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Though DST is common in Europe and North America, most of the world's population does not use it.
  DST used
  DST no longer used
  DST never used

Daylight saving time (DST), also called 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; the ancients lengthened summer daylight by the individual hours instead. Presaged by a 1784 satire by Benjamin Franklin, modern DST was first proposed in 1907 by Briton William Willett, and in 1916 saw its first widespread use as a wartime measure aimed at conserving coal. Despite controversy, many countries have used it since then; details vary by location and change occasionally.

Adding daylight to afternoons generally benefits retailing, sports, and other activities that exploit sunlight after working hours,[1] but it can cause problems for farmers and others whose hours depend on the sun.[2] Extra afternoon daylight appears to cut traffic fatalities;[3] its effect on health and crime is less clear. DST is said to save electricity by reducing the need for artificial evening lighting,[4] but the evidence for this is weak,[5] and DST can boomerang by boosting peak demand, increasing overall electricity costs.[6]

DST's clock shifts complicate timekeeping and can disrupt meetings, travel, billing, recordkeeping, medical devices, and heavy equipment.[7] Many computer-based systems can adjust their clocks automatically, but this can be limited and error-prone, particularly when DST rules change.[8]

Origin

In this ancient water clock, a series of gears rotated a cylinder to display hour lengths appropriate for each day's date.

Though not punctual in the modern sense, the ancients adjusted daily schedules to the sun more flexibly than modern DST does, often dividing daylight into twelve hours regardless of day length, so that daylight hours were longer during summer.[9] 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 in modern terms 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.[10] After ancient times equal-length hours eventually supplanted unequal, so civil time no longer varied by season.

Benjamin Franklin suggested firing cannons at sunrise to waken Parisians.

While an American envoy to France, Benjamin Franklin anonymously published a letter in 1784 suggesting that Parisians economize on candles by arising earlier to use morning sunlight.[11] Franklin's mild satire proposed taxing shutters, rationing candles, and waking the public by ringing church bells and firing cannons at sunrise, in the spirit of his earlier proverb "Early to bed and early to rise / Makes a man healthy, wealthy, and wise."[12] Franklin did not propose shifting clocks; 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.[13]

William Willett invented DST and advocated it tirelessly.

In 1905, the English builder and outdoorsman William Willett invented DST during a pre-breakfast horseback ride where he was dismayed by how many Londoners slept through the best part of a summer day.[14] An avid golfer, he disliked cutting short his round at dusk. Two years later he published his proposal,[15] but his idea was not acted on immediately. Germany, its allies, and their occupied zones were the first European countries to use DST, starting 30 April 1916. Most belligerents and many European neutrals soon followed suit, but 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.[16]

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.[15]

However, many people ignore DST by altering their nominal work schedules to coordinate with daylight, TV broadcasts, or remote colleagues.[17] 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.[5]

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, energy is conserved if the evening reduction outweighs the morning increase, which can happen if more people need evening light than morning. However, statistically significant evidence for any such effect 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,[4] but the National Bureau of Standards (NBS) reviewed the DOT study in 1976 and found no significant energy savings.[5] 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.[6] In North America, there is no clear evidence that electricity will be saved by the extra DST introduced in 2007[18] and though one utility did report a decrease in March 2007 five others did not.[19] DST may increase gasoline consumption: U.S. gasoline demand grew an extra 1% during the newly introduced DST in March 2007.[20]

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.[21] Conversely, DST can adversely affect farmers and others whose hours are set by the sun. 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.[2] DST also hurts prime-time broadcast ratings[22] and theaters, especially drive-ins.[23]

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.[24] 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.[25]

Public safety

Clock shifts affect apparent sunrise and sunset times in Greenwich in 2007.[26]

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%,[4] but the 1976 NBS review of the DOT study found no differences in traffic fatalities.[5] In 1995 the Insurance Institute for Highway Safety estimated a reduction of 1.2%, including a 5% reduction in crashes fatal to pedestrians.[3] Others have found similar reductions.[27] SDST has been projected to reduce traffic fatalities by 3% to 4% in the UK, compared to ordinary DST.[28] It is not clear whether sleep disruption contributes to fatal accidents immediately after the spring and autumn clock shifts. A correlation between clock shifts and accidents has been observed in the U.S. but not in Sweden. If this twice-yearly effect exists, it is far smaller than the overall reduction in fatalities.[29][30]

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."[31] Although outdoor lighting makes potential crime victims feel safer, it may actually encourage crime.[32]

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.[33][34][35] This is not an essential function of DST, as locations without DST can instead use the first days of spring and autumn as reminders.[36]

Health

The William Willett Memorial Sundial is always on DST.

