Daylight saving time

Un article de Vev.

Daylight saving time (DST; also summer time in British English) is the convention of advancing clocks so that afternoons have more daylight and mornings have less. Typically clocks are adjusted forward one hour near the start of spring and are adjusted backward in autumn. Modern DST was first proposed in 1907 by William Willett. Many countries have used it since then; details vary by location and change occasionally.

The practice can be controversial.<ref name=Downing-Prerau/> Adding daylight to afternoons benefits retailing, sports, and other activities that exploit sunlight after working hours,<ref name=Benfield/> but causes problems for farming, entertainment and other occupations tied to the sun.<ref name=farming/><ref name=Kissell/> Extra afternoon daylight reduces traffic fatalities;<ref name=Ferguson/> its effect on health and crime is less clear. An early goal of DST was to reduce evening usage of incandescent lighting, formerly a primary use of electricity;<ref name=Bartlett/> changes in usage patterns have cast doubt on whether DST now saves energy.<ref name=Kellogg/>

DST's clock shifts can serve as fire safety reminders,<ref name=USFA/> but they complicate timekeeping and can disrupt meetings, travel, billing, recordkeeping, medical devices, and heavy equipment.<ref name=Neumann/> Many computer-based systems can adjust their clocks automatically, but this can be limited and error-prone, particularly when DST rules change.<ref name=Tong/>

Origin

Although not punctual in the modern sense, the ancients adjusted daily schedules to the sun more flexibly than modern DST does, often dividing daylight into 12 equal hours regardless of day length, so that each daylight hour was longer during summer.<ref>Modèle:Cite journal</ref> For example, Roman water clocks had different scales for different months of the year: at Rome's latitude the third hour from sunrise, hora tertia, started by modern standards 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.<ref>Modèle:Cite book</ref> After ancient times, equal-length civil hours eventually supplanted unequal, so civil time no longer varies by season. Unequal hours are still used in a few traditional settings, such as some Mount Athos monasteries.<ref>Modèle:Cite journal</ref>

During his time as an American envoy to France, Benjamin Franklin, author of the proverb "Early to bed, and early to rise, makes a man healthy, wealthy and wise," anonymously published a letter suggesting that Parisians economize on candles by rising earlier to use morning sunlight.<ref>Modèle:Cite journal</ref> This 1784 satire proposed taxing shutters, rationing candles, and waking the public by ringing church bells and firing cannons at sunrise.<ref name='Franklin'>Modèle:Cite journal Its first publication was in the journal's "Économie" section. The revised English version (retrieved on 2007-05-26) is commonly called "An Economical Project", a title that is not Franklin's; see Modèle:Cite journal</ref> Franklin did not propose DST; like ancient Rome, 18th-century Europe did not keep accurate schedules. However, this soon changed as rail and communication networks came to require a standardization of time unknown in Franklin's day.<ref>Modèle:Cite journal</ref>

The prominent English builder and outdoorsman William Willett invented DST in 1905 during one of his pre-breakfast horseback rides, when he observed with dismay how many Londoners slept through the best part of a summer day.<ref> David Rooney

     (2007)
   
.    Royal Observatory commemorates William Willett 
. Petts Wood & District Residents' Association 
   

. Retrieved on 2007-03-27. </ref> An avid golfer, he also disliked cutting short his round at dusk. His solution was to advance the clock during the summer months, a proposal he published two years later.<ref name='Willett'>Modèle:Cite paper | Modèle:Cite paper</ref> He lobbied unsuccessfully for the proposal until his death in 1915; see Politics for more details. Wartime Germany, its allies, and their occupied zones were the first European countries to use DST, starting April 30, 1916. Britain, most other belligerents, and many European neutrals soon followed suit, Russia and a few other countries waited until the next year, and the United States did not use it until 1918. Since then the world has seen many enactments, adjustments, and repeals.<ref>Modèle:Cite book</ref>

