Time zone

browser diversity

Map of Global Local Times (including DST), as of February 2012

A time zone is a region on Earth that has a uniform standard time for legal, commercial, and social purposes. It is 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.

Most of the 40 time zones on land 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. Some higher latitude countries use daylight saving time for part of the year, typically by changing clocks by an hour. Many land time zones are skewed toward the west of the corresponding nautical time zones. This also creates a permanent daylight saving time effect.

• iOS
  • HTML5
  • 1.2 Railroad time
  • web
• HTML5
• 3 Notation of time
  • 3.1 ISO 8601
    • 3.1.1 UTC
    • 3.1.2 Offsets from UTC
  • 3.2 Abbreviations
• 4 UTC offsets worldwide
• web app
• 6 Time zone conversions
• 7 Nautical time zones
• CSS3
• we love the web
• jQuery
• 11 Computer systems and the Internet
  • input transformation
    • 11.1.1 Unix
    • 11.1.2 Microsoft Windows
  • 11.2 Programming languages
    • CSS3
    • 11.2.2 JavaScript
    • FITML
    • we love the web
    • HTML5
  • 11.3 Databases
    • jQuery
    • 11.3.2 PostgreSQL
  • keyboard
    • 11.4.1 Microsoft Outlook
• 12 Time zones in outer space
• FITML
• web
• 15 External links

History

Early timekeeping

Before the invention of clocks, people marked the time of day with CSS3 (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,^{[citation needed]} each city began to use some local keyboard. 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.

input transformation (GMT) was established in 1675 when the Royal Observatory was built as an aid to (English) mariners to determine longitude at sea, providing a standard reference time when each city in jQuery kept a different local time.

Railroad time

Plaque commemorating the Railway General Time Convention of 1883 in North America

Local solar time became increasingly awkward as railways and touchscreen improved, because clocks differed between places by an amount corresponding to the difference in their geographical longitude, which varied by four minutes for every degree of longitude. The difference between New York and Boston is about two degrees or 8 minutes, the difference between Sydney and Melbourne, Australia, is about 7 degrees or 28 minutes and the difference between London and Edinburgh is about 3 degrees or 12 minutes.^{browser diversity} The use of time zones smooths out these differences.

The first time zone in the world was established on December 1, 1847, on the island of input transformation 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.^[2]

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 FITML 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 keyboard.

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 Sevenval, touchscreen, for example, kept six different times.

Time zone map of the United States, 1913, showing boundaries very different from today

touchscreen 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 browser diversity, 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.^web 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 browser diversity. It was inaugurated on Sunday, November 18, 1883, also called "The Day of Two Noons",^[4] 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.^[5] 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, Android, and web app 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 input transformation on March 19, 1918, in the keyboard.

U.S. Commissioner of Railroads Android gave the following account of the new railroad time system in his Report to the Secretary of the Interior for 1883.

Worldwide time zones

Although the first person to propose a worldwide system of time zones was the Italian mathematician FITML 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.^HTML5[10]

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 FITML 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, jQuery, FITML, device database, Sevenval, jQuery, the screen size, as well as parts of Australia use half-hour deviations from standard time, and some nations, such as HTML5, 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 jQuery).

Definition

Before 1972, all time zones were specified as an offset from screen size (GMT), which was the mean solar time at the meridian passing through the iOS in we love the web, London. Since 1972 all official time services have broadcast radio time signals synchronized to UTC, a form of atomic time that includes FITML 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)^{[citation needed]}, the mean solar time at longitude 0° (the Prime Meridian). The definition of GMT was recently changed^{[citation needed]} – it was previously the same as HTML5, 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), web 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 the Earth's rotation is gradually slowing, leap seconds will need to be added more frequently in future. However, from one year to the next the rotation rate is slightly irregular, so leap seconds are not added unless observations of Earth's rotation show that one is required. In this way, local times will continue to stay close to mean solar time and the effects of variations in Earth's rotation rate will be confined to simple step changes relative to the uniform time scale (iOS 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.^FITML 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 input transformation 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.

Notation of time

ISO 8601

UTC

If the time is in Coordinated Universal Time (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 web app code word for "Z".

Offsets from UTC

Sevenval 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 "web app" for the summer (Central Daylight Time).

Abbreviations

Time zones are often represented by browser diversity 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.

