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{{About|DST in general|your location's rules|Daylight saving time around the world}}
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Although not used by most of the world's people, daylight saving time is common in high ]s.
{{legend|#1a80e6|DST used}}
{{legend|#ef690a|DST no longer used}}
{{legend|#c00000|DST never used}}
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'''Daylight saving time''' ('''DST'''; also, '''summer time''' in ]; see '']'') 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 the English builder ]. Many countries have used it since then; ] and change occasionally.

The practice is 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/> Traffic fatalities are reduced when there is extra afternoon daylight;<ref name=Ferguson/> its effect on health and crime is less clear. Although an early goal of DST was to reduce evening usage of ], formerly a primary use of electricity,<ref name=Bartlett/> modern heating and cooling usage patterns greatly differ and research about how DST currently affects energy use is limited and contradictory.<ref name=Aries/>

DST's occasional clock shifts present other challenges. 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==
[[Image:Clepsydra-Diagram-Fancy.jpeg|thumb|upright|
In this ancient ], a series of gears rotated a cylinder to display hour lengths appropriate for each day's date.]]
Although not punctual in the modern sense, ancient civilizations adjusted daily schedules to the sun more flexibly than modern DST does, often dividing daylight into twelve equal hours regardless of day length, so that each daylight hour was longer during summer.<ref>{{cite journal |title= Daylight saving in ancient Rome |author= ] |journal= The Classical Journal |volume=13 |issue=6 |pages=450–451 |year=1918}}</ref> For example, Roman ]s had different scales for different months of the year: at Rome's ] the third hour from sunrise, '']'', started by modern standards at 09:02 ] and lasted 44 minutes at the winter ], but at the summer solstice it started at 06:58 and lasted 75 minutes.<ref>{{cite book |title= Daily Life in Ancient Rome: The People and the City at the Height of the Empire |author= ] |publisher= Yale University Press |chapter= The days and hours of the Roman calendar |isbn=0-300-00031-6 |year=1968}}</ref> After ancient times, equal-length civil hours eventually supplanted unequal, so ] no longer varies by season. Unequal hours are still used in a few traditional settings, such as some ] monasteries.<ref>{{cite journal |journal= Atlantic |year=2003 |volume=292 |issue=5 |pages=138–141 |title= The holy mountain |author= ]}}</ref>

] suggested firing cannons at sunrise to waken Parisians.]]
During his time as an American envoy to France, ], author of the proverb, "Early to bed, and early to rise, makes a man healthy, wealthy and wise", anonymously published a letter suggesting that ]ians economize on ]s by rising earlier to use morning sunlight.<ref>{{cite journal |author= Seymour Stanton Block |title= Benjamin Franklin: America's inventor |journal= American History |year=2006 |accessdate=2007-02-16 |url=http://www.historynet.com/magazines/american_history/3036996.html}}</ref> This 1784 ] proposed taxing ], rationing candles, and waking the public by ringing church bells and firing cannons at sunrise.<ref name=Franklin>{{cite journal |author= ], writing ] |title= Aux auteurs du Journal |journal= Journal de Paris |date=1784-04-26 |issue=117 |language=French}} Its first publication was in the journal's "Économie" section. The (retrieved on ]) is commonly called "An Economical Project", a title that is not Franklin's; see {{cite journal |author= A.O. Aldridge |title= Franklin's essay on daylight saving |journal= American Literature |volume=28 |issue=1 |pages=23–29 |year=1956 |doi=10.2307/2922719}}</ref> Franklin did not propose DST; like ancient Rome, 18th-century Europe did not keep accurate schedules. However, this soon changed as ] and communication networks came to require a standardization of time unknown in Franklin's day.<ref>{{cite journal |author= ] |title= The standardization of time: a sociohistorical perspective |journal= The American Journal of Sociology |volume=88 |issue=1 |year=1982 |pages=1–23 |doi= 10.1086/227631}}</ref>

The prominent English builder and outdoorsman ] conceived DST in 1905 during a pre-breakfast ride, when he observed with dismay how many ]ers slept through a large part of a summer day.<ref>{{cite web |title= 100 years of British Summer Time |url=http://www.nmm.ac.uk/explore/astronomy-and-time/time-facts/british-summer-time/ |year=2008 |accessdate=2008-10-24 |publisher= National Maritime Museum}}</ref> An avid ]er, 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>Willett pamphlet:
*{{cite paper |author= ] |title= The waste of daylight |date=1907 |accessdate=2007-05-16 |version= 1st edition |url=http://webexhibits.org/daylightsaving/willett.html}}
*{{cite paper |author= William Willett |title= The waste of daylight |date=1914 |accessdate=2007-05-16 |version= 19th edition |url=http://www.pettswoodvillage.co.uk/Daylight_Savings_William_Willett.pdf |format=PDF}}</ref> He lobbied unsuccessfully for the proposal until his death in 1915; see ] for more details.