Benefits and drawbacks

Willett's 1907 proposal argued that DST increases opportunities for outdoor leisure activities during afternoon sunlight hours. Obviously it does not change the length of the day; the longer days nearer the summer solstice in high latitudes merely offer more room to shift apparent daylight from morning to evening so that early morning daylight is not wasted.<ref name='Willett'/> DST is commonly not observed during most of winter, because its mornings are darker: workers may have no sunlit leisure time, and children may need to leave for school in the dark.<ref name='CRS'>Modèle:Cite paper</ref>

General agreement about the day's layout confers so many advantages that a standard DST schedule usually outranks ad hoc efforts to get up earlier, even for people who personally dislike the DST schedule.<ref>Modèle:Cite book Retrieved on 2007-05-26.</ref> The advantages of coordination are so great that many people ignore whether DST is in effect by altering their nominal work schedules to coordinate with daylight, television broadcasts, or remote colleagues.<ref>Modèle:Cite paper</ref>

Energy use

Delaying the nominal time of sunrise and sunset increases the use of artificial light in the morning and reduces it in the evening. As Franklin's 1784 satire pointed out, lighting costs are reduced if the evening reduction outweighs the morning increase, as in high-latitude summer when most people wake up well after sunrise. An early goal of DST was to reduce evening usage of incandescent lighting, formerly a primary use of electricity.<ref name=Bartlett>Modèle:Cite book</ref> Although energy conservation remains an important goal,<ref>Modèle:Cite journal</ref> statistically significant evidence for it has proved elusive. The U.S. Dept. of Transportation (DOT) concluded in 1975 that DST might reduce the country's electricity usage by 1% during March and April,<ref name=Lawson>Modèle:Cite book</ref> but the National Bureau of Standards (NBS) reviewed the DOT study in 1976 and found no significant energy savings.<ref name='CRS'/> In 2000 when parts of Australia began DST in late winter, overall electricity consumption did not decrease, but the morning peak load and prices increased.<ref name=Kellogg>Modèle:Cite paper</ref> Although a 2007 study estimated that introducing DST to Japan would reduce household lighting energy consumption,<ref name=Fong>Modèle:Cite journal</ref> a 2007 simulation estimated that DST would increase overall energy use in Osaka residences by 0.13%, with a 0.02% decrease due to less lighting more than outweighed by a 0.15% increase due to extra cooling; neither study examined non-residential energy use.<ref>Modèle:Cite journal</ref> A 2007 California study found no clear evidence that electricity would be saved through the earlier start to DST that year,<ref>Modèle:Cite paper</ref> and although one utility did report a decrease in March 2007, five others did not.<ref> Cindy Cline

  . 
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   Early daylight saving time hasn't saved energy in Ottawa 
     
 " , CFRA , 2007-03-21
 
  . Retrieved on 2007-05-16
 .  |    Jim Goddard
     
   
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   No saving to hydro and gas bill with earlier time change 
     
 " , News1130 , 2007-04-02
 
  . Retrieved on 2007-05-16
 .  |    Lisa Lee
     
   
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   Early U.S. Daylight Savings a bust in power savings 
     
 " , Reuters , 2007-04-02
 
  . Retrieved on 2007-05-16
 .  | "
   Daylight saving time reduces power consumption 
     
 " , AM770CHQR , 2007-04-03
 
  . Retrieved on 2007-05-16
 .  |    Ned Potter
     
   
  . 
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   Final word on daylight saving—not 
     
 " , ABC News , 2007-04-04
 
  . Retrieved on 2007-05-16
 . </ref> DST may increase gasoline consumption: U.S. gasoline demand grew an extra 1% during the newly introduced DST in March 2007.<ref>   Shawn McCarthy
     
   
  . 
 "
   Has daylight saving time fuelled gasoline consumption?
   