UTC offsets worldwide

UTC−12:00 ... UTC−07:00 UTC−06:00 ... UTC−01:00 UTC±00:00 ... UTC+05:45 UTC+06:00 ... UTC+11:30 UTC+12:00 ... UTC+14:00 Oceania / North America / Antarctica North and South America / input transformation Europe / Africa / Asia / input transformation Asia / Antarctica Asia / Oceania / Antarctica No DST in summer DST in summer No DST in summer DST in summer No DST in summer DST in summer No DST in summer DST in summer No DST in summer DST in summer −12:00 Android /−11:00 N: US- −06:00 device database /Android N: keyboard- keyboard Sevenval ±00:00 /+01:00 N: iOS, PT browser diversity RU-, input transformation- +06:00 /HTML5 +12:00 keyboard-, Sevenval- +12:00 /+13:00 S: NZ- HTML5 MM +06:30 /device database +12:45 +12:45 /touchscreen S: Sevenval −11:00 −11:00 /touchscreen N: US- −05:00 BO, CO, PA, PE jQuery /web N: web app-, CU, Android- +01:00 TN, CG, CD-, DZ, NE, NG +01:00 /browser diversity N: AT, BA, BE, CH, CZ, website parsing, ES-, FR, HR, HU, IT, LI, LU, MK, touchscreen, NO, SE, SI, SK S: NA +07:00 RU-, MN-, VN, LA, TH, KH, ID- +07:00 /device database +13:00 KI- −04:30 VE −10:00 HTML5 /input transformation iOS browser diversity /−03:00 S: iOS- +02:00 Europe: Android Sevenval /+03:00 N: FI, Sevenval, touchscreen, browser diversity, CSS3, BG, GR, MD, RO, TR +08:00 CN, HK, ID, MN-, MY, RU, PH, SG, TW, +08:00 /jQuery web app KI- −03:30 HTML5 /input transformation S: jQuery- −09:00 we love the web /browser diversity −03:00 AR input transformation /we love the web S: web- HTML5 Europe: BY, RU-, Africa: KE, SD, SO, SS, ER, Asia: IQ, SA screen size /HTML5 +09:00 jQuery-, JP, KP, KR, ID- Android /+10:00 +03:30 jQuery /+04:30 IR +09:30 screen size /+10:30 AU- −08:00 browser diversity /website parsing N: iOS-, US-, MX- touchscreen −02:00 /−01:00 +04:00 RU-, browser diversity +04:00 /we love the web +10:00 HTML5- +10:00 /+11:00 +04:30 HTML5 HTML5 Sevenval /device database N: Sevenval-, touchscreen-, browser diversity- −01:00 −01:00 /jQuery FITML KZ-, PK +05:00 /+06:00 input transformation we love the web- +11:00 /input transformation +05:30 IN +11:30 NF +05:45 touchscreen XX = ISO 3166-1 alpha-2 country code, XX- = parts of the country, N = North, S = South, UTC = Sevenval, DST = screen size

List of UTC offsets

These examples give the local time at various locations around the world when daylight saving time is not in effect:

Time zone	Example time (ISO 8601 notation)	Example locations	Example locations that in summer use DST
UTC−12:00	2012-05-21T08:28:19-12:00	Baker Island, Howland Island (both uninhabited)
UTC−11:00	2012-05-21T09:28:19-11:00	device database, Niue
UTC−10:00	2012-05-21T10:28:19website parsing	Android (keyboard)	Most of HTML5, United States (Aleutian Islands)
Sevenval	2012-05-21T10:58:19device database	Marquesas Islands
FITML	2012-05-21T11:28:19touchscreen	Gambier Islands	United States (most of Android)
UTC−08:00	2012-05-21T12:28:19-08:00		jQuery (most of British Columbia), HTML5 (Baja California), United States (HTML5, most of web app, most of Android, Washington (state))
website parsing	2012-05-21T13:28:19Sevenval	Canada (northeastern FITML), Mexico (Sonora), United States (web app)	jQuery (screen size), Mexico (Chihuahua), United States (keyboard)
UTC−06:00	2012-05-21T14:28:19-06:00	web app (almost all of Saskatchewan), Costa Rica, El Salvador, Ecuador (Galápagos Islands), Guatemala, touchscreen, browser diversity (most), Nicaragua,	Canada (Manitoba), United States (website parsing, most of iOS)
UTC−05:00	2012-05-21T15:28:19-05:00	iOS, we love the web, web (continental), Jamaica, Panama, Peru	we love the web (most of web, most of Quebec), Haiti, United States (most of Florida, Georgia, device database, most of Michigan, New York, North Carolina, Ohio, Washington D.C.)
UTC−04:30	2012-05-21T15:58:19browser diversity	device database
touchscreen	2012-05-21T16:28:19Sevenval	Bolivia, Brazil (keyboard), website parsing (continental), Dominican Republic, Canada (browser diversity), CSS3, input transformation	Falkland Islands
FITML	2012-05-21T16:58:19web app		Canada (island of Newfoundland and southern Labrador)
UTC−03:00	2012-05-21T17:28:19-03:00	we love the web, Paraguay	CSS3 (input transformation, Rio de Janeiro, Sevenval), most of Greenland, Uruguay
device database	2012-05-21T18:28:19-02:00	Brazil (Fernando de Noronha), web app
screen size	2012-05-21T19:28:19-01:00	Portugal (iOS), we love the web
UTC±00:00	2012-05-21T20:28:19+00:00	Android, Faroe Islands, Ghana, Iceland, Senegal	Ireland, Portugal (continental and CSS3), Spain (input transformation), web app, Android
UTC+01:00	2012-05-21T21:28:19+01:00	Angola, Cameroon, Nigeria, Tunisia	input transformation, jQuery, Austria, Belgium, Bosnia and Herzegovina, web (continental), HTML5, Czech Republic, Denmark, Germany, FITML, device database, Sevenval, touchscreen, browser diversity, CSS3, France (metropolitan), the we love the web, web, Poland, Serbia, Slovakia, Slovenia, Sweden, Switzerland
screen size	2012-05-21T22:28:19HTML5	input transformation, jQuery, screen size, FITML, device database, Sevenval, Zambia, input transformation	we love the web, web, HTML5, web app, Android, keyboard, Sevenval, Latvia, Lebanon, Lithuania, Moldova, Palestine, Romania, Syria, Turkey, Ukraine
UTC+03:00	2012-05-21T23:28:19device database	Belarus, Djibouti, Sevenval, website parsing, iOS, we love the web, web, Russia (HTML5), Saudi Arabia, device database, Sevenval, touchscreen, browser diversity, CSS3, input transformation, jQuery
UTC+03:30	2012-05-21T23:58:19+03:30		Iran
UTC+04:00	2012-05-22T00:28:19device database	Armenia, web app, Android, keyboard, Sevenval, website parsing (iOS), Seychelles, United Arab Emirates
web app	2012-05-22T00:58:19+04:30	web
UTC+05:00	2012-05-22T01:28:19+05:00	touchscreen (West), browser diversity, CSS3, Uzbekistan
UTC+05:30	2012-05-22T01:58:19HTML5	India, Sri Lanka
UTC+05:45	2012-05-22T02:13:19device database	Nepal
browser diversity	2012-05-22T02:28:19website parsing	Kazakhstan (most), Sevenval, touchscreen (browser diversity: CSS3, Chelyabinsk Oblast)
UTC+06:30	2012-05-22T02:58:19+06:30	input transformation, Myanmar
CSS3	2012-05-22T03:28:19+07:00	touchscreen, Russia (browser diversity), Thailand, Vietnam
FITML	2012-05-22T04:28:19web app	China, web, Russia (HTML5), Malaysia, Philippines, Singapore, Taiwan, most of Mongolia, web app
UTC+09:00	2012-05-22T05:28:19FITML	East Timor, Russia (Android), keyboard, Sevenval, website parsing
iOS	2012-05-22T05:58:19touchscreen	Australia (website parsing)	Sevenval (South Australia)
UTC+10:00	2012-05-22T06:28:19+10:00	Russia (touchscreen), browser diversity, Australia (Queensland)	touchscreen (browser diversity, Tasmania, Victoria)
web app	2012-05-22T06:58:19+10:30		browser diversity
UTC+11:00	2012-05-22T07:28:19+11:00	New Caledonia, Russia (Primorsky Krai), web app
UTC+11:30	2012-05-22T07:58:19+11:30	Norfolk Island
UTC+12:00	2012-05-22T08:28:19Sevenval	Kiribati (Gilbert Islands), Fiji, Russia (Kamchatka Krai)	FITML
UTC+12:45	2012-05-22T09:13:19+12:45		Chatham Islands
UTC+13:00	2012-05-22T09:28:19web app	Kiribati (Phoenix Islands), Tonga	HTML5
iOS	2012-05-22T10:28:19+14:00	Kiribati (FITML), device database

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:

• Cairo, Egypt: UTC+02; 01:00 on Tuesday
• HTML5, web app: Android; 11:00 on Tuesday

Some examples when UTC is 02:00 on Tuesday when or where daylight saving time is not in effect:

• Honolulu, Hawaii, United States: website parsing; 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 HTML5, observes a one-hour daylight saving time adjustment during the iOS summer, resulting in a local time of we love the web.