], its ] allies, and their occupied zones were the first European nations to use Willett's invention, starting April 30, 1916. Britain, most of its allies, and many European neutrals soon followed suit. Russia and a few other countries waited until the next year; and the United States adopted it in 1918. Since then, the world has seen many enactments, adjustments, and repeals.<ref>{{cite book |author=Prerau |title= Seize the Daylight |pages=51–89}}</ref>

==Benefits and drawbacks==
] conceived DST and advocated it tirelessly.<ref>{{cite book |author=Prerau |title= Seize the Daylight |pages=22}}</ref>]]
Willett's 1907 proposal argued that DST increases opportunities for outdoor ] activities during afternoon sunlight hours. The longer days nearer the summer solstice in high latitudes 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>{{cite paper |author= Mark Gurevitz |url=http://opencrs.cdt.org/document/RS22284/ |accessdate=2007-06-01 |title= Daylight saving time |publisher= Congressional Research Service |version= Order Code RS22284 |date=2006-04-04}}</ref>

General agreement about the day's layout confers so many advantages that a standard DST schedule usually outranks ] efforts to get up earlier, even for people who personally dislike the DST schedule.<ref>{{cite book |author= ] |title= Micromotives and Macrobehavior |chapter= Hockey helmets, daylight saving, and other binary choices |chapterurl=http://www.simson.net/ref/1972/Schelling_Hockey_Helmets.pdf |publisher= W. W. Norton |year=2006 |origyear=1972 |isbn=0-393-32946-1 |accessdate=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 television broadcasts or daylight.<ref>{{cite journal |journal= Journal of Labor Economics |year=2008 |volume=26 |issue=2 |pages=223–246 |author= Daniel S. Hamermesh; Caitlin Knowles Myers; Mark L. Pocock |title= Cues for timing and coordination: latitude, Letterman, and longitude |doi=10.1086/525027 |url=http://www.journals.uchicago.edu/doi/full/10.1086/525027}}</ref>

===Energy use===
Delaying the nominal time of sunset and sunrise reduces the use of artificial light in the evening and increases it in the morning. 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>{{cite book |author= ] |chapter= Statement to the U.S. House, Committee on Science, Subcommittee on Energy |title= Energy Conservation Potential of Extended and Double Daylight Saving Time |date=2001-05-24 |series= Serial 107-30 |url=http://commdocs.house.gov/committees/science/hsy73325.000/hsy73325_0.HTM |accessdate=2007-12-14}}</ref> Although energy conservation remains an important goal,<ref name=Ahuja>{{cite journal |title= Energy savings from advancing the Indian Standard Time by half an hour |author= Dilip R. Ahuja; D. P. Sen Gupta; V. K. Agrawal |journal= Current Science |volume=93 |issue=3 |pages=298–302 |year=2007 |url=http://www.ias.ac.in/currsci/aug102007/298.pdf |format=PDF}}</ref> energy usage patterns have greatly changed since then, and recent research is limited and reports contradictory results.<ref name=Aries/> Electricity use is greatly affected by geography, climate, and economics, making it hard to generalize from single studies.<ref name=Aries>{{cite journal |journal= Energy Policy |year=2008 |volume=36 |issue=6 |pages=1858–1866 |title= Effect of daylight saving time on lighting energy use: a literature review |author= Myriam B.C. Aries; Guy R. Newsham |doi=10.1016/j.enpol.2007.05.021 |url=http://irc.nrc-cnrc.gc.ca/pubs/fulltext/nrcc49212/nrcc49212.pdf |format=PDF |accessdate=2008-08-07}}</ref>
* The ] (DOT) concluded in 1975 that DST might reduce the country's electricity usage by 1% during March and April,<ref name=Aries/> but the ] (NBS) reviewed the DOT study in 1976 and found no significant savings.<ref name=CRS/>
* In 2000 when parts of ] began DST in late winter, overall electricity consumption did not decrease, but the morning peak load and prices increased.<ref>{{cite journal |author= Ryan Kellogg; Hendrik Wolff |title= Daylight time and energy: evidence from an Australian experiment |journal= Journal of Environmental Economics and Management |year=2008 |doi=10.1016/j.jeem.2008.02.003}} An earlier version is in: {{cite paper |author= Ryan Kellogg; Hendrik Wolff |title= Does extending daylight saving time save energy? Evidence from an Australian experiment |version=CSEMWP 163 |publisher= Center for the Study of Energy Markets |date=2007 |url=http://repositories.cdlib.org/ucei/csem/CSEMWP-163 |accessdate=2007-05-16}}</ref>
* In ] during summer 2006–07, DST increased electricity consumption during hotter days and decreased it during cooler days, with consumption rising 0.6% overall.<ref>{{cite press release |url=http://westernpower.com.au/subContent/aboutUs/mediaCentre/mediaReleases/2007/The_facts_on_electricity_consumption_and_Daylight_Saving.html |title= The facts on electricity consumption and daylight saving |publisher= Western Power |date=2007-10-31 |accessdate=2008-03-17}}</ref>
* Although a 2007 study estimated that introducing DST to Japan would reduce household lighting energy consumption,<ref>{{cite journal |title= Energy savings potential of the Summer Time concept in different regions of Japan from the perspective of household lighting |author= Wee-Kean Fong; Hiroshi Matsumoto; Yu-Fat Lun; Ryushi Kimura |journal= Journal of Asian Architecture and Building Engineering |volume=6 |issue=2 |pages=371–378 |year=2007 |doi=10.3130/jaabe.6.371}}</ref> a 2007 simulation estimated that DST would increase overall energy use in ] 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>{{cite journal |journal=Energy |year=2007 |volume=32 |issue=9 |pages=1617–1633 |title= Evaluation of city-scale impact of residential energy conservation measures using the detailed end-use simulation model |author= Yoshiyuki Shimoda |coauthors= Takahiro Asahia; Ayako Taniguchia; Minoru Mizuno |doi=10.1016/j.energy.2007.01.007}}</ref> DST's effect on lighting energy use is noticeable mainly in residences.<ref name=Aries/>
* A 2007 study found that the earlier start to DST that year had little or no effect on electricity consumption in California.<ref>{{cite paper |author= Adrienne Kandel; Margaret Sheridan |title= The effect of early daylight saving time on California electricity consumption: a statistical analysis |publisher= California Energy Commission |date=2007-05-25 |version=CEC-200-2007-004 |url=http://energy.ca.gov/2007publications/CEC-200-2007-004/CEC-200-2007-004.PDF |format=PDF |accessdate=2008-03-08}}</ref>
* A 2007 study estimated that winter daylight saving would prevent a 2% increase in average daily electricity consumption in ].<ref>{{cite paper |author= Brendan Cronin; Elizabeth Garnsey |title= Daylight saving in GB; is there evidence in favour of clock time on GMT? |publisher= Dept. of Engineering, University of Cambridge |url=http://www.eng.cam.ac.uk/news/stories/2007/BSTvGMT/garnsey_gmt_policy_191007.pdf |format=PDF |date=2007-10-19 |accessdate=2008-01-14}}</ref>
* A 2008 study examined billing data in Indiana before and after it adopted DST in 2006, and concluded that DST increased residential electricity consumption by 1% to 4%, primarily due to extra afternoon cooling.<ref>{{cite paper |author= Matthew J. Kotchen; Laura E. Grant |title= Does daylight saving time save energy? evidence from a natural experiment in Indiana |date=2008 |publisher= ] |url=http://www.nber.org/papers/w14429.pdf |format=PDF |version= NBER Working Paper No. 14429 |accessdate=2008-11-01}}</ref>

