A time zone is a region on Earth, more or less bounded by lines of longitude, that has a uniform, legally mandated standard time, usually referred to as the local time. By convention, the 24 main time zones on Earth compute their local time as an offset from UTC (see also Greenwich Mean Time). Local time in each time zone is UTC plus the current time zone offset for the location in question. In theory, the increase proceeds eastward from the eastern boundary of the UTC time zone centered on 0°, increasing by one hour for each 15°, up to the International Date Line (longitude 180°). A corresponding one hour decrease relative to UTC occurs every 15° heading westward from the western boundary of the UTC time zone, up to the International Date Line.
Time zones are adjusted seasonally into standard and daylight saving (or summer) variants. Daylight saving time zones (or summer time zones) include an offset (typically +1 hour) 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. Thus, the advance of technology has both forced (rail transport) and enabled (modern timepieces) the development of arbitrary official "time."
Time zones are based on Greenwich Mean Time (GMT), the mean solar time at longitude 0° (the Prime Meridian). The definition of GMT was recently changed – it was previously the same as UT1, a mean solar time calculated directly from the rotation of the Earth. As the rate of rotation of the Earth is not constant, the time derived from atomic clocks was adjusted to closely match UT1. In January 1972, however, the length of the second in both Greenwich Mean Time and atomic time was equalised. The readings of participating atomic clocks are averaged out to give a uniform time scale.
Because of the secular (long term) slowing down of the Earth's rotation leap seconds are periodically inserted into Greenwich Mean Time to make it approximate to UT1. Because of the method of calculation this new time system is also 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 the uniform time scale (International Atomic Time or TAI). All local times differ from TAI by an integral number of seconds. With the implementation of UTC, nations began to use it in the definition of their time zones. As of 2005, most but not all nations had altered the definition of local time in this way.
In England, this involved redefining Greenwich Mean Time to make it the same as UTC. British Summer Time (BST) is still one hour in advance of Greenwich Mean Time and is therefore also one hour in advance of Coordinated Universal Time. Thus Greenwich Mean Time is the local time at the Royal Observatory, Greenwich between 0100 hours GMT on the last Sunday in October and 0100 hours GMT on the last Sunday in March. Similar circumstances apply in many other places.
Looking to the future, leap seconds are considered by many to be a nuisance, and ways to abolish them are being considered. One suggestion is to let the time difference accumulate and insert a "leap-hour" in about 5,000 years. In practice this would mean not putting the clocks forward for daylight saving time in the year of adjustment. This proposal is likely to be resisted by astronomers, who like the stars to appear in the expected positions at the same time each night, but might appeal to daylight saving enthusiasts as Summer Time begins to approximate to Double Summer Time.
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 ICAO spelling alphabet code word for "Z".
Time zone are written as offset from UTC in the format ±[hh]:[mm], ±[hh][mm], or ±[hh]. 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, which is in the North American Central Time Zone, would be "−06:00" for the winter (Central Standard Time) and "−05:00" for the summer (Central Daylight Time).
Time zones are often represented by abbreviations such as "EST, WST, CST" but these are not part of the international time and date standard ISO 8601 and their use as sole designator for a time zone is not recommended. Such designations can be ambiguous. For example, "BST", which is British Summer Time, was re - named "British Standard Time" between 1968 and 1971 when Central European Time was in force because legislators objected to calling it Central European Time. The same legislation affirmed that the Standard Time within the United Kingdom was, and would continue to be, Greenwich Mean Time. Unlike Americans, Australians name their time zones according to the states using them.
