A time zone is a region on Earth that has a uniform standard time for legal, commercial, and social purposes. In order for the same clock time to always correspond to the same portion of the day as the Earth rotates (for example, the sun being at its highest point every day around noon), different places on the Earth need to have different clock times. Time zones have been used in modern times so similarly situated cities can keep exactly the same time, for simplicity and ease of communication.
Standard time zones could 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, and the variation in the position of the sun from one end of the zone to the other (east vs. west) would be at most 1/24th of the sky. Most of the 25 nautical time zones (specifically UTC−11 to UTC+11) are indeed defined this way, and are 15° of longitude wide. An hourly zone in the central Pacific Ocean is split into two 7.5° wide zones (UTC±12) by the 180th meridian, part of which coincides with the International Date Line.
On land, it is more convenient for areas in close commercial or other communication to keep the same time, so time zones tend to follow the boundaries of countries and their subdivisions instead. Of the 40 time zones on land, most are offset from Coordinated Universal Time (UTC) by a whole number of hours (UTC−12 to UTC+14), but a few are offset by 30 or 45 minutes from a nearby hourly zone. Daylight saving time is used in some higher-latitude countries to manipulate clock time with respect to the position of the sun for parts of the year, typically by changing clocks by an hour. Many land time zones are skewed toward the west relative to the corresponding nautical time zones, which also creates a permanent daylight saving time-like offset. Computer operating systems use either UTC or a local time zone to time stamp events.
Before the invention of clocks, people marked the time of day with apparent solar time (or "true" solar time) - for example, the time on a sundial - which was typically different for every settlement.
When well-regulated mechanical clocks became widespread in the early 19th century, each city began to use some local mean solar time. Apparent and mean solar time can differ by up to around 15 minutes (as described by the equation of time) due to the non-circular shape of the Earth's orbit around the sun. Mean solar time has days of equal length, and the difference between the two averages to zero after a year.
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, providing a reference time at a point in history when each city in England kept a different local time.
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.
The first time zone in the world was established on December 1, 1847, on the island of Great Britain by railway companies using 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 British clocks from this period have two minute hands—one for the local time, one for GMT.[1]
The increase in worldwide communication had further increased the need for interacting parties to communicate mutually comprehensible time references to one another. The problem of differing local times could be solved across larger areas by synchronizing clocks worldwide, but in many places the local time would then differ markedly from the solar time to which people were accustomed. Time zones were a compromise, relaxing the complex geographic dependence while still allowing local time to approximate the mean solar time.
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.[2] 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",[3] 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.[4] 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.
U.S. Commissioner of Railroads William H. Armstrong gave the following account of the new railroad time system in his Report to the Secretary of the Interior for 1883.
The question of uniform time standards for railways of the United States has long attracted the attention of railway managers, but Mr. W. F. Allen, editor of the Traveler's Official Guide, and secretary of the time conventions, is entitled to the credit of having perfected the admirable system which was adopted by the general time convention of railway managers, held at Chicago, October 11, 1883, and ratified by the southern railway time convention, held at New York, October 17, 1883.
As this is a subject of great interest to the entire country, a brief synopsis of the general principles governing the proposed plan is deemed appropriate in this report.
Under the present system each railway is operated independently on the local time of some principal point or points on said road, but this plan was found to be highly objectionable, owing to the fact that some fifty standards, intersecting and interlacing each other, were in use throughout the country. By the plan which has been adopted this number will be reduced to four, the difference in time being one hour between each, viz, the 75th, 90th, 105th, and 120th degrees of longitude west from Greenwich. The adoption of these standards will not cause a difference of more than thirty minutes from the local time at any point which is now used as a standard. The new arrangement goes into effect November 18, 1883, and all changes of time are to occur at the termini of roads, or at the ends of divisions. The seventy-fifth meridian being almost precisely the central meridian for the system of roads now using standards based upon the time of the Eastern cities, and the ninetieth meridian being equally central for roads now running by the time of Western cities, the time of these meridians has been adopted for the territory which includes 90 per cent. of the whole railway system of the country. Nearly all of the larger cities have abolished local time and adopted that of the nearest standard meridian in use by the railways.[5][6][7]
Although 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.[8][9]
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".
By about 1900, almost all time on Earth was in the form of standard time zones, only some of which used an hourly offset from GMT. Many applied the time at a local astronomical observatory to an entire country, without any reference to GMT. It took many decades before all time on Earth was in the form of time zones referred to some "standard offset" from GMT/UTC. Most major countries had adopted hourly time zones by 1929. Nepal was the last country to adopt a standard offset, shifting slightly to UTC+5:45 in 1986.
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).
Before 1972, all time zones were specified as an offset from Greenwich Mean Time (GMT), which was the mean solar time at the meridian passing through the Royal Observatory in Greenwich, London, United Kingdom. Since 1972, all official time services have broadcast radio time signals synchronized to UTC, a form of atomic time that includes leap seconds to keep it within 0.9 seconds of this former GMT, now called UT1. Many countries now legally define their standard time relative to UTC, although some still legally refer to GMT, including the United Kingdom itself. UTC, also called Zulu time, is used everywhere on Earth by astronomers and others who need to state the time of an event unambiguously.