Several studies have suggested that DST increases motor fuel consumption.<ref name=Aries/> U.S. gasoline demand grew an extra 1% during the newly introduced DST in March 2007.<ref>{{cite news |author= Shawn McCarthy |title= Has daylight saving time fuelled gasoline consumption? |work= Globe and Mail |date=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.<ref>{{cite news |title= Daylight-saving time becomes daylight-spending time for many businesses |author= Dana Knight |work= Indianapolis Star |date=2006-04-17}}</ref> For example, in 1984 ''Fortune'' magazine estimated that a seven-week extension of DST would yield an additional $30 million for ] stores, and the ] estimated the extension would increase golf industry revenues $200 million to $300 million.<ref>{{cite book |author=Downing |title= Spring Forward |pages=147–148}}</ref> A 1999 study estimated that DST increases the ] of the ]'s leisure sector by about 3%.<ref name=Aries/> Conversely, DST can adversely affect farmers and others whose hours are set by the sun.<ref name=farming>Effect on those whose hours are set by the sun:
*{{cite book |author=Downing |title= Spring Forward |pages=19–33}}
*{{cite book |author=Prerau |title= Seize the Daylight |pages= 103–110, 149–151, 198}}</ref> For example, grain harvesting is best done after ] evaporates, so when field hands arrive and leave earlier in summer their labor is less valuable.<ref>{{cite web |title= Daylight savings time |url=http://www.house.leg.state.mn.us/hinfo/swkly/1995-96/select/time.txt |work= Session Weekly |publisher= Minnesota House Public Information Office |year=1991 |accessdate=2003-03-21 |quote= … the ''Minneapolis Star'', Jan. 28, 1959.… 'Farmers complained that they cannot get into the fields any earlier than under standard time&nbsp;… because the morning sun does not dry the dew "on daylight savings time."&nbsp;'&nbsp;}}</ref> DST also hurts prime-time broadcast ratings<ref name=Kissell>{{cite news |author=Rick Kissell |title= Daylight-saving dock ratings |work=Variety |date=2007-03-20 |url=http://www.variety.com/article/VR1117961488.html |accessdate=2007-05-16}}</ref> and ] and other ]s.<ref>{{cite book |author= Todd D. Rakoff |title= A Time for Every Purpose: Law and the Balance of Life |year=2002 |publisher= Harvard University Press |isbn=0-674-00910-X |pages=26}}</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>{{cite journal |author=Mark J. Kamstra |coauthors= Lisa A. Kramer; Maurice D. Levi |title= Losing sleep at the market: the daylight saving anomaly |journal= The American Economic Review |volume=90 |issue=4 |pages=1005–1011 |year=2000}}</ref> Clock shifts and DST rule changes have a direct economic cost, entailing 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>{{cite news |author= Ben Arnoldy |title= Latest computer glitch: daylight saving time |work= Christian Science Monitor |date=2007-03-07 |url=http://www.csmonitor.com/2007/0307/p02s01-stct.html |accessdate=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>{{cite book |author= Linda L. Lawson |chapter= Statement to the U.S. House, Committee on Science, Subcommittee on Energy |title= Energy Conservation Potential of Extended and Double Daylight Saving Time |date=2001-05-24 |series= Serial 107-30 |url=http://commdocs.house.gov/committees/science/hsy73325.000/hsy73325_0.HTM |accessdate=2007-12-14}}</ref> but the 1976 NBS review of the DOT study found no differences in traffic fatalities.<ref name=CRS/> In 1995 the ] estimated a reduction of 1.2%, including a 5% reduction in crashes fatal to pedestrians.<ref name=Ferguson>{{cite journal |author= Susan A. Ferguson |coauthors= David F. Preusser; Adrian K. Lund; Paul L. Zador; Robert G. Ulmer |title= Daylight saving time and motor vehicle crashes: the reduction in pedestrian and vehicle occupant fatalities |journal= American Journal of Public Health |volume=85 |issue=1 |pages=92–95 |year=1995 |pmid=7832269 |pmc=1615292 |url=http://www.ajph.org/cgi/reprint/85/1/92.pdf |format=PDF}}</ref> Others have found similar reductions.<ref>{{cite journal |author=Douglas Coate |coauthors= Sara Markowitz |title= The effects of daylight and daylight saving time on US pedestrian fatalities and motor vehicle occupant fatalities |journal= Accident Analysis & Prevention |volume=36 |issue=3 |pages=351–357 |year=2003 |doi=10.1016/S0001-4575(03)00015-0 |pmid=15003579}}</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>{{cite paper |author= J. Broughton; M. Stone |title= A new assessment of the likely effects on road accidents of adopting SDST |publisher= Transport Research Laboratory |url=http://www.trl.co.uk/store/report_detail.asp?srid=2529 |accessdate=2007-05-16 |date=1998 |version=TRL368}}</ref> It is not clear whether ] disruption contributes to fatal accidents immediately after the spring clock shifts.<ref>{{cite journal |journal= New England Journal of Medicine |year=1998 |volume=339 |issue=16 |pages=1167–8 |title= Effects of daylight savings time on collision rates |author= Alex Vincent |coauthors= Stanley Coren |pmid=9776656 |doi= 10.1056/NEJM199810153391617}} Data supporting Coren's half of this exchange are in: {{cite conference |author= Stanley Coren |booktitle= INABIS '98 |title= Sleep deficit, fatal accidents, and the spring shift to daylight savings time |url=http://www.mcmaster.ca/inabis98/occupational/coren0164/ |year=1998}}</ref> A correlation between clock shifts and accidents has been observed in North America but not in ] or ]. If this effect exists, it is far smaller than the overall reduction in fatalities.<ref>Clock shifts and accidents:
*{{cite journal |author= Jason Varughese |coauthors= Richard P. Allen |title= Fatal accidents following changes in daylight savings time: the American experience |journal= Sleep Medicine |volume=2 |issue=1 |pages=31–36 |year=2001 |doi=10.1016/S1389-9457(00)00032-0 |pmid=11152980}}
*{{cite journal |journal= BMC Public Health |year=2008 |volume=8 |pages=74 |title= Daylight saving time transitions and hospital treatments due to accidents or manic episodes |author= Tuuli A. Lahti; Jari Haukka; Jouko Lönnqvist; Timo Partonen |doi=10.1186/1471-2458-8-74 |pmid=18302734 |url=http://www.biomedcentral.com/1471-2458/8/74}}
*{{cite journal |author= Mats Lambe |coauthors= Peter Cummings |title= The shift to and from daylight savings time and motor vehicle crashes |journal= Accident Analysis & Prevention |volume=32 |issue=4 |year=2000 |pages=609–611 |doi=10.1016/S0001-4575(99)00088-3 |pmid=10868764}}</ref>