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:
|Baker Island, Howland Island (both uninhabited)||UTC−12||00:00|
|Samoa, American Samoa||UTC−11||01:00|
|Anchorage, Fairbanks, Juneau||UTC−09||03:00|
|Vancouver, Washington (U.S. state), Portland, Las Vegas, California, Baja California||UTC−08||04:00|
|Alberta, Colorado, Arizona, Chihuahua, Sonora||UTC−07||05:00|
|Costa Rica , Dallas, El Salvador, Guatemala, Honduras, Houston, Illinois, Manitoba, Mexico City, Nicaragua, Saskatchewan||UTC−06||06:00|
|Ottawa, Toronto, Montreal, Boston, New York, North Carolina, Washington D.C., Georgia, Miami, Cuba, Jamaica, Haiti, Panama, Colombia, Continental Ecuador, Peru||UTC−05||07:00|
|Nova Scotia, Dominican Republic, Puerto Rico, Trinidad and Tobago, Amazonas, Bolivia, Continental Chile, Paraguay||UTC−04||08:00|
|Rio de Janeiro, São Paulo, Argentina, Uruguay, Nuuk||UTC−03||09:00|
|Fernando de Noronha, South Georgia and the South Sandwich Islands||UTC−02||10:00|
|Azores, Cape Verde||UTC−01||11:00|
|Iceland, United Kingdom, Ireland, Continental Portugal, Morocco, Senegal, Ghana, Côte d'Ivoire||UTC||12:00|
Albania, Slovenia, Macedonia, Norway, Sweden, Denmark, Germany, the Netherlands, Belgium, Metropolitan France, Switzerland, Austria, Poland, Czech Republic, Slovakia, Hungary, European Spain, Italy, Croatia, Serbia, Tunisia, Algeria, Nigeria, Cameroon, Angola, Kinshasa
|Moscow, Saint Petersburg, Samara, Iraq, Saudi Arabia, Yemen, Sudan, Ethiopia, Somalia, Kenya, Uganda, Tanzania, Madagascar||UTC+03||15:00|
|Georgia, Armenia, Azerbaijan, United Arab Emirates, Oman, Seychelles, Mauritius||UTC+04||16:00|
|Sverdlovsk, Uzbekistan, Pakistan, Maldives, Kazakhstan||UTC+05||17:00|
|India, Sri Lanka||UTC+05:30||17:30|
|Novosibirsk, Almaty, Bangladesh||UTC+06||18:00|
|Myanmar, Cocos Islands||UTC+06:30||18:30|
|Krasnoyarsk, Thailand, Vietnam, Jakarta||UTC+07||19:00|
|Irkutsk, Ulan Bator, China, Taiwan, Hong Kong, Philippines, Malaysia, Singapore, Western Australia||UTC+08||20:00|
|Zabaykalsky, Japan, North Korea, South Korea, East Timor||UTC+09||21:00|
|Northern Territory, South Australia||UTC+09:30||21:30|
|Primorsky, New South Wales,Queensland, Victoria||UTC+10||22:00|
|Lord Howe Island||UTC+10:30||22:30|
|Kamchatka, Solomon Islands, New Caledonia||UTC+11||23:00|
|Fiji, New Zealand||UTC+12||00:00 (the following day)|
|Chatham Islands||UTC+12:45||00:45 (the following day)|
|Tonga||UTC+13||01:00 (the following day)|
|Line Islands||UTC+14||02:00 (the following day)|
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:
Some examples when UTC is 02:00 on Tuesday when daylight saving time is not in effect:
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.
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).
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.
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., but by 1872 the first was centered 75°W of Greenwich, with geographic borders (for example, sections of the Appalachian Mountains). Dowd's system was never accepted by American railroads. Instead, U.S. and Canadian railroads implemented a version proposed by William F. Allen, the editor of the Traveler's Official Railway Guide. The borders of its time zones ran through railroad stations, often in major cities. For example, the border between its Eastern and Central time zones ran through Detroit, Buffalo, Pittsburgh, Atlanta, and Charleston. It was inaugurated on Sunday, November 18, 1883, also called "The Day of Two Noons", when each railroad station clock was reset as standard-time noon was reached within each time zone. The zones were named Intercolonial, Eastern, Central, Mountain, and Pacific. Within one year, 85% of all cities with populations over 10,000, about 200 cities, were using standard time. A notable exception was Detroit (which is about half-way between the meridians of eastern time and central time), which kept local time until 1900, then tried Central Standard Time, local mean time, and Eastern Standard Time before a May 1915 ordinance settled on EST and was ratified by popular vote in August 1916. The confusion of times came to an end when Standard zone time was formally adopted by the U.S. Congress on March 19, 1918, in the Standard Time Act.
While the first person to propose a worldwide system of time zones was the Italian mathematician Quirico Filopanti, in his book Miranda! published in 1858, his idea was unknown outside the pages of his book until long after his death, so it did not influence the adoption of time zones during the 19th century. He proposed 24 hourly time zones, which he called "longitudinal days", the first centered on the meridian of Rome. He also proposed a universal time to be used in astronomy and telegraphy.
Canadian Sir Sandford Fleming proposed a worldwide system of time zones in 1879. He advocated his system at several international conferences, thus is widely credited with their invention. In 1876, his first proposal was for a global 24-hour clock, conceptually located at the center of the Earth and not linked to any surface meridian. In 1879 he specified that his universal day would begin at the anti-meridian of Greenwich (180th meridian), while conceding that hourly time zones might have some limited local use. He also proposed his system at the International Meridian Conference in October 1884, but it did not adopt his time zones because they were not within its purview. The conference did adopt a universal day of 24 hours beginning at Greenwich midnight, but specified that it "shall not interfere with the use of local or standard time where desirable".
Nevertheless, most major countries had adopted hourly time zones by 1929. Today, all nations use standard time zones for secular purposes, but they do not all apply the concept as originally conceived. Newfoundland, India, Iran, Afghanistan, Venezuela, Burma, the Marquesas, as well as parts of Australia use half-hour deviations from standard time, and some nations, such as Nepal, and some provinces, such as the Chatham Islands, use quarter-hour deviations. Some countries, most notably China and India, use a single time zone, even though the extent of their territory far exceeds 15° of longitude. Before 1949 China used five time zones (see Time in China).
