Daylight saving time
Daylight saving time (DST), also daylight savings time or daylight time (the United States and Canada) and summer time (United Kingdom, European Union, and others), is the practice of advancing clocks (typically by one hour) during warmer months so that darkness falls at a later clock time. The typical implementation of DST is to set clocks forward by one hour in the spring ("spring forward") and set clocks back by one hour in autumn ("fall back") to return to standard time. As a result, there is one 23-hour day in late winter or early spring and one 25-hour day in the autumn.

upright=1.5|thumb
|alt=World map. Europe, most of North America, parts of southern South America and southeastern Australia, and a few other places use DST. Most of equatorial Africa and a few other places near the equator have never used DST. The rest of the landmass is marked as formerly using DST.|Daylight saving time regions:
George Hudson proposed the idea of daylight saving in 1895. The German Empire and Austria-Hungary organized the first nationwide implementation starting on April 30, 1916. Many countries have used it at various times since then, particularly since the 1970s energy crisis. DST is generally not observed near the equator, where sunrise and sunset times do not vary enough to justify it. Some countries observe it only in some regions; for example, parts of Australia observe it, while other parts do not, and the United States observes it, except Arizona (excepting Navajo Tribal lands, who conform to the national practice), and Hawaii, which do not. Only a minority of the world's population uses DST; Asia and Africa generally do not observe it.

DST clock shifts sometimes complicate timekeeping and can disrupt travel, billing, record keeping, medical devices, heavy equipment, and sleep patterns. Computer software generally adjusts clocks automatically.

Rationale
thumb|upright|alt=A water clock. A small human figurine holds a pointer to a cylinder marked by the hours. The cylinder is connected by gears to a water wheel driven by water that also floats, a part that supports the figurine.|
An ancient water clock that lets hour lengths vary with season.
Industrialized societies usually follow a clock-based schedule for daily activities that do not change throughout the course of the year. The time of day that individuals begin and end work or school, and the coordination of mass transit, for example, usually remain constant year-round. In contrast, an agrarian society's daily routines for work and personal conduct are more likely governed by the length of daylight hours and by solar time, which change seasonally because of the Earth's axial tilt. North and south of the tropics daylight lasts longer in summer and shorter in winter, with the effect becoming greater the further one moves away from the tropics.

By synchronously resetting all clocks in a region to one hour ahead of standard time, individuals who follow such a year-round schedule will wake an hour earlier than they would have otherwise; they will begin and complete daily work routines an hour earlier, and they will have available to them an extra hour of daylight after their workday activities. However, they will have one less hour of daylight at the start of each day, making the policy less practical during winter.

While the times of sunrise and sunset change at roughly equal rates as the seasons change, proponents of daylight saving time argue that most people prefer a greater increase in daylight hours after the typical "nine to five" workday. Supporters have also argued that DST decreases energy consumption by reducing the need for lighting and heating, but the actual effect on overall energy use is heavily disputed.

The shift in apparent time is also motivated by practicality.  In American temperate latitudes, for example, the sun rises around 04:30 on the summer solstice and sets around 19:30.  Since most people are sound asleep at 04:30, it makes more sense to pretend that 04:30 is actually 05:30, attendantly enabling people to be active in the evening light until what we decide to call 20:30 or later.  Of course, these times apply only in the center of the time bands at multiples of fifteen degrees longitude.  Farther east within a time zone, the sun rises and sets earlier than 12:00 (rechristened 13:00); farther west, later.

The manipulation of time at higher latitudes (for example Iceland, Nunavut, Scandinavia or Alaska) has little impact on daily life, because the length of day and night changes more extremely throughout the seasons (in comparison to other latitudes), and thus sunrise and sunset times are significantly out of phase with standard working hours regardless of manipulations of the clock. DST is also of little use for locations near the equator, because these regions see only a small variation in daylight in the course of the year. The effect also varies according to how far east or west the location is within its time zone, with locations farther east inside the time zone benefiting more from DST than locations farther west in the same time zone.  Neither is daylight savings of much practicality in such places as China, whichdespite its width of thousands of milesis all located within a single time zone per government mandate.