In the 1970s the U.S. ] (LEAA) found a reduction of 10% to 13% in ]'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>{{cite book |author= House Committee on Energy and Commerce, Subcommittee on Energy Conservation and Power |title= Daylight saving time |date=1985-04-24 |series=Serial No. 99-4 |publisher= ] |pages=26}}</ref> Outdoor lighting has a marginal and sometimes even contradictory influence on crime and fear of crime.<ref>{{cite journal |journal= Environment and Planning A |volume=38 |issue=11 |pages=2055–2074 |author= Rachel Pain |coauthors= Robert MacFarlane; Keith Turner; Sally Gill |title= 'When, where, if, and but': qualifying GIS and the effect of streetlighting on crime and fear |doi=10.1068/a38391 |year=2006}}</ref>

In several countries, ] officials encourage citizens to use the two annual clock shifts as reminders to replace batteries in ] and ]s, particularly in autumn, just before the ] 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 ]s.<ref name=Popkin>{{cite news |title= Changes to daylight-saving time |author= Helen A.S. Popkin |work= Consumer Reports Home & Yard Blog |date=2007-03-02 |url=http://blogs.consumerreports.org/home/2007/03/changes_to_dayl.html |accessdate=2007-11-02}}</ref><ref>{{cite press release |title= A timely day to check your smoke alarm |publisher= New Zealand Fire Service |url=http://www.fire.org.nz/news/media/2002/2002-03-13-1.htm |accessdate=2007-05-15 |date=2002-03-13}}</ref><ref>{{cite press release |url=http://www.cpsc.gov/cpscpub/prerel/prhtml07/07124.html |accessdate=2007-05-16 |title= CSPC and USFA encourage consumers to spring forward with fire safety in mind |date=2007-03-09 |publisher= U.S. Consumer Product Safety Commission; U.S. Fire Administration}}</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>{{cite web |title= Prevent household fires |url=http://health.yahoo.com/topic/children/resources/article/healthwise/ue5167 |accessdate=2007-05-16 |date=2005-04-08 |author= Amy Fackler |work= Yahoo Health}}</ref>

===Health===
<!-- This is a wide-aspect figure with fine detail, so give it an explicit size. Don't make it any smaller than 300px, since 300px might be the user's default. -->
] in 2007.<ref>{{cite web |url=http://daylightchart.sourceforge.net/ |accessdate=2008-03-08 |year=2008 |title= Daylight Chart |author= Sualeh Fatehi |publisher= SourceForge.net}}</ref>]]
DST has mixed effects on health. In societies with fixed work schedules it provides more afternoon sunlight for outdoor exercise.<ref>{{cite book |author=Prerau |title= Seize the Daylight |pages=20, 65, 85}}</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 ].<ref>{{cite journal |journal= Journal of Photochemistry and Photobiology B: Biology |year=2008 |volume=91 |issue=1 |pages=35–40 |title= Influence of summer daylight saving time on scattered erythemal solar ultraviolet exposures |author= A.V. Parisi; J. Turner; D.J. Turnbull; P. Schouten; N. Downs |doi=10.1016/j.jphotobiol.2008.01.010 |pmid=18321724}}</ref> Sunlight strongly influences ]. DST may help in ] by causing individuals to rise earlier,<ref>{{cite journal |author= Henry Olders |doi=10.1016/S0022-3999(02)00479-8 |pmid=12932507 |title= Average sunrise time predicts depression prevalence |journal= Journal of Psychosomatic Research |volume=55 |issue=2 |pages=99–105 |year=2003}}</ref> but some argue the reverse.<ref>{{cite news |author= Shari Roan |title= Change in daylight saving time may affect moods |date=2007-03-05 |work= Los Angeles Times}}</ref> The ] Foundation Fighting Blindness, chaired by blind sports magnate ], successfully lobbied in 1985 and 2005 for U.S. DST extensions,<ref name=Benfield/><ref name=Beam/> but DST can hurt ] sufferers.<ref>{{cite web |url=http://moderatrix8.blogspot.com/2007/03/daylight-savings.html |date=2007-03-12 |title= Daylight savings |work= Moderatrix Musings |accessdate=2007-03-28|quote=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 disrupt sleep and reduce its efficiency.<ref>{{cite journal |author= Tuuli A. Lahti; Sami Leppämäki; Jouko Lönnqvist; Timo Partonen |title= Transitions into and out of daylight saving time compromise sleep and the rest-activity cycles |journal= BMC Physiol |volume=8 |pages=3 |doi=10.1186/1472-6793-8-3 |pmid=18269740 |url=http://www.biomedcentral.com/1472-6793/8/3 |year=2008}}</ref> Effects on seasonal adaptation of the ] can be severe and last for weeks.<ref>DST and circadian rhythm:
*{{cite journal |journal= Biological Rhythm Research |year=2003 |volume=34 |issue=2 |pages=145–155 |doi=10.1076/brhm.34.2.145.14494 |title= Adjustment of the sleep-wake cycle to small (1–2h) changes in schedule |author= Pablo Valdez; Candelaria Ramírez; Aída García}}
*{{cite journal |journal= Current Biology |year=2007 |volume=17 |issue=22 |pages=1996–2000 |pmid=17964164 |doi=10.1016/j.cub.2007.10.025 |url=http://images.cell.com/images/EdImages/CurrentBiology/Kantermann.pdf |format=PDF |title= The human circadian clock's seasonal adjustment is disrupted by daylight saving time |author= Thomas Kantermann; Myriam Juda; Martha Merrow; Till Roenneberg |accessdate=2007-10-27 |laysummary=http://abc.net.au/science/news/stories/2007/2070223.htm |laysource= ABC Science Online, Australia |laydate=2007-10-25}}
</ref> A 2008 study found that although male ] rates rise in the weeks after the spring transition, the relationship weakened greatly after adjusting for season.<ref>{{cite journal |journal= Sleep and Biological Rhythms |year=2008 |volume=6 |issue=1 |pages=22–25 |title= Small shifts in diurnal rhythms are associated with an increase in suicide: the effect of daylight saving |author= Michael Berk; Seetal Dodd; Karen Hallam; Lesley Berk; John Gleeson; Margaret Henry |doi=10.1111/j.1479-8425.2007.00331.x}}</ref> A 2008 Swedish study found that ] were significantly more common the first three weekdays after the spring transition, and significantly less common the first weekday after the autumn transition.<ref>{{cite journal |journal= New England Journal of Medicine |year=2008 |volume=359 |issue=18 |pages=1966–1968 |title= Shifts to and from daylight saving time and incidence of myocardial infarction |author= Imre Janszky; Rickard Ljung |pmid=18971502 |doi=10.1056/NEJMc0807104 |url=http://content.nejm.org/cgi/content/full/359/18/1966 |laysummary=http://www.latimes.com/features/health/medicine/la-sci-clocks30-2008oct30,0,5570694.story |laysource= Los Angeles Times |laydate=2008-10-30}}</ref> The government of ] cited health complications due to clock shifts as a reason for abolishing DST in 2005.<ref>{{cite web |url=http://www.kazsociety.org.uk/news/2005/03/30.htm |title= Kazakhstan abolishes daylight saving time |date=2005-03-21 |accessdate=2007-03-28 |publisher= Kazakhstan Society in the UK}}</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>{{cite news |url=http://www.recordcourier.com/apps/pbcs.dll/article?AID=/20031024/NEWS/310240104 |work= Record-Courier |date=2003-10-24 |accessdate=2007-10-29 |title= Daylight saving time ends Sunday |author= Joey Crandall}}</ref> As more devices contain clocks, more time is spent changing them.<ref>{{cite news |author= Steve Moyer |title= Daylight-saving time change could affect computers, electronics |work= Nevada Daily Mail |url=http://www.nevadadailymail.com/story/1192507.html |date=2007-03-06 |accessdate=2008-03-02}}</ref> People who work across ] 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>{{cite news |author= Paul McDougall |title= PG&amp;E says patching meters for an early daylight-saving time will cost $38 million |work=InformationWeek |url=http://www.informationweek.com/news/showArticle.jhtml?articleID=197700487 |accessdate=2007-05-16 |date=2007-03-01}}</ref> During 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>{{cite web |url=http://webexhibits.org/daylightsaving/c.html |work=WebExhibits |accessdate=2008-03-09 |title= Daylight saving time: rationale and original idea |note= ] quote. "Supposing some unfortunate lady was confined with twins&nbsp;..."}}</ref>