Since the 1920s a nautical standard time system has been in operation for ships on the high seas. Nautical time zones are an ideal form of the terrestrial time zone system. Under the system, a time change of one hour is required for each change of longitude by 15°. The 15° gore that is offset from GMT or UT1 (not UTC) by twelve hours is bisected by the nautical date line into two 7.5° gores that differ from GMT by ±12 hours. A nautical date line is implied but not explicitly drawn on time zone maps. It follows the 180th meridian except where it is interrupted by territorial waters adjacent to land, forming gaps: it is a pole-to-pole dashed line.
A ship within the territorial waters of any nation would use that nation's standard time, but would revert to nautical standard time upon leaving its territorial waters. The captain was permitted to change the ship's clocks at a time of the captain’s choice following the ship's entry into another time zone. The captain often chooses midnight.
Ideal time zones, such as nautical time zones, are based on the mean solar time of a particular meridian located in the middle of that zone with boundaries located 7.5 degrees east and west of the meridian. In practice, zone boundaries are often drawn much farther to the west with often irregular boundaries, and some locations base their time on meridians located far to the east.
For example, even though the Prime Meridian (0°) passes through Spain and France, they use the mean solar time of 15 degrees east (Central European Time) rather than 0 degrees (Greenwich Mean Time). France previously used GMT, but was switched to CET (Central European Time) during the German occupation of the country during World War II and did not switch back after the war.
There is a tendency to draw time zone boundaries far to the west of their meridians. Many of these locations also use daylight saving time. As a result, in the summer, solar noon in the Spanish town of Muxia occurs on 14:37 (2:37pm) by the clock. This area of Spain never experiences sunset before 18:00 (6pm) local time even in midwinter, despite its lying more than 40 degrees north of the equator. Near the summer solstice, Muxia has sunset times similar to those of Stockholm, which is in the same time zone and 16 degrees further north.
A more extreme example is Nome, Alaska, which is at 165°24′W longitude—just west of center of the idealized Samoa Time Zone (165°W). Nevertheless, Nome observes Alaska Time (135°W) with DST so it is slightly more than two hours ahead of the sun in winter and over three in summer. Kotzebue, Alaska, also near the same meridian but north of the Arctic Circle, has an annual event on 9 August to celebrate two sunsets in the same 24-hour day, one shortly after midnight at the start of the day, and the other shortly before midnight at the end of the day.
Many countries, and sometimes just certain regions of countries, adopt daylight saving time (also known as "Summer Time") during part of the year. This typically involves advancing clocks by an hour near the start of spring and adjusting back in autumn ("spring" forward, "fall" back). Some countries also use backward daylight saving over the winter period. Modern DST was first proposed in 1907 and was in widespread use in 1916 as a wartime measure aimed at conserving coal. Despite controversy, many countries have used it since then; details vary by location and change occasionally. Most countries around the equator do not observe daylight saving time, since the seasonal difference in sunlight is minimal.
UTC is often used on the Internet for meetings (i.e. IRC chats, news, shows and so on). For e-mail, the sender time zone is used to calculate the send time, but this time is recalculated by the receiver mail client, and shown according to the receiver time zone.
On websites with mainly domestic audiences local time is often used, sometimes with UTC in brackets: e.g. the international English-language version of CNN includes GMT and Hong Kong Time, whilst the US version shows Eastern Time. US Eastern Time and Pacific Time are also used fairly commonly on many US-based English-language websites with global readership.
The format is based in the W3C Note "datetime".
On the other hand, most modern computer operating systems include information about time zones, including the capability to automatically change the local time when daylight saving starts and finishes (see the article on daylight saving time for more details on this aspect).
Most Unix-like systems, including Linux and Mac OS X, keep system time as UTC (Coordinated Universal Time). Rather than having a single time zone set for the whole computer, timezone offsets can vary for different processes. Standard library routines are used to calculate the local time based on the current timezone, normally supplied to processes through the TZ environment variable. This allows users in multiple timezones to use the same computer, with their respective local times displayed correctly to each user. Timezone information is most commonly stored in a timezone database known as tz database (or sometimes zoneinfo or Olson format). In fact, many systems, including anything using the GNU C Library, can make use of this database.
Windows-based computer systems normally keep system time as local time in a particular time zone. A system database of timezone information includes the offset from UTC and rules that indicate the start and end dates for daylight saving in each zone. Application software is able to calculate the time in various zones. Terminal Servers allow remote computers to redirect their time zone settings to the Terminal Server so that users see the correct time for their time zone in their desktop/application sessions. Terminal Services uses the server base time on the Terminal Server and the client time zone information to calculate the time in the session.