 " , Globe and Mail , 2007-04-18
 
 . </ref>

Economic effects

Retailers, sporting goods makers, and other businesses benefit from extra afternoon sunlight, as it induces customers to shop and to participate in outdoor afternoon sports. For example, in 1984 Fortune magazine estimated that a seven-week extension of DST would yield an additional $30 million for 7-Eleven stores, and the National Golf Foundation estimated the extension would increase golf industry revenues $200 million to $300 million.<ref>Modèle:Cite book</ref> Conversely, DST can adversely affect farmers and others whose hours are set by the sun.<ref name=farming>Modèle:Cite book | Modèle:Cite book</ref> For example, grain harvesting is best done after dew evaporates, so when field hands arrive and leave earlier in summer their labor is less valuable.<ref> Daylight savings time

. Session Weekly
. Minnesota House Public Information Office 
 
 (1991)
   

. Retrieved on 2003-03-21.

 “… the Minneapolis Star, Jan. 28, 1959.… [wrote] 'Farmers complained that they cannot get into the fields any earlier than under standard time … because the morning sun does not dry the dew "on daylight savings time." ' ”

</ref> DST also hurts prime-time broadcast ratings<ref name=Kissell> Rick Kissell

  . 
 "
   Daylight-saving dock ratings 
     
 " , Variety , 2007-03-20
 
  . Retrieved on 2007-05-16
 . </ref> and theaters, especially drive-ins.<ref>Modèle:Cite book</ref>

Clock shifts correlate with decreased economic efficiency. In 2000 the daylight-saving effect implied an estimated one-day loss of $31 billion on U.S. stock exchanges.<ref>Modèle:Cite journal</ref> Clock shifts and DST rule changes have a direct economic cost, since they entail extra work to support remote meetings, computer applications and the like. For example, a 2007 North American rule change cost an estimated $500 million to $1 billion.<ref> Ben Arnoldy

  . 
 "
   Latest computer glitch: daylight saving time 
     
 " , Christian Science Monitor , 2007-03-07
 
  . Retrieved on 2007-05-16
 . </ref>

Public safety

In 1975 the U.S. DOT conservatively identified a 0.7% reduction in traffic fatalities during DST, and estimated the real reduction to be 1.5% to 2%,<ref name='Lawson'/> but the 1976 NBS review of the DOT study found no differences in traffic fatalities.<ref name='CRS'/> In 1995 the Insurance Institute for Highway Safety estimated a reduction of 1.2%, including a 5% reduction in crashes fatal to pedestrians.<ref name='Ferguson'>Modèle:Cite journal</ref> Others have found similar reductions.<ref>Modèle:Cite journal</ref> Single/Double Summer Time (SDST), a variant where clocks are one hour ahead of the sun in winter and two in summer, has been projected to reduce traffic fatalities by 3% to 4% in the UK, compared to ordinary DST.<ref>Modèle:Cite paper</ref> It is not clear whether sleep disruption contributes to fatal accidents immediately after the spring and autumn clock shifts.<ref>Modèle:Cite journal Data supporting Coren's half of this exchange are in: Modèle:Cite conference</ref> A correlation between clock shifts and accidents has been observed in North America but not in Sweden. If this twice-yearly effect exists, it is far smaller than the overall reduction in fatalities.<ref>Modèle:Cite journal | Modèle:Cite journal</ref>

In the 1970s the U.S. Law Enforcement Assistance Administration (LEAA) found a reduction of 10% to 13% in Washington, D.C.'s violent crime rate during DST. However, the LEAA did not filter out other factors, and it examined only two cities and found crime reductions only in one and only in some crime categories; the DOT decided it was "impossible to conclude with any confidence that comparable benefits would be found nationwide."<ref>Modèle:Cite book</ref> Outdoor lighting has a marginal and sometimes even contradictory influence on crime and fear of crime.<ref>Modèle:Cite journal</ref>