History

Ancient civilizations adjusted daily schedules to the sun more flexibly than DST does, often dividing daylight into 12 hours regardless of daytime, so that each daylight hour became progressively longer during spring and shorter during autumn. For example, the Romans kept time with water clocks that had different scales for different months of the year; at Rome's latitude, the third hour from sunrise (hora tertia) started at 09:02 solar time and lasted 44 minutes at the winter solstice, but at the summer solstice it started at 06:58 and lasted 75 minutes. From the 14th century onwards, equal-length civil hours supplanted unequal ones, so civil time no longer varied by season. Unequal hours are still used in a few traditional settings, such as some monasteries of Mount Athos and all Jewish ceremonies.
thumb|upright|right|alt= Fuzzy head-and-shoulders photo of a 40-year-old man with a mustache.|George Hudson invented modern DST, proposing it first in 1895.

Benjamin Franklin published the proverb "early to bed and early to rise makes a man healthy, wealthy, and wise", and published a letter in the Journal de Paris during his time as an American envoy to France (1776–1785) suggesting that Parisians economize on candles by rising earlier to use morning sunlight. This 1784 satire proposed taxing window shutters, rationing candles, and waking the public by ringing church bells and firing cannons at sunrise. Its first publication was in the journal's "Économie" section in a French translation. The revised English version [cited February 13, 2009] is commonly called "An Economical Project", a title that is not Franklin's; see  Despite common misconception, Franklin did not actually propose DST; 18th-century Europe did not even keep precise schedules. However, this changed as rail transport and communication networks required a standardization of time unknown in Franklin's day.

In 1810, the Spanish National Assembly Cortes of Cádiz issued a regulation that moved certain meeting times forward by one hour from May 1 to September 30 in recognition of seasonal changes, but it did not actually change the clocks. It also acknowledged that private businesses were in the practice of changing their opening hours to suit daylight conditions, but they did so of their own volition.

New Zealand entomologist George Hudson first proposed modern DST. His shift-work job gave him leisure time to collect insects and led him to value after-hours daylight. In 1895, he presented a paper to the Wellington Philosophical Society proposing a two-hour daylight-saving shift, and considerable interest was expressed in Christchurch; he followed up with an 1898 paper. Many publications credit the DST proposal to prominent English builder and outdoorsman William Willett, who independently conceived DST in 1905 during a pre-breakfast ride when he observed how many Londoners slept through a large part of a summer day. Willett also was an avid golfer who disliked cutting short his round at dusk. His solution was to advance the clock during the summer months, and he published the proposal two years later. Liberal Party member of parliament Robert Pearce took up the proposal, introducing the first Daylight Saving Bill to the House of Commons on February 12, 1908. A select committee was set up to examine the issue, but Pearce's bill did not become law and several other bills failed in the following years. Willett lobbied for the proposal in the UK until his death in 1915.

Port Arthur, Ontario, Canada was the first city in the world to enact DST, on July 1, 1908. This was followed by Orillia, Ontario, introduced by William Sword Frost while mayor from 1911 to 1912. The first states to adopt DST () nationally were those of the German Empire and its World War I ally Austria-Hungary commencing April 30, 1916, as a way to conserve coal during wartime. Britain, most of its allies, and many European neutrals soon followed. Russia and a few other countries waited until the next year, and the United States adopted daylight saving in 1918. Most jurisdictions abandoned DST in the years after the war ended in 1918, with exceptions including Canada, the UK, France, Ireland, and the United States.  It became common during World War II, and was widely adopted in America and Europe from the 1970s as a result of the 1970s energy crisis. Since then, the world has seen many enactments, adjustments, and repeals.

Procedure

The relevant authorities usually schedule clock changes to occur at (or soon after) midnight, and on a weekend, in order to lessen disruption to weekday schedules. A one-hour change is customary, but twenty-minute and two-hour changes have been used in the past. In all countries that observe daylight saving time seasonally (i.e. during summer and not winter), the clock is advanced from standard time to daylight saving time in the spring, and they are turned back from daylight saving time to standard time in the autumn. The practice, therefore, reduces the number of civil hours in the day of the springtime change, and it increases the number of civil hours in the day of the autumnal change. For a midnight change in spring, a digital display of local time would appear to jump from 23:59:59.9 to 01:00:00.0. For the same clock in autumn, the local time would appear to repeat the hour preceding midnight, i.e. it would jump from 23:59:59.9 to 23:00:00.0.