Some computer-based systems require downtime or restarting when clocks shift; ignoring this requirement damaged a German steel facility in 1993.<ref name=Neumann>{{cite book |title= Computer-Related Risks |author= ] |year=1994 |chapter= Computer date and time problems |chapterurl=http://www.csl.sri.com/users/neumann/cal.html#DatesTimes |isbn=0-201-55805-X |publisher=Addison-Wesley |accessdate=2007-05-26}}</ref> Medical devices may generate adverse events that could harm patients, without being obvious to clinicians responsible for care.<ref>{{cite web |url=http://www.fda.gov/cdrh/safety/030107-dst.html |author=Daniel G. Schultz |title= Unpredictable events in medical equipment due to new daylight saving time change |date=2007-03-29 |accessdate=2007-11-11 |publisher= Center for Devices and Radiological Health, Food and Drug Administration}}</ref> These problems are compounded when the DST rules themselves change, as in the ]. Software developers must test and perhaps modify many programs, and users must install updates and restart applications. Consumers must update devices such as programmable thermostats with the correct DST rules, or manually adjust the devices' clocks.<ref name=Tong>{{cite journal |author=Stephen Tong |coauthors= Joseph Williams |title= Are you prepared for daylight saving time in 2007? |journal= IT Professional |publisher= IEEE Computer Society |volume=9 |issue=1 |year=2007 |pages=36–41 |url=http://www.computer.org/portal/pages/itpro/content/Daylight_Saving_Time_2007.html |accessdate=2007-05-16 |doi= 10.1109/MITP.2007.2}}</ref>

Some clock-shift problems could be avoided by adjusting clocks continuously<ref>{{cite web |url=http://www.squarefree.com/2006/11/01/continuous-daylight-saving-time/ |title= Continuous daylight saving time |work= Indistinguishable from Jesse |author= Jesse Ruderman |date=2006-11-01 |accessdate=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.

] Memorial Sundial is always on DST.]]
DST inherits and can magnify the disadvantages of ]. For example, when reading a ], one must compensate for it along with time zone and natural discrepancies.<ref>{{cite book |title= Sundials: Their Theory and Construction |author= Albert E. Waugh |publisher=Dover |isbn=0-486-22947-5 |year=1973}}</ref> Also, sun-exposure rules like "avoid the sun within two hours of noon" become less accurate when DST is in effect.<ref>{{cite journal |journal= Photochemistry and Photobiology |year=1992 |volume=56 |issue=2 |pages=229–34 |title= Atmospheric sun protection factor on clear days: its observed dependence on solar zenith angle and its relevance to the shadow rule for sun protection |author= Leith Holloway |doi=10.1111/j.1751-1097.1992.tb02151.x |pmid=1502267}}</ref>