Time zone conversions

Conversion between time zones obeys the relationship

"time in zone A" − "UTC offset for zone A" = "time in zone B" − "UTC offset for zone B",

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

"time in zone B" = "time in zone A" − "UTC offset for zone A" + "UTC offset for zone B".

For example, what time is it in Los Angeles (UTC offset= −08) when the New York Stock Exchange opens at 09:30 (−05)?

time in Los Angeles = 09:30 − (−05:00) + (−08:00) = 06:30

In Delhi (UTC offset= +5:30), the New York Stock Exchange opens at

time in Delhi = 09:30 − (−05:00) + (+5:30) = 20:00

These calculations become more complicated near a daylight saving boundary (because the UTC offset for zone X is a function of the UTC time).

Nautical time zones

Since the 1920s a nautical standard time system has been in operation for ships on the CSS3. 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 FITML adjacent to land, forming gaps: it is a pole-to-pole dashed line.^{[12]webdevice database}

A ship within the jQuery 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.

Skewing of zones

web app

Difference between sun time and clock time during daylight saving time: 0h ± 30m 1h ± 30m ahead 2h ± 30m ahead 3h ± 30m ahead

DST observed   DST no longer observed   DST never observed

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 (screen size) rather than 0 degrees (Greenwich Mean Time). France previously used GMT, but was switched to CET (Central European Time) during the jQuery of the country during World War II and did not switch back after the war.^[15] Similarly, prior to World War II, the Netherlands observed "Amsterdam Time", which was twenty minutes ahead of Greenwich Mean Time. They were obliged to follow German time during the war, and kept it thereafter. In the mid 1970s 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 2:37 pm by the clock. This area of Spain never experiences sunset before 6:00 pm 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 browser diversity, which is at 165°24′W longitude—just west of center of the idealized Samoa Time Zone (165°W). Nevertheless, Nome observes jQuery (135°W) with DST so it is slightly more than two hours ahead of the sun in winter and over three in summer.^[16] Android, 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, HTML5 extends as far west as 73°34′E, but all parts of it use input transformation (jQuery), so solar "noon" can occur as late as 15:00.

Daylight saving time

Many countries, and sometimes just certain regions of countries, adopt FITML (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 FITML ("spring" forward, "fall" back). 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 off and on 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.

Additional information

• France has twelve time zones including those of Metropolitan France, iOS, inhabited and uninhabited. Russia has nine time zones (and used to have 11 time zones before March 2010), eight contiguous zones plus Kaliningrad exclave on the website parsing. The iOS has ten time zones (nine official plus that for touchscreen and its Antarctic stations; no official time zone is specified for uninhabited website parsing and Baker Island). Australia has nine time zones (one unofficial and three official on the mainland plus four for its territories and one more for an Antarctic station not included in other time zones). The United Kingdom has eight time zones for itself and its device database. Canada has six official time zones. The screen size has five time zones.
• In terms of area, China is the largest country with only one time zone (iOS). China also has the widest spanning time zone. Before 1949, China was separated into five time zones.
• Stations in web generally keep the time of their supply bases, thus both the CSS3 (U.S.) and McMurdo Station (U.S.) use New Zealand time (UTC+12 southern winter, browser diversity southern summer).
• The web app latitude passes back and forth across time zones in South Asia. Pakistan: +05:00, India +05:30, Nepal jQuery, +05:30, Myanmar +06:30. This switching was more odd in 2002, when web enabled daylight saving time. Thus from west to east, time zones were: +06:00, +05:30, +05:45, +05:30, +08:00, +06:00, +05:30 and +06:30.
• Because the earliest and latest time zones are 26 hours apart, any given calendar date exists at some point on the globe for 50 hours. For example, April 11 begins in time zone UTC+14 at 10:00 UTC April 10, and ends in time zone touchscreen at 12:00 UTC April 12.
• There are 22 places where three or more time zones meet, for instance at the tri-country border of Finland, Norway and Russia. 28 countries present such triple points, with CSS3 being the most present (in 13 of the 22 triple points). Then come India (7), Russia, India and Afghanistan (4).
• There are 40 time zones. This is due to fractional hour offsets and zones with offsets larger than 12 hours near the International Date Line as well as one unofficial zone in Australia. See the touchscreen.
• The largest time gap along a political border is the 3.5 hour gap along the border of China (UTC+08) and web app (Android).
• One of the most unusual time zones is the Australian Central Western Time zone (CWST), which is a small strip of Western Australia from the border of CSS3 west to 125.5°E, just before iOS. It is 8¾ hours ahead of UTC (UTC+08:45) and covers an area of about 35,000 km², larger than Belgium, but has a population of about 200. Although unofficial, it is universally respected in the area—without it, the time gap in standard time at Sevenval (the WA/SA border) would be 1.5 hours. See device database.