In most countries that observe seasonal daylight saving time, the clock observed in winter is legally named "standard time," in accordance with the standardization of time zones to agree with the local mean time near the center of each region. An exception exists in Ireland, where its winter clock has the same offset (UTC±00:00) and legal name as that in Britain (Greenwich Mean Time)—but while its summer clock also has the same offset as Britain's (UTC+01:00), its legal name is Irish Standard Time as opposed to British Summer Time.

While most countries that change clocks for daylight saving time observe standard time in winter and DST in summer, Morocco observes (since 2019) daylight saving time every month but Ramadan. During the holy month (the date of which is determined by the lunar calendar and thus moves annually with regard to the Gregorian calendar), the country's civil clocks observe Western European Time (UTC+00:00, which geographically overlaps most of the nation). At the close of this month, its clocks are turned forward to Western European Summer Time (UTC+01:00), where they remain until the return of the holy month the following year.

The time at which to change clocks differs across jurisdictions. Members of the European Union conduct a coordinated change, changing all zones at the same instant, at 01:00 Coordinated Universal Time (UTC), which means that it changes at 02:00 Central European Time (CET), equivalent to 03:00 Eastern European Time (EET). As a result, the time differences across European time zones remain constant. North America coordination of the clock change differs, in that each jurisdiction change at 02:00 local time, which temporarily creates unusual differences in offsets. For example, Mountain Time is, for one hour in the autumn, zero hours ahead of Pacific Time instead of the usual one hour ahead, and, for one hour in the spring, it is two hours ahead of Pacific Time instead of one. Also, during the autumn shift from daylight saving to standard time, the hour between 01:00 and 01:59:59 occurs twice in any given time zone, whereasduring the late winter or spring shift from standard to daylight saving timethe hour between 02:00 and 02:59:59 disappears.

The dates on which clocks change vary with location and year; consequently, the time differences between regions also vary throughout the year. For example, Central European Time is usually six hours ahead of North American Eastern Time, except for a few weeks in March and October/November, while the United Kingdom and mainland Chile could be five hours apart during the northern summer, three hours during the southern summer, and four hours for a few weeks per year. Since 1996, European Summer Time has been observed from the last Sunday in March to the last Sunday in October; previously the rules were not uniform across the European Union. Starting in 2007, most of the United States and Canada observed DST from the second Sunday in March to the first Sunday in November, almost two-thirds of the year. Moreover, the beginning and ending dates are roughly reversed between the northern and southern hemispheres because spring and autumn are displaced six months. For example, mainland Chile observes DST from the second Saturday in October to the second Saturday in March, with transitions at 24:00 local time. In some countries time is governed by regional jurisdictions within the country such that some jurisdictions change and others do not; this is currently the case in Australia, Canada, Mexico, and the United States (formerly in Brazil, etc.).

From year to year, the dates on which to change clock may also move for political or social reasons. The Uniform Time Act of 1966 formalized the United States' period of daylight saving time observation as lasting six months (it was previously declared locally); this period was extended to seven months in 1986, and then to eight months in 2005. The 2005 extension was motivated in part by lobbyists from the candy industry, seeking to increase profits by including Halloween (October 31) within the daylight saving time period. In recent history, Australian state jurisdictions not only changed at different local times but sometimes on different dates. For example, in 2008 most states there that observed daylight saving time changed clocks forward on October 5, but Western Australia changed on October 26.

 Politics, religion and sport 
The concept of daylight saving has caused controversy since its early proposals.DST practices and controversies:
 
  Winston Churchill argued that it enlarges "the opportunities for the pursuit of health and happiness among the millions of people who live in this country" and pundits have dubbed it "Daylight Slaving Time". Retailing, sports, and tourism interests have historically favored daylight saving, while agricultural and evening-entertainment interests have opposed it; energy crises and war prompted its initial adoption.