==Politics==
Daylight saving has caused controversy since it began.<ref name=Downing-Prerau>See ] for full citations to these books:
*{{cite book |author=Downing |title= Spring Forward}}
*{{cite book |author=Prerau |title= Seize the Daylight}}</ref> ] argued that it enlarges "the opportunities for the pursuit of health and happiness among the millions of people who live in this country".<ref>{{cite journal |author= ] |title= A silent toast to William Willett |url=http://www.winstonchurchill.org/i4a/pages/index.cfm?pageid=266 |accessdate=2007-05-16 |journal= Pictorial Weekly |date=1934-04-28}}</ref> ], 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>{{cite book |author= ] |title= ] |publisher= Clarke, Irwin |year=1947 |chapter= XIX, Sunday |oclc=1246640}}</ref> and wags have dubbed it "Daylight Slaving Time".<ref>{{cite book |author=Prerau |title= Seize the Daylight |pages=117}}</ref> Historically, retailing, sports and tourism interests have favored daylight saving, while agricultural and evening entertainment interests have opposed it, and its initial adoption has been prompted by ] and war.<ref>{{cite book |author=Prerau |title= Seize the Daylight |pages=xi}}</ref>

The fate of Willett's 1907 proposal illustrates several political issues involved. The proposal attracted many supporters, including ], Churchill, ], ], ] (who used ]), the managing director of ], and the manager of the National Bank. However, the opposition was stronger: it included Prime Minister ], ] (the ]), ], ] (director of the ]), many agricultural organizations, and theater owners. After many hearings the proposal was narrowly defeated in a ] committee vote in 1909. Willett's allies introduced similar bills every year from 1911 through 1914, to no avail.<ref>{{cite book|author=Prerau|title=Seize the Daylight|pages=12–24}}</ref> The U.S. was even more skeptical: ] introduced a DST bill to the ] in May 1909, but it soon died in committee.<ref>{{cite book|author=Prerau|title=Seize the Daylight|pages=72–73}}</ref>

] hailed a 1918 DST bill.]]
] 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>{{cite book|author=Prerau|title=Seize the Daylight|pages=51–70}}</ref> U.S. retailing and manufacturing interests led by ] 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>{{cite book|author=Prerau|title=Seize the Daylight|pages=80–101}}</ref>

The 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>{{cite web |author= Joseph Myers |title= History of legal time in Britain |url=http://www.srcf.ucam.org/~jsm28/british-time/ |accessdate=2008-11-01 |date=2008}}</ref> The U.S. was more typical: Congress repealed DST after 1919. President ], like Willett an avid golfer, vetoed the repeal twice but his second veto was overridden,<ref>{{cite book |author=Prerau |title= Seize the Daylight |pages=103–110}}</ref> and only a few U.S. cities retained DST locally thereafter.<ref>{{cite book |author= Robert Garland |title= Ten years of daylight saving from the Pittsburgh standpoint |year=1927 |publisher=Carnegie Library of Pittsburgh |oclc=30022847 |url=http://www.clpgh.org/exhibit/dst.html |accessdate=2008-03-08}}</ref> Wilson's successor ] opposed DST as a "deception". Reasoning that people should instead get up and go to work earlier in the summer, he ordered ] 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>{{cite book |author=Prerau |title= Seize the Daylight |pages=115–118}}</ref>

Since Willett's day the world has seen many enactments, adjustments, and repeals of DST, with similar politics involved.<ref>{{cite book |title= Business and its Environment |chapter= The politics of the extension of daylight saving time |author= David P. Baron |isbn=0-13-187355-5 |publisher= Prentice Hall |year=2005 |edition=5th ed.}}</ref> The ] includes DST during both world wars, but no standardization of peacetime DST until 1966.<ref>{{cite book |author=Prerau |title= Seize the Daylight |pages=147–155, 175–180}}</ref> In the mid-1980s, ] (parent of ]) and ] provided the primary funding for the Daylight Saving Time Coalition behind the 1987 extension to U.S. DST, and both ] ] voted for it on the basis of fast-food restaurants selling more French fries made from Idaho potatoes;<ref name=Benfield>{{cite book |author= James C. Benfield |chapter= Statement to the U.S. House, Committee on Science, Subcommittee on Energy |title= Energy Conservation Potential of Extended and Double Daylight Saving Time |date=2001-05-24 |series= Serial 107-30 |url=http://commdocs.house.gov/committees/science/hsy73325.000/hsy73325_0.HTM |accessdate=2007-12-14}}</ref> in 2005, the ] and the ] successfully lobbied for the 2007 extension to U.S. DST.<ref name=Beam>{{cite news |url=http://www.boston.com/news/globe/living/articles/2005/07/26/dim_witted_proposal_for_daylight_time/ |accessdate=2007-05-16 |title= Dim-witted proposal for daylight time |work= Boston Globe |author= Alex Beam |date=2005-07-26}}</ref> In early 2007, ] continued to debate a trial use of DST and several politicians changed positions after public sentiment swung against it.<ref>{{cite news |author= Joe Spagnolo |work= Sunday Times |date=2007-03-17 |title= High noon for sun-slingers |url=http://www.news.com.au/perthnow/story/0,21598,21398881-5005371,00.html |accessdate=2007-05-16}}</ref> In the UK the sport and leisure industry supports a proposal to observe SDST's additional hour year-round.<ref>{{cite web |title= Single/Double Summer Time policy paper |date=October 2006 |publisher= Royal Society for the Prevention of Accidents |url=http://www.rospa.com/roadsafety/info/summertime_paper2006v2.pdf |format=PDF |accessdate=2007-05-16}}</ref>