In several countries, fire safety officials encourage citizens to use the two annual clock shifts as reminders to replace batteries in smoke and carbon monoxide detectors. This is especially important in autumn, just before the heating and candle season causes an increase in home fires. Similar twice-yearly tasks include reviewing and practicing fire escape and family disaster plans, inspecting vehicle lights, checking storage areas for hazardous materials, and reprogramming thermostats.<ref> Helen A.S. Popkin

  . 
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   Changes to daylight-saving time 
     
 " , Consumer Reports Home & Yard Blog , 2007-03-02
 
  . Retrieved on 2007-11-02
 . </ref><ref>Modèle:Cite press release</ref><ref name='USFA'>Modèle:Cite press release</ref> This is not an essential function of DST, as locations without DST can instead use the first days of spring and autumn as reminders.<ref>   Amy Fackler
   
 

     (2005-04-08)
   
.    Prevent household fires 
. Yahoo Health

. Retrieved on 2007-05-16. </ref>

Health

DST has mixed effects on health. In societies with fixed work schedules it provides more afternoon sunlight for outdoor exercise.<ref>Modèle:Cite book</ref> It alters sunlight exposure; whether this is beneficial depends on one's location and daily schedule, as sunlight triggers vitamin D synthesis in the skin, but overexposure can lead to skin cancer. Sunlight strongly influences seasonal affective disorder; DST may help in depression by causing individuals to rise earlier,<ref>Modèle:Cite journal</ref> but some argue the reverse.<ref> Shari Roan

  . 
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   Change in daylight saving time may affect moods
   
 " , Los Angeles Times , 2007-03-05
 
 . </ref> The Retinitis Pigmentosa Foundation Fighting Blindness, chaired by blind sports magnate Gordon Gund, successfully lobbied in 1985 and 2005 for U.S. DST extensions,<ref name='Benfield'/><ref name='Beam'/> but DST can hurt night blindness sufferers.<ref>   Daylight savings 
. Moderatrix Musings
 (2007-03-12)
   

. Retrieved on 2007-03-28.

 “I now have to drive to work in the dark. This is a real problem for me as I've got terrible night blindness and avoid driving in the dark at all costs. It's just not safe (especially for the other drivers since I can't tell where they are.)”

</ref>

Clock shifts reduce sleep duration and efficiency,<ref>Modèle:Cite journal</ref> particularly for late-risers after clocks spring forward; effects can last for weeks.<ref>Modèle:Cite journal</ref> The government of Kazakhstan cited health complications due to clock shifts as a primary reason for abolishing DST in 2005.<ref> Kazakhstan abolishes daylight saving time

. Kazakhstan Society in the UK 
 
 (2005-03-21)
   

. Retrieved on 2007-03-28. </ref>

Complexity

DST's clock shifts have the obvious disadvantage of complexity. People must remember to change their clocks; this consumes time, particularly for mechanical clocks that cannot be moved backward safely.<ref> Joey Crandall

  . 
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   Daylight saving time ends Sunday 
     
 " , Record-Courier , 2003-10-24
 
  . Retrieved on 2007-10-29
 . </ref> 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 day becomes variable. Disruption to meetings, travel, broadcasts, billing systems, and records management is common, and can be expensive.<ref>   Paul McDougall
     
   
  . 
 "
   PG&E says patching meters for an early daylight-saving time will cost $38 million 
     
 " , InformationWeek , 2007-03-01
 
  . Retrieved on 2007-05-16
 . </ref> Near an autumn transition from 02:00 to 01:00, a clock reads times from 01:00 to 02:00 twice, possibly leading to confusion.<ref>   Daylight saving time: rationale and original idea 
. WebExhibits

 

. Retrieved on 2007-02-06. </ref>

Computer-based systems may also require downtime or restarting when clocks shift. Ignoring this requirement damaged a German steel facility in 1993.<ref name='Neumann'>Modèle:Cite book Retrieved on 2007-05-26.</ref> Medical devices may generate adverse events that could harm patients, without being obvious to clinicians responsible for care.<ref> Daniel G. Schultz