DST has mixed effects on health. In societies with fixed work schedules it provides more afternoon sunlight for outdoor exercise, which can contribute greatly to health. 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 arise earlier[37] but some argue the reverse.[38] The Retinitis Pigmentosa Foundation Fighting Blindness, chaired by blind sports magnate Gordon Gund, successfully lobbied in 1985 and 2005 for DST extensions,[1][39] but DST can hurt night blindness sufferers.[40]

Clock shifts reduce sleep duration and efficiency,[41] and the government of Kazakhstan cited health complications due to clock shifts as a primary reason for abolishing DST in 2005.[42]

Complexity

DST's clock shifts have the obvious disadvantage of complexity. People must remember to change their clocks. 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.[43] 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.[44]

Computer-based systems may also require downtime or restarting when clocks shift. Ignoring this requirement damaged a German steel facility in 1993.[7] Medical devices may generate adverse events that could harm patients and not be obvious to clinicians responsible for care.[45] 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.[8]

Some clock-shift problems could be avoided by adjusting clocks continuously[46] 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.[47]

Politics

Retailers generally favor DST. United Cigar Stores hailed a 1918 DST bill.

Daylight saving has caused controversy since it began. Proponents argue that it helps to "enlarge the opportunities for the pursuit of health and happiness among the millions of people who live in this country."[48] Critics "detect 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."[49] 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.[50] The U.S. was even more skeptical: Andrew Peters (R-MA) introduced a DST bill in May 1909 but it soon died in committee.[51]

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 21 May 1916.[52] 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.[53]

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.[54] The U.S. was more typical: Congress repealed DST after 1919. Woodrow Wilson, like Willett an avid golfer, vetoed the repeal twice but his second veto was overridden,[55] and only a few U.S. cities retained DST locally thereafter.[56]

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, but many did not, and critics gave the scheduling mess names like "Ragtime" and "Daylight Slaving Time"; the experiment was not repeated.[57] More-recent economic theory suggests that 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 schedule.[58]

Since Willett's day the world has seen many enactments, adjustments, and repeals of DST, with similar politics involved.[59] In the UK the sport and leisure industry supports a proposal to observe Single/Double Summer Time (SDST), a variant where clocks are one hour ahead of the sun in winter and two in summer.[60] In the U.S. the Sporting Goods Manufacturers Association and the National Association of Convenience Stores successfully lobbied for the 2007 extension to DST;[39] 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, and both Idaho senators voted for it on the basis of fast-food restaurants selling more French fries made from Idaho potatoes.[1] In early 2007, Western Australia continued to debate a trial use of DST and several politicians changed positions after public sentiment swung against it.[61]

Observance practices

Main article: Daylight saving time around the world
Clocks advance when DST starts.

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.[62] 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.[54] Most of North America shifts at 02:00 local time, so adjacent zones do not shift simultaneously; 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 October 1, Western Australia on December 3, and the remaining DST-observing areas on October 29.[63]

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.[54] Starting in 2007, most of the United States and Canada observe DST from the second Sunday in March to the first Sunday in November.[64] 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.[65]

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.[66] The time difference between the United Kingdom and mainland Chile may therefore be three, four, or five hours, depending on the time of year.

Time zones often lie west of their idealized boundaries, resulting in year-round DST.

Argentina, 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.[67] 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.[68]

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.[69]

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.[70] 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.[71]

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, Central European Time (CET) becomes Central European Summer Time (CEST). 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 advances clocks before the vernal equinox, so the mnemonic works with the dictionary definition of spring but not an astronomical one, but a proposed substitute "March forward…"[72] works only in the northern hemisphere, and is less robust against future rule changes.

Computing

A 2001 public service announcement reminded people to adjust clocks manually.

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;[73] HP's "tztab" database is similar but incompatible.[74] 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.[75]

Microsoft Windows

The procedure for adjusting and patching the DST configuration of Microsoft Windows varies with release.[76] 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.[77]

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, causing 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 include manually switching zone files every year[78] and a Microsoft tool that switches zones automatically.[79]

Notes

Franklin's letter about daylight is often miscited. It never mentioned DST and its first publication had neither title nor byline.[11]
William Willett's pamphlet promoting DST went through nineteen editions.[15]
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  21. ^ Downing. Spring Forward. pp. 147–148.
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  23. ^ Todd D. Rakoff (2002). A Time for Every Purpose: Law and the Balance of Life. Harvard University Press. p. 26. ISBN 067400910X.
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  31. ^ House Committee on Energy and Commerce, Subcommittee on Energy Conservation and Power (1985-04-24). Daylight saving time. Serial No. 99-4. U.S. GPO. p. 26. {{cite book}}: Check date values in: |date= (help)
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References

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