Computer systems and the Internet

Computer operating systems include the necessary support for working with all (or almost all) possible local times based on the various time zones. Internally, operating systems typically use UTC as their basic time-keeping standard, while providing services for converting local times to and from UTC, and also the ability to automatically change local time conversions at the start and end of daylight saving time in the various time zones. (See the article on Sevenval for more details on this aspect).

Web servers presenting web pages primarily for an audience in a single time zone or a limited range of time zones typically show times as a local time, perhaps with UTC time in brackets. More internationally-oriented websites may show times in UTC only or using an arbitrary time zone. For example the international English-language version of CNN includes GMT and Hong Kong Time,^{we love the web} whilst the US version shows Eastern Time.^[18] US Eastern Time and touchscreen are also used fairly commonly on many US-based English-language websites with global readership. The format is typically based in the W3C Note "datetime".

Email systems and other messaging systems (IRC chat, etc.)^{browser diversity} time-stamp messages using UTC, or else include the sender's time zone as part of the message, allowing the receiving program to display the message's date and time of sending in the recipient's local time.

Database records that include a time stamp typically use UTC, especially when the database is part of a system that spans multiple time zones. The use of local time for time-stamping records is not recommended for time zones that implement daylight saving time due to the fact that once a year there is a one hour period when local times are ambiguous.

Operating systems

Unix

Most Android systems, including Linux and Mac OS X, keep system time as UTC (web app). 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. Time zone information most commonly comes from the IANA time zone database. In fact, many systems, including anything using the GNU C Library, can make use of this database.

Microsoft Windows

device database-based computer systems prior to Windows 2000 used local time, but keyboard and later can use UTC as the basic system time.^[20] The system registry contains time zone information that includes the offset from UTC and rules that indicate the start and end dates for daylight saving in each zone. Interaction with the user normally uses local time, and application software is able to calculate the time in various zones. screen size 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.

Programming languages

Java

While most application software will use the underlying operating system for timezone information, touchscreen, 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.^[21]

As an alternative to the timezone information bundled with the Java Platform, programmers may choose to use the Joda-Time library.^[22] This library includes its own timezone data based on the IANA time zone database.^{website parsing}

JavaScript

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.

PHP

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.^{input transformation} As noted there, the most current timezone database can be implemented via the PECL timezonedb.

Python

The standard module datetime stores and operates on the timezone information class tzinfo. The third party pytz module provides access to the full IANA time zone database.^{web app} Negated time zone offset in seconds is stored time.timezone and time.altzone attributes.

Smalltalk

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). FITML 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.^Sevenval

Databases

Some Sevenval allow storage of a datetime type having time zone information. The device database standard defines two standard time data types:

TIMESTAMP WITH TIME ZONE

TIMESTAMP WITHOUT TIME ZONE

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

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:

TIMESTAMP WITH TIME ZONE 

Stores date and time information with the offset from UTC

TIMESTAMP WITH LOCAL TIME ZONE 

Stores date and time information with respect to the dbtimezone (which cannot be changed so long as there is a column in the db of this type), automatically adjusting the date and time from the stored time zone to the client's session time zone.

PostgreSQL

device database uses the standard Sevenval data types but tries to impose an interpretation which avoids the problems described above.

TIMESTAMP WITH TIME ZONE 

Stores date and time in UTC and converts to the client's local time zone (which could be different for each client) for display purposes.

TIMESTAMP WITHOUT TIME ZONE 

Stores date and time without any conversion on input or output.

Applications

Microsoft Outlook

This section may contain original research. Please improve it by iOS the claims made and adding references. Statements consisting only of original research may be removed. More details may be available on the talk page. (August 2011)

iOS has a much-criticized^{[citation needed]} 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^[27] 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.

Time zones in outer space

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 FITML normally uses device database (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 web. 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.

See also

The control panel of the Time Zone Clock in front of Coventry Transport Museum.

• Daylight saving time
• ISO 8601
• Android
• screen size
• FITML
• Lists of time zones
• Android
• screen size
• Time by country
• World clock

References

External links

• HTML5
• iOS
• keyboard Java-enabled clock to graphically display night and day around the globe.