     (2007-03-29)
   
.    Unpredictable events in medical equipment due to new daylight saving time change 
. Center for Devices and Radiological Health, Food and Drug Administration 
   

. Retrieved on 2007-11-11. </ref> These problems are compounded when the DST rules themselves change, as in the Year 2007 problem. Software developers must test and perhaps modify many programs, and users must install updates and restart applications.<ref name='Tong'>Modèle:Cite journal</ref>

Some clock-shift problems could be avoided by adjusting clocks continuously<ref> Jesse Ruderman

     (2006-11-01)
   
.    Continuous daylight saving time 
. Indistinguishable from Jesse

. Retrieved on 2007-03-21. </ref> or at least more gradually—for example, Willett originally 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.<ref> Ann Parker

  . 
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   Sundials put old slant on telling time 
     
 " , Santa Cruz Sentinel , 2007-03-11
 
  . Retrieved on 2007-05-16
 . </ref> Also, DST complicates time-of-day approaches for avoiding sun exposure, necessitating even more the shadow rule of avoiding sun exposure whenever your shadow is shorter than you are.<ref>Modèle:Cite journal</ref>

Politics

Daylight saving has caused controversy since it began.<ref name=Downing-Prerau>Modèle:Cite book | Modèle:Cite book See Further reading for full citations.</ref> 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".<ref>Modèle:Cite journal</ref> Robertson Davies, however, detected "the bony, blue-fingered hand of Puritanism, eager to push people into bed earlier, and get them up earlier, to make them healthy, wealthy and wise in spite of themselves,"<ref>Modèle:Cite book</ref> and wags have dubbed it "Daylight Slaving Time"<ref>Modèle:Cite book</ref> or "Daylight Stupid Time".<ref> Diana M. Estill

  . 
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   On the clock
   
 " , Dallas Morning News , 2007-11-08
 
  . Retrieved on 2007-11-11
 . </ref> Historically, retailing, sports and tourism interests have favored daylight saving, while agricultural and evening entertainment interests have opposed it, and a war or economic crisis is often associated with its adoption.

The fate of Willett's 1907 proposal illustrates several political issues involved. The proposal attracted many supporters, including Balfour, Churchill, Lloyd George, MacDonald, Edward VII (who used half-hour DST at Sandringham), the managing director of Harrods, and the manager of the National Bank. However, the opposition was stronger: it included Prime Minister Asquith, Christie (the Astronomer Royal), George Darwin, Napier Shaw (director of the Meteorological Office), many agricultural organizations, and theater owners. After many hearings the proposal was narrowly defeated in a Parliament committee vote in 1909. Willett's allies introduced similar bills every year from 1911 through 1914, to no avail.<ref>Modèle:Cite book</ref> The U.S. was even more skeptical: Andrew Peters introduced a DST bill to the U.S. House in May 1909, but it soon died in committee.<ref>Modèle:Cite book</ref>

World War I changed the political equation, as DST was promoted as a way to alleviate hardships from wartime coal shortages and air raid blackouts. After Germany led the way, the United Kingdom first used DST on May 21, 1916.<ref>Modèle:Cite book</ref> U.S. retailing and manufacturing interests led by Pittsburgh industrialist Robert Garland soon began lobbying for DST, but were opposed by railroads. The U.S.'s 1917 entry to the war overcame objections, and DST was established in 1918.<ref>Modèle:Cite book</ref>

War's end swung the pendulum back. Farmers continued to dislike DST, and many countries repealed it after the war. Britain was an exception: it retained DST nationwide but over the years adjusted transition dates for several reasons, including special rules during the 1920s and 1930s to avoid clock shifts on Easter mornings.<ref name='Myers'> Joseph Myers

     (2007-09-16)
   