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 equalized. The readings of participating atomic clocks are averaged out to give a uniform time scale.
Because the length of the average day is a small fraction of a second more than 24 hours (slightly more than 86400 seconds), leap seconds are periodically inserted into Greenwich Mean Time to make it approximate to UT1. This new time system is also called Coordinated Universal Time (UTC). Leap seconds are inserted to keep UTC within 0.9 seconds of UT1. Because of the secular (long term) slowing down of the Earth's rotation, leap seconds will gradually need to be added more and more often. But on short time scales (from one year to the next) the rotation rate is irregular, so leap seconds are not added unless observations of Earth's rotation show that one is required. 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 nations had altered the definition of local time in this way.
In the United Kingdom, this involved redefining Greenwich Mean Time to make it the same as UTC.[10] 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. This means letting the time difference accumulate. One suggestion is to insert a "leap-hour" in about 5,000 years. For more on this discussion read Proposal to abolish leap seconds.
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".
Offsets from UTC are written 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 renamed "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.
These examples give the local time at various locations around the world when daylight saving time 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 or where daylight saving time is not in effect:
Some examples when UTC is 02:00 on Tuesday when or where 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).
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.[11][12][13]
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 is 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. Ships going in shuttle traffic over a time zone border often keeps the same time zone all the time, to avoid confusion about work, meal and shop opening hours. Still the time table for port calls must follow the land time zone.
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.. Similarly, the Netherlands prior to World war Two observed "Amsterdam Time" which was twenty minutes fast on Greenwich Mean Time. They were obliged to follow German time during the war and stayed with it thereafter. In the mid 1970's the Netherlands, as with other European states, began observing daylight saving (summer) time.
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 at 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.[14] 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.
Also, China extends as far west as 73°34′E, but all parts of it use UTC+08:00 (120°E), so solar "noon" can occur as late as 15:00.
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 (e.g. IRC chats, news, shows and so on).[15] 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,[16] whilst the US version shows Eastern Time.[17] 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.[18]
As an alternative to the timezone information bundled with the Java Platform, programmers may choose to use the Joda-Time library.[19] This library includes its own timezone data based on the frequently updated tz database.[20]
There is very little in the way of timezone support for JavaScript. Essentially the programmer has to extract the UTC offset by instantiating a time object, getting a GMT time from it, and differencing the two. This does not provide a solution for daylight savings variations.
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.[21] 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.[22] 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 two 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, Gemstone, or Dolphin.[23]
Some databases allow storage of a datetime type having time zone information. The SQL standard defines two standard time data types:
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:
PostgreSQL uses the standard SQL data types but tries to impose an interpretation which avoids the problems described above.
Microsoft Outlook has a much-criticized behavior regarding time zone handling. Appointments stored in Outlook move when the computer changes time zone, since they are assumed to be fixed against UTC not against the hour number. As a consequence, someone who inserts an appointment requiring a travel into another timezone will not get a correct time for the appointment after travelling to the other time zone. For example, a New Yorker plans to meet someone in Los Angeles at 9:00 AM. He inserts an appointment at 9:00 AM in Outlook while his computer is on New York time. He travels to Los Angeles and adjusts his computer time zone, which causes the meeting to show up at 6:00 AM (9:00 New York time) in Outlook. One workaround is to adjust the clock but not the timezone of the computer when travelling. This will give sent e-mail wrong time stamp, and new meeting invitations will be wrong. Microsoft recommends[24] to not change the clock at all and show a second time scale in the calendar. This will give reminder popups at the wrong time, since the clock does not match local time. The Outlook functionality will give correct time if the organizer invites the guest to a meeting using the "invite attendees" feature (the Los Angeles meeting will show up as 12:00 noon in the New Yorkers calender, before he adjusted the time zone), but only if the time zone is adjusted when travelling. The Outlook functionality will also give correct time for telephone meetings. For Outlook 2010 a new feature has been added, the possibility to specify which time zone an event occurs in. This solves most of these problems if properly used. An appointment at 9:00 AM Los Angeles time will show up as 12 AM but at 9 AM on the secondary scale if used.
Orbiting spacecraft typically experience many sunrises and sunsets in a 24-hour period, or in the case of Apollo program astronauts travelling to the moon, none. Thus it is not possible to calibrate time zones with respect to the sun, and still respect a 24-hour sleep/wake cycle. A common practice for space exploration is to use the Earth-based time zone of the launch site or mission control. This keeps the sleeping cycles of the crew and controllers in sync. The International Space Station normally uses Coordinated Universal Time (UTC).
Timekeeping on Mars can be more complex, since the planet has a solar day of approximately 24 hours and 39 minutes, known as a sol. Earth controllers for some Mars missions have synchronized their sleep/wake cycles with the Martian day, because solar-powered rover activity on the surface was tied to periods of light and dark. The difference in day length caused the sleep/wake cycles to slowly drift with respect to the day/night cycles on Earth, repeating approximately once every 36 days.
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