The fate of Willett's 1907 proposal illustrates several political issues. It attracted many supporters, including Arthur Balfour, Churchill, David Lloyd George, Ramsay MacDonald, King Edward VII (who used half-hour DST or "Sandringham time" at Sandringham, the managing director of Harrods, and the manager of the National Bank. However, the opposition proved stronger, including Prime Minister H. H. Asquith, William Christie (the Astronomer Royal), George Darwin, Napier Shaw (director of the Meteorological Office), many agricultural organizations, and theatre-owners. After many hearings, a parliamentary committee vote narrowly rejected the proposal in 1909. Willett's allies introduced similar bills every year from 1911 through 1914, to no avail. People in the U.S. demonstrated even more skepticim;  Andrew Peters introduced a DST bill to the House of Representatives in May 1909, but it soon died in committee.

thumb|upright|alt= Poster titled "VICTORY! CONGRESS PASSES DAYLIGHT SAVING BILL" showing Uncle Sam turning a clock to daylight saving time as a clock-headed figure throws his hat in the air. The clock face of the figure reads "ONE HOUR OF EXTRA DAYLIGHT". The bottom caption says "Get Your Hoe Ready!"| Retailers generally favor DST; United Cigar Stores hailed a 1918 DST bill.

Germany together with its allies led the way in introducing DST () during World War I on April 30, 1916, aiming to alleviate hardships due to wartime coal-shortages and from air-raid blackouts. The political equation changed in other countries; the United Kingdom used DST first on May 21, 1916. U.S. retailing and manufacturing interests - led by Pittsburgh industrialist Robert Garland - soon began lobbying for DST, but railroads opposed the idea. The U.S.'s 1917 entry into the war overcame objections, and DST started in 1918.

The war's end swung the pendulum back. Farmers continued to dislike DST, and many countries repealed it after the war - like Germany itself, which dropped DST from 1919 to 1939 and from 1950 to 1979. Britain proved an exception; it retained DST nationwide but adjusted transition dates over the years for several reasons, including special rules during the 1920s and 1930s to avoid clock shifts on Easter mornings.  summer time began annually on the last Sunday in March under a European Community directive, which may be Easter Sunday (as in 2016). The U.S. dithered more; Congress repealed DST after 1919. President Woodrow Wilson - an avid golfer like Willett - vetoed the repeal twice, but his second veto was overridden. Only a few U.S. cities retained DST locally, including New York (so that its financial exchanges could maintain an hour of arbitrage trading with London), and Chicago and Cleveland (to keep pace with New York). Wilson's successor as President, Warren G. Harding, opposed DST as a "deception", reasoning that people should instead get up and go to work earlier in the summer. He ordered District of Columbia federal employees to start work at 8 am rather than 9 am during the summer of 1922. Some businesses followed suit, though many others did not; the experiment was not repeated.

Since Germany's adoption of DST in 1916, the world has seen many enactments, adjustments, and repeals of DST, with similar politics involved. The history of time in the United States features DST during both world wars, but no standardization of peacetime DST until 1966. St. Paul and Minneapolis, Minnesota, kept different times for two weeks in May 1965: the capital city decided to switch to daylight saving time, while Minneapolis opted to follow the later date set by state law. In the mid-1980s, Clorox and 7-Eleven provided the primary funding for the Daylight Saving Time Coalition behind the 1987 extension to U.S. DST. Both senators from Idaho, Larry Craig and Mike Crapo, voted for it based on the premise that fast-food restaurants sell more French fries (made from Idaho potatoes) during DST.

A  referendum on the introduction of daylight saving took place in Queensland, Australia, in 1992, after a three-year trial of daylight saving. It was defeated with a 54.5% "no" vote, with regional and rural areas strongly opposed, and those in the metropolitan southeast in favor. In 2005 the Sporting Goods Manufacturers Association and the National Association of Convenience Stores successfully lobbied for the 2007 extension to U.S. DST. In December 2008 the Daylight Saving for South East Queensland (DS4SEQ) political party was officially registered in Queensland, advocating the implementation of a dual-time-zone arrangement for daylight saving in South East Queensland, while the rest of the state maintained standard time. DS4SEQ contested the March 2009 Queensland state election with 32 candidates and received one percent of the  statewide primary vote, equating to around 2.5% across the 32 electorates contested. After a three-year trial, more than 55% of Western Australians voted against DST in 2009, with rural areas strongly opposed. Queensland Independent member Peter Wellington introduced the Daylight Saving for South East Queensland Referendum Bill 2010 into the Queensland parliament on April 14, 2010, after being approached by the DS4SEQ political party, calling for a referendum at the next state election on the introduction of daylight saving into South East Queensland under a dual-time-zone arrangement. The Queensland parliament rejected Wellington's bill on June 15, 2011.