While most application software will use the underlying operating system for timezone information, the Java Platform, from version 1.3.1, has maintained its own timezone database. This database will need to be updated whenever timezone rules change. Sun provides an updater tool for this purpose.
As an alternative to the timezone information bundled with the Java Platform, programmers may choose to use the Joda-Time library. This library includes its own timezone data based on the frequently updated tz database.
The DateTime objects and related functions have been compiled into the PHP core since 5.2. This includes the ability to get and set the default script timezone, and DateTime is aware of its own timezone internally. PHP.net provides extensive documentation on this. As noted there, the most current timezone database can be implemented via the PECL timezonedb.
The standard module datetime stores and operates on the timezone information class tzinfo. The third party pytz module provides access to the full tz database.. Negated time zone offset in seconds is stored time.timezone and time.altzone attributes.
Each Smalltalk dialect comes with its own built-in classes for dates, times and timestamps, only a few of which implement the DateAndTime and Duration classes as specified by the ANSI Smalltalk Standard. VisualWorks provides a TimeZone class that supports up to 2 annually recurring offset transitions, which are assumed to apply to all years (same behavior as Windows time zones.) Squeak provides a Timezone class that does not support any offset transitions. Dolphin Smalltalk does not support time zones at all.
For full support of the tz database (zoneinfo) in a Smalltalk application (including support for any number of annually recurring offset transitions, and support for different intra-year offset transition rules in different years) the third-party, open-source, ANSI-Smalltalk-compliant Chronos Date/Time Library is available for use with any of the following Smalltalk dialects: VisualWorks, Squeak or Dolphin.
However, the standard has a somewhat naive understanding of time zones. It generally assumes a time zone can be specified by a simple offset from GMT. This causes problems when trying to do arithmetic on dates which span daylight saving time transitions or which span political changes in time zone rules.
Oracle Database is configured with a database time zone, and connecting clients are configured with session time zones. Oracle Database uses two data types to store time zone information:
af:Tydsone als:Zeitzone am:ሰዓት ክልል ab:Асааҭтә зонақәа ar:منطقة زمنية an:Zona horaria ast:Fusu horariu az:Saat qurşağı bn:সময় অঞ্চল zh-min-nan:Sî-khu be:Часавы пояс be-x-old:Часавы пас bar:Zeitzone bs:Vremenska zona bg:Часова зона ca:Zona horària cv:Вăхăт тăрăхĕ cs:Časové pásmo cy:Cylchfa amser da:Tidszone de:Zeitzone et:Ajavöönd el:Ώρα ζώνης es:Huso horario eo:Horzono eu:Ordu-eremu fa:منطقه زمانی fr:Fuseau horaire fy:Tiidsône fur:Fûs orari ga:Amchrios gv:Cryss hraa gl:Fuso horario ko:시간대 hy:Ժամային գոտի hi:समय मण्डल hsb:Časowe pasmo hr:Vremenska zona bpy:সময়র লয়া id:Zona waktu ia:Fuso horari is:Tímabelti it:Fuso orario he:אזור זמן jv:Zona wektu krc:Сагъат бёлге ka:სასაათო სარტყელი kg:Mukaba ya ntangu ku:Navçeya demjimêrî la:Zona temporalis lv:Laika josla lb:Zäitzon lt:Laiko juosta li:Tiedzaone lmo:Füüs urari hu:Időzóna mk:Временска зона ml:സമയമേഖല mr:आंतरराष्ट्रीय कालविभाग ms:Zon waktu mn:Цагийн бүс nl:Tijdzone ja:標準時 ce:Хан pih:Tiemsoen no:Tidssone nn:Tidssone nov:Tempe-sone oc:Fus orari mhr:Шагат ӱштӧ pnb:ویلہ تھاں pap:Zona di tempu nds:Tietrebeet pl:Strefa czasowa pt:Fuso horário crh:Saat quşağı ksh:Zickzon ro:Fus orar qu:Pacha suyu ru:Часовой пояс sah:Кэм зоната se:Áigeavádat sco:Time zone stq:Tiedzone scn:Fusu orariu simple:Time zone sk:Časové pásmo sl:Časovni pas szl:Czasowo strefa sr:Временска зона sh:Vremenska zona su:Zona wanci fi:Aikavyöhyke sv:Tidszon tl:Sona ng oras ta:நேர வலயம் te:కాలాంశం th:เขตเวลา tr:Saat dilimi udm:Дырлэн зонаез uk:Часовий пояс ur:منطقۂ وقت vec:Fuso orario vi:Múi giờ fiu-vro:Aovüü war:Zona hin oras yi:צייט זאנע yo:Agbègbè àkókò zh-yue:時區 bat-smg:Čiesa zuona zh:时区