World Clock
Time and Time Zones
Information and resources about time, sunrise, sunset, and time zones. Find out the exact time in your city or around the world, learn about Daylight Saving Time, time zones, and ...
Time Of Day : exact time with daylight saving time in ...
Time Of Day : exact time with daylight saving time North America, in all time zones. ... What time is it in an other city, when it's e.g. 8 am in Washington DC ?
City Time Zones
About City Time Zones This site is an informational resource. It's purpose is to prodive a FREE DATABASE of city time zones. Maintenance of this database is done in real time by ...
World Time Zones
Includes information for all the worlds time zones! 387 time zones 2,000,000 cities 242 countries. Check out our list of guides! Facts and Figures about time zones
The World Clock – Time Zones
The World Clock shows current local time in cities and countries, in all time zones.
The World Time Server: correct current local in any ...
World Time Server shows current local time in cities and countries in all time zones, adjusted for Daylight Saving Time rules automatically. Even check current international ...
The current time in Holy See (Vatican City) (UTC+01:00)
The current time in Holy See (Vatican City) - Central European Time (CET).
Add Your City To Linux's List of Time Zones | Linux ...
Ever feel like you're Rodney Dangerfield? You live someplace and Linux shows you no respect, time-zone-wise that is. What I mean is that you live somewhere that's not one of the ...
Downtown Time Zones - City of Fargo
Downtown Time Zones. The City of Fargo, along with the Downtown Community Partnership (DCP), Downtown Neighborhood Association (DNA) and Fargo Downtowners are working together to ...