In the UK, the Royal Society for the Prevention of Accidents supports a proposal to observe SDST's additional hour year-round, but that is opposed by some industries, by some postal workers and farmers, and particularly by those living in the northern regions of the UK. In some Muslim countries, DST is temporarily abandoned during Ramadan (the month when no food should be eaten between sunrise and sunset), since the DST would delay the evening dinner. Iran maintains DST during Ramadan, but most Muslim countries do not use DST, partially for this reason.

Russia declared in 2011 that it would stay in DST all year long; Belarus followed with a similar declaration. (The Soviet Union had operated under permanent  "summer time" from 1930 to at least 1982.) Russia's plan generated widespread complaints due to the dark of winter-time mornings, and thus was abandoned in 2014. The country changed its clocks to standard time on October 26, 2014, intending to stay there permanently.

Impacts

thumb|right|upright|alt=A standing man in three-piece suit, facing camera. He is about 60 and is bald with a mustache. His left hand is in his pants pocket, and his right hand is in front of his chest, holding his pocket watch.|William Willett independently proposed DST in 1907 and advocated it tirelessly.
 or is good for business.

A 2017 meta-analysis of 44 studies found that DST leads to electricity savings of 0.3% during the days when DST applies. Several studies have suggested that DST increases motor fuel consumption, but a 2008 United States Department of Energy report found no significant increase in motor gasoline consumption due to the 2007 United States extension of DST. An early goal of DST was to reduce evening usage of incandescent lighting, once a primary use of electricity. Although energy conservation remains an important goal, energy usage patterns have greatly changed since then. Electricity use is greatly affected by geography, climate, and economics, so the results of a study conducted in one place may not be relevant to another country or climate.

Later sunset times from DST are thought to affect behavior; for example, increasing participation in after-school sports programs or outdoor afternoon sports such as golf, and attendance at professional sporting events. Advocates of daylight saving time argue that having more hours of daylight between the end of a typical workday and evening induces people to consume other goods and services.

Many farmers oppose DST, particularly dairy farmers as the milking patterns of their cows do not change with the time. and others whose hours are set by the sun.Effect on those whose hours are set by the sun:

 
  Young children often have difficulty getting enough sleep at night when the evenings are bright. DST also hurts prime-time television broadcast ratings, drive-ins and other theaters.

It has been argued that clock shifts correlate with decreased economic efficiency, and that in 2000 the daylight-saving effect implied an estimated one-day loss of $31 billion on U.S. stock exchanges, Others have asserted that the observed results depend on methodology and disputed the findings, though the original authors have refuted points raised by disputers.

A correlation between clock shifts and traffic accidents has been observed in North America and the UK but not in Finland or Sweden. Four reports have found that this effect is smaller than the overall reduction in traffic fatalities. DST likely reduces some kinds of crime, such as robbery and sexual assault, as fewer potential victims are outdoors after dusk. Artificial outdoor lighting has a marginal and sometimes even contradictory influence on crime and fear of crime. A 2017 study in the American Economic Journal: Applied Economics estimated that "the transition into DST caused over 30 deaths at a social cost of $275 million annually," primarily by increasing sleep deprivation.

Opponents argue that DST disrupts human circadian rhythms (negatively impacting human health in the process), that it increases fatal traffic collisions, that the actual energy savings are inconclusive, and that DST increases health risks such as heart attack. Year-round standard time (not year-round DST) is proposed to be the preferred option for public health and safety. Clock shifts were found to increase the risk of heart attack by 10 percent, and to disrupt sleep and reduce its efficiency. Effects on seasonal adaptation of the circadian rhythm can be severe and last for weeks.DST and circadian rhythm:

 
 

DST's clock shifts have the obvious disadvantage of complexity. People must remember to change their clocks; this can be time-consuming, particularly for mechanical clocks that cannot be moved backward safely. People who work across time zone boundaries need to keep track of multiple DST rules, as not all locations observe DST or observe it the same way. The length of the calendar day becomes variable; it is no longer always 24 hours. Disruption to meetings, travel, broadcasts, billing systems, and records management is common, and can be expensive. During an autumn transition from 02:00 to 01:00, a clock reads times from 01:00:00 through 01:59:59 twice, possibly leading to confusion.