.    History of legal time in Britain 

. Retrieved on 2007-10-24. </ref> The U.S. was more typical: Congress repealed DST after 1919. President Woodrow Wilson, like Willett an avid golfer, vetoed the repeal twice but his second veto was overridden,<ref>Modèle:Cite book</ref> and only a few U.S. cities retained DST locally thereafter.<ref>Modèle:Cite book</ref> Wilson's successor 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 08:00 rather than 09:00 during summer 1922. Many businesses followed suit though many others did not; the experiment was not repeated.<ref>Modèle:Cite book</ref>

Since Willett's day the world has seen many enactments, adjustments, and repeals of DST, with similar politics involved.<ref>Modèle:Cite book</ref> The history of time in the United States includes DST during both world wars, but no standardization of peacetime DST until 1966.<ref>Modèle:Cite book</ref> In the mid-1980s, Clorox (parent of Kingsford Charcoal) and 7-Eleven provided the primary funding for the Daylight Saving Time Coalition behind the 1987 extension to U.S. DST, and both Idaho senators voted for it on the basis of fast-food restaurants selling more French fries made from Idaho potatoes;<ref name=Benfield>Modèle:Cite book</ref> in 2005, the Sporting Goods Manufacturers Association and the National Association of Convenience Stores successfully lobbied for the 2007 extension to U.S. DST.<ref name=Beam> Alex Beam

  . 
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   Dim-witted proposal for daylight time 
     
 " , Boston Globe , 2005-07-26
 
  . Retrieved on 2007-05-16
 . </ref> In early 2007, Western Australia continued to debate a trial use of DST and several politicians changed positions after public sentiment swung against it.<ref>   Joe Spagnolo
     
   
  . 
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   High noon for sun-slingers 
     
 " , Sunday Times , 2007-03-17
 
  . Retrieved on 2007-05-16
 . </ref> In the UK the sport and leisure industry supports a proposal to observe SDST's additional hour year-round.<ref>   Single/Double Summer Time policy paper 
 (PDF)
. Royal Society for the Prevention of Accidents 
 
 (Oct. 2006)
   

. Retrieved on 2007-05-16. </ref>

Observance practices

Main article: Daylight saving time around the world

In a typical case where a one-hour shift occurs at 02:00 local time, in spring the clock jumps forward from 02:00 standard time to 03:00 DST and the day has 23 hours, whereas in autumn the clock jumps backward from 02:00 DST to 01:00 standard time, repeating that hour, and the day has 25 hours. A digital display of local time does not read 02:00 exactly, but instead jumps from 01:59:59.9 either forward to 03:00:00.0 or backward to 01:00:00.0. In this example, a location observing [[UTC+10]] during standard time is at [[UTC+11]] during DST; conversely, a location at UTC−10 during standard time is at UTC−9 during DST.

Clock shifts are usually scheduled near a weekend midnight to lessen disruption to weekday schedules. A one-hour shift is customary, but Australia's Lord Howe Island uses a half-hour shift.<ref> Colin Oriti

     (2007-12-12)
   
.    Getting to Lord Howe Island 
. Lord Howe Island Tourism Association 
   

. Retrieved on 2007-12-14. </ref> Twenty-minute and two-hour shifts have been used in the past.

Coordination strategies differ when adjacent time zones shift clocks. The European Union shifts all at once, at 01:00 UTC; for example, Eastern European Time is always one hour ahead of Central European Time.<ref name='Myers'/> Most of North America shifts at 02:00 local time, so its zones do not shift at the same time; for example, Mountain Time can be temporarily either zero or two hours ahead of Pacific Time. Australian districts go even further and do not always agree on start and end dates; for example, to start DST in 2006 Tasmania shifted clocks forward on 1 October, Western Australia on 3 December, and the remaining DST-observing areas on 29 October.<ref> Implementation dates of daylight saving time within Australia

. Bureau of Meteorology 
 
 (2007-10-30)
   

. Retrieved on 2007-11-06. </ref>

Start and end dates vary with location and 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.<ref name='Myers'/> Starting in 2007, most of the United States and Canada observe DST from the second Sunday in March to the first Sunday in November, almost two-thirds of the year.<ref> Tom Baldwin

  . 
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   US gets summertime blues as the clocks go forward 3 weeks early 
     
 " , The Times , 2007-03-12
 
  . Retrieved on 2007-05-16
 . </ref> The 2007 U.S. change was part of the Energy Policy Act of 2005; previously, from 1987 through 2006, the start and end dates were the first Sunday in April and the last Sunday in October, and Congress retains the right to go back to the previous dates once an energy-consumption study is done.<ref>   Energy Policy Act of 2005, Public Law 109-58 § 110 
 (2005-08-08)
   

. Retrieved on 2007-05-10. </ref>

Beginning and ending dates are the reverse in the southern hemisphere. 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.<ref> Historia de la hora oficial de Chile

 (Spanish) 
. Chilean Hydrographic and Oceanographic Service 
 
 (2007-05-17)
   

. Retrieved on 2007-11-06. </ref> The time difference between the United Kingdom and mainland Chile may therefore be three, four, or five hours, depending on the time of year.

Western China, Iceland, and other areas skew time zones westward, in effect observing DST year round without complications from clock shifts. For example, Saskatoon, Saskatchewan is at 106°39′W longitude, slightly west of center of the idealized Mountain Time Zone (105°W), but Saskatchewan observes Central Standard Time (90°W) year-round so Saskatoon is always about 67 minutes ahead of mean solar time.<ref> Time zones & daylight saving time

. Institute for National Measurement Standards 
 
 (2007-05-05)
   

. Retrieved on 2007-05-10. </ref> The United Kingdom and Ireland experimented with year-round DST from 1968 to 1971 but abandoned it because of its unpopularity, particularly in northern regions.<ref> Philip Johnston

  . 
 "
   Is it time to lighten our darkness? 
     
 " , Daily Telegraph , 2007-01-22
 
  . Retrieved on 2007-05-16
 . </ref>

Western France, Spain, and other areas skew time zones and shift clocks, in effect observing DST in winter with an extra hour in summer. For example, Nome, Alaska is at 165°24′W longitude, which is just west of center of the idealized Samoa Time Zone (165°W), but Nome observes Alaska Time (135°W) with DST, so it is slightly more than two hours ahead of the sun in winter and three in summer.<ref> Doug O'Harra

     (2007-04-01)
   
.    Alaska: daylight stealing time 
. Far North Science

. Retrieved on 2007-12-14. </ref>

DST is generally not observed near the equator, where sunrise times do not vary enough to justify it. Some countries observe it only in some regions; for example, southern Brazil observes it while equatorial Brazil does not.<ref> Decretos sobre o Horário de Verão no Brasil

 (Portuguese) 
. Time Service Dept., National Observatory, Brazil 
 
 (2007)
   

. Retrieved on 2007-11-06. </ref> Only a minority of the world's population uses DST because Asia and Africa generally do not observe it.

Terminology

In the normative form, daylight saving time uses the present participle saving as an adjective, as in labor saving device; the first two words are sometimes hyphenated. Daylight savings time and daylight time are common variants.<ref>Modèle:Cite journal | Modèle:Cite book Retrieved on 2007-05-26. | daylight saving time

. Merriam-Webster's Online Dictionary

 

. Retrieved on 2007-03-08.

 “called also daylight saving, daylight savings, daylight savings time, daylight time”
|    15 U.S.C. § 260a notes 

 

. Retrieved on 2007-05-09.

 “Congressional Findings; Expansion of Daylight Saving Time”

</ref>

Time zone names typically change when DST is observed. American English replaces standard with daylight: for example, Pacific Standard Time (PST) becomes Pacific Daylight Time (PDT). British English uses summer: for example, Greenwich Mean Time (GMT) becomes British Summer Time (BST). Abbreviations do not always change: for example, many (though not all) Australians say that Eastern Standard Time (EST) becomes Eastern Summer Time (also EST).

The American English mnemonic "spring forward, fall back" (also "spring ahead …", "spring up …", and "… fall behind") helps people remember which direction to shift clocks. Much of North America now advances clocks before the vernal equinox, so the mnemonic disagrees with the astronomical definition of spring, but a proposed substitute "March forward …"<ref> Time talk

. Language Realm
 (2007)
   

. Retrieved on 2007-10-24. </ref> works only in the northern hemisphere, and is less robust against future rule changes.

Computing

Many computer-based systems can shift their clocks automatically when DST starts and finishes, based on their time zone settings. Two implementations in wide use today are zoneinfo and Microsoft Windows.

Zoneinfo

The zoneinfo 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 Oracle;<ref name='zoneinfo'> Paul Eggert

 ; Arthur David Olson 

     (2007-11-29)
   
.    Sources for time zone and daylight saving time data 

. Retrieved on 2007-12-14. </ref> HP's "tztab" database is similar but incompatible.<ref> tztab(4)

. HP-UX Reference: HP-UX 11i Version 3
. Hewlett-Packard Co. 
 
 (2007)
   

. Retrieved on 2007-05-16. </ref> When temporal authorities change DST rules, zoneinfo updates are installed as part of ordinary system maintenance. In Unix-like systems a process's TZ environment variable specifies the location name, as in TZ='America/New_York'.

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 eastern North America starting in 2007. TZ must be changed whenever DST rules change, and the new TZ value applies to all years, mishandling some older time stamps.<ref> Other environment variables

. IEEE Std 1003.1-2004
. The Open Group 
 
 (2004)
   

. Retrieved on 2007-05-16. </ref>

Microsoft Windows

As with zoneinfo, a user of Microsoft Windows configures DST by specifying the name of a location, and the operating system then consults a table of rule sets that must be updated when DST rules change. Procedures for specifying the name and updating the table vary with release. Updates are not issued for older versions of Microsoft Windows.<ref> Daylight saving time help and support center

. Microsoft Corp. 
 
 (2007-09-14)
   

. Retrieved on 2007-10-24. </ref> Windows Vista supports at most two start and end rules per time zone setting. In a Canadian location observing DST, a single Vista setting supports both 1987–2006 and post-2006 time stamps, but mishandles some older time stamps. Older Microsoft Windows systems usually store only a single start and end rule for each zone, so that the same Canadian setting reliably supports only post-2006 time stamps.<ref> Visual Studio and daylight saving time change

. Microsoft Corp. 
 
 (2007)
   

. Retrieved on 2007-03-22. </ref>

These limitations have caused problems. For example, before 2005, DST in Israel varied each year and was skipped some years. Windows 95 used rules correct for 1995 only, causing problems in later years. In Windows 98 Microsoft gave up and marked Israel as not having DST, forcing Israeli users to shift their computer clocks manually twice a year. The 2005 Israeli Daylight Saving Law established predictable rules but Windows zone files cannot represent the rules' dates in a year-independent way. Partial workarounds, which mishandle older time stamps, include manually switching zone files every year<ref> Windows daylight savings timezones for Israel

. Lingnu Open Source Consulting 
 
 (2005-04-20)
   

. Retrieved on 2007-03-22. </ref> and a Microsoft tool that switches zones automatically.<ref> Microsoft support entry regarding daylight saving time in Israel

 (Hebrew) 
. Microsoft Corp. 
 
 (2007)
   

. Retrieved on 2007-03-22. </ref>

References

Further reading

• Modèle:Cite book
• Modèle:Cite book The British version, focusing on the UK, is Modèle:Cite book

External links

• Daylight Saving Time - general history and anecdotes
• Saving Time, Saving Energy - North American viewpoint
• Summer Time - European viewpoint
• Sources for time zone and daylight saving time data - technical resources

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