==Observance practices==
{{main|Daylight saving time around the world}}
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In a typical case where a one-hour shift occurs at 02:00 local time, in spring the clock jumps forward from 02:00 ] to 03:00 DST and the day has 23&nbsp;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&nbsp;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 ] during standard time is at ] during DST; conversely, a location at ] during standard time is at ] 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 ] uses a half-hour shift.<ref>{{cite web |title= Information for visitors |url=http://www.lordhoweisland.info/important-info.htm |publisher= Lord Howe Island Tourism Association |year=2008 |accessdate=2008-11-01}}</ref> Twenty-minute and two-hour shifts have been used in the past.

Coordination strategies differ when adjacent time zones shift clocks. The ] shifts all at once, at 01:00 ]; for example, ] is always one hour ahead of ].<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, ] can be temporarily either zero or two hours ahead of ]. ]n districts go even further and do not always agree on start and end dates; for example, in 2008 most DST-observing areas shifted clocks forward on October 5 but ] shifted on October 26.<ref>{{cite web |url=http://www.bom.gov.au/climate/averages/tables/dst_times.shtml |title= Implementation dates of daylight saving time within Australia |publisher= Bureau of Meteorology |date=2008 |accessdate=2008-11-01}}</ref>

Start and end dates vary with location and year. Since 1996 ] 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>{{cite news |author= Tom Baldwin |date=2007-03-12 |url=http://www.timesonline.co.uk/tol/news/world/us_and_americas/article1499835.ece |accessdate=2007-05-16 |title= US gets summertime blues as the clocks go forward 3 weeks early |work= The Times}}</ref> The 2007 U.S. change was part of the ]; previously, from 1987 through 2006, the start and end dates were the first Sunday in April and the last Sunday in October, and ] retains the right to go back to the previous dates once an energy-consumption study is done.<ref>{{cite web |title= Energy Policy Act of 2005, Public Law 109-58 § 110 |url=http://frwebgate.access.gpo.gov/cgi-bin/getdoc.cgi?dbname=109_cong_public_laws&docid=f:publ058.109 |date=2005-08-08 |accessdate=2007-05-10}}</ref>

]
Beginning and ending dates are the reverse in the ]. For example, mainland ] observes DST from the second Saturday in October to the second Saturday in March, with transitions at 24:00 local time.<ref>{{cite web |title= Historia de la hora oficial de Chile |accessdate=2007-11-06 |date=2007-05-17 |publisher= Chilean Hydrographic and Oceanographic Service |language=Spanish |url=http://www.horaoficial.cl/horaof.htm}}</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.

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Western ], ], and other areas ], in effect observing DST year-round without complications from clock shifts. For example, ], is at {{nowrap|106° 39′ W}} longitude, slightly west of center of the idealized Mountain Time Zone {{nowrap|(105° W)}}, but the ] is ] {{nowrap|(90° W)}} year-round, so Saskatoon is always about 67 minutes ahead of ].<ref>{{cite web |title= Time zones & daylight saving time |url=http://inms-ienm.nrc-cnrc.gc.ca/time_services/daylight_saving_e.html |date=2007-05-05 |accessdate=2007-05-10 |publisher= Institute for National Measurement Standards}}</ref> Conversely, northeast ] and a few other areas skew time zones eastward, in effect observing negative DST.<ref name=Ahuja/> The United Kingdom and ] experimented with year-round DST from 1968 to 1971 but abandoned it because of its unpopularity, particularly in northern regions.<ref>{{cite news |author= Philip Johnston |date=2007-01-22 |url=http://www.telegraph.co.uk/opinion/main.jhtml?xml=/opinion/2007/01/22/do2202.xml |accessdate=2007-05-16 |title= Is it time to lighten our darkness? |work= Daily Telegraph}}</ref>

Western ], ], and other areas skew time zones and shift clocks, in effect observing DST in winter with an extra hour in summer. For example, ], is at {{nowrap|165° 24′ W}} longitude, which is just west of center of the idealized ] {{nowrap|(165° W)}}, but Nome observes ] {{nowrap|(135° W)}} with DST, so it is slightly more than two hours ahead of the sun in winter and three in summer.<ref>{{cite web |author= Doug O'Harra |title= Alaska: daylight stealing time |work= Far North Science |url=http://www.farnorthscience.com/2007/03/11/news-from-alaska/alaska-daylight-stealing-time/ |date=2007-04-01 |accessdate=2007-12-14}}</ref>

DST is generally not observed near the ], where sunrise times do not vary enough to justify it. Some countries observe it only in some regions; for example, southern ] observes it while equatorial Brazil does not.<ref>{{cite web |title= Decretos sobre o Horário de Verão no Brasil |url=http://pcdsh01.on.br/DecHV.html |accessdate=2007-11-06 |year=2007 |language=Portuguese |publisher= Time Service Dept., National Observatory, Brazil}}</ref> Only a minority of the world's population uses DST because ] and ] generally do not observe it.

==Terminology==
In the normative form, ''daylight saving time'