Some clock-shift problems could be avoided by adjusting clocks continuously or at least more gradually—for example, Willett at first suggested weekly 20-minute transitions—but this would add complexity and has never been implemented. DST inherits and can magnify the disadvantages of standard time. For example, when reading a sundial, one must compensate for it along with time zone and natural discrepancies. Also, sun-exposure guidelines such as avoiding the sun within two hours of noon become less accurate when DST is in effect.

Terminology
As explained by Richard Meade in the English Journal of the (American) National Council of Teachers of English, the form daylight savings time (with an "s") was already in 1978 much more common than the older form daylight saving time in American English ("the change has been virtually accomplished"). Nevertheless, even dictionaries such as Merriam-Webster's, American Heritage, and Oxford, which describe actual usage instead of prescribing outdated usage (and therefore also list the newer form), still list the older form first. This is because the older form is still very common in print and preferred by many editors. ("Although daylight saving time is considered correct, daylight savings time (with an "s") is commonly used.") The first two words are sometimes hyphenated (daylight-saving(s) time). Merriam-Webster's also lists the forms daylight saving (without "time"), daylight savings (without "time"), and daylight time.Daylight saving time and its variants:
 
 
 
  "also daylight savings time"
  The Oxford Dictionary of American Usage and Style explains the development and current situation as follows: "Although the singular form daylight saving time is the original one, dating from the early 20th century—and is preferred by some usage critics—the plural form is now extremely common in AmE. [...] The rise of daylight savings time appears to have resulted from the avoidance of a miscue: when saving is used, readers might puzzle momentarily over whether saving is a gerund (the saving of daylight) or a participle (the time for saving). [...] Using savings as the adjective—as in savings account or savings bond—makes perfect sense. More than that, it ought to be accepted as the better form."

In Britain, Willett's 1907 proposal used the term daylight saving, but by 1911 the term summer time replaced daylight saving time in draft legislation. The same or similar expressions are used in many other languages: Sommerzeit in German, zomertijd in Dutch, kesäaika in Finnish, horario de verano or hora de verano in Spanish, and heure d'été in French.

The name of local time typically changes when DST is observed. American English replaces standard with daylight: for example, Pacific Standard Time (PST) becomes Pacific Daylight Time (PDT). In the United Kingdom, the standard term for UK time when advanced by one hour is British Summer Time (BST), and British English typically inserts summer into other time zone names, e.g. Central European Time (CET) becomes Central European Summer Time (CEST).

The North American English mnemonic "spring forward, fall back" (also "spring ahead ...", "spring up ...", and "... fall behind") helps people remember in which direction to shift the clocks.

Computing
thumb|upright|
alt=Strong man in sandals and with shaggy hair, facing away from audience/artist, grabbing a hand of a clock bigger than he is and attempting to force it backwards. The clock uses Roman numerals and the man is dressed in stripped-down Roman gladiator style. The text says "You can't stop time... But you can turn it back one hour at 2 a.m. on Oct. 28 when daylight-saving time ends and standard time begins."|
A 2001 US public service advertisement reminded people to adjust clocks.

Changes to DST rules cause problems in existing computer installations. For example, the 2007 change to DST rules in North America required that many computer systems be upgraded, with the greatest impact on e-mail and calendar programs. The upgrades required a significant effort by corporate information technologists.

Some applications standardize on UTC to avoid problems with clock shifts and time zone differences.
Likewise, most modern operating systems internally handle and store all times as UTC and only convert to local time for display.

However, even if UTC is used internally, the systems still require external leap second updates and time zone information to correctly calculate local time as needed. Many systems in use today base their date/time calculations from data derived from the tz database also known as zoneinfo.

IANA time zone database
The tz database maps a name to the named location's historical and predicted clock shifts. This database is used by many computer software systems, including most Unix-like operating systems, Java, and the Oracle RDBMS; HP's "tztab" database is similar but incompatible. When temporal authorities change DST rules, zoneinfo updates are installed as part of ordinary system maintenance. In Unix-like systems the TZ environment variable specifies the location name, as in TZ=':America/New_York'. In many of those systems there is also a system-wide setting that is applied if the TZ environment variable is not set: this setting is controlled by the contents of the  file, which is usually a symbolic link or hard link to one of the zoneinfo files. Internal time is stored in time-zone-independent Unix time; the TZ is used by each of potentially many simultaneous users and processes to independently localize time display.

Older or stripped-down systems may support only the TZ values required by POSIX, which specify at most one start and end rule explicitly in the value. For example, TZ='EST5EDT,M3.2.0/02:00,M11.1.0/02:00' specifies time for the eastern United States starting in 2007. Such a TZ value must be changed whenever DST rules change, and the new value applies to all years, mishandling some older timestamps.

Permanent daylight saving time

thumb|upright|alt=A standing stone in a grassy field surrounded by trees. The stone contains a vertical sundial centered on 1 o'clock, and is inscribed "HORAS NON NUMERO NISI ÆSTIVAS" and "SUMMER TIME ACT 1925"|The William Willett Memorial Sundial in Petts Wood, south London, is always on DST.
A move to permanent daylight saving time (staying on summer hours all year with no time shifts) is sometimes advocated and is currently implemented in some jurisdictions such as Argentina, Belarus, Saskatchewan, Yukon, Iceland, Kyrgyzstan, Magallanes and Chilean Antarctica Region, Malaysia, Morocco, Namibia, Singapore, Turkey, Turkmenistan and Uzbekistan. Advocates cite the same advantages as normal DST without the problems associated with the twice yearly time shifts. However, many remain unconvinced of the benefits, citing the same problems and the relatively late sunrises, particularly in winter, that year-round DST entails.

Russia switched to permanent DST from 2011 to 2014, but the move proved unpopular because of the late sunrises in winter, so in 2014, Russia switched permanently back to standard time. The United Kingdom and Ireland also experimented with year-round summer time between 1968 and 1971, and put clocks forward by an extra hour during World War II.

In the United States, the Florida, Washington, California, and Oregon legislatures have all passed bills to enact permanent DST, but the bills require Congressional approval in order to take effect. Maine, Massachusetts, New Hampshire, and Rhode Island have also introduced proposals or commissions to that effect. Although 26 states have considered making DST permanent, unless Congress changes federal law, states cannot implement permanent DST—states can only opt out of DST, not standard time.

In September 2018, the European Commission proposed to end seasonal clock changes as of 2019. Member states would have the option of observing either daylight saving time all year round or standard time all year round. In March 2019, the European Parliament approved the commission's proposal, while deferring implementation from 2019 until 2021. , the decision has not been confirmed by the Council of the European Union. The council has asked the commission to produce a detailed impact assessment, but the Commission considers that the onus is on the Member States to find a common position in Council. As a result, progress on the issue is effectively blocked.

Experts in circadian rhythms and sleep caution against permanent daylight saving time, recommending year-round standard time as the preferred option for public health and safety.

Perceived problems with permanent DST
Since daylight saving time creates the illusion of the sun rising and setting one hour later on the clock, but does not add any additional daylight, the already later sunrise times under standard time are pushed an hour later on the clock with daylight saving time. Late sunrise times can become unpopular in the winter months which essentially forces workers and schoolchildren to begin the day in darkness. In 1974 following the enactment of the Emergency Daylight Saving Time Act in the United States, there were complaints of children going to school in the dark and working people commuting and starting their work day in pitch darkness during the winter months. The complaints led to the repeal of the act in October 1974 when standard time was restored until February 23, 1975. In 1976, the United States returned to the schedule set under the Uniform Time Act of 1966. In 1971, year-round daylight time in the United Kingdom was abandoned after a 3-year experiment because of complaints about winter sunrise times. The same complaints also led to Russia abandoning DST and instituting standard time year round in 2014.

By country and region

Africa
Asia
Europe
North and South America
Oceania

See also

 Winter time (clock lag)

References

Sources

 
  The British version, focusing on the UK, is 

Further reading

 

External links

 Daylight Saving Time Congressional Research Service
 Information about the Current Daylight Saving Time (DST) Rules, U.S. National Institute of Standards and Technology
 "Legal Time 2015", Telecommunications Standardization Bureau of the ITU
 Sources for time zone and daylight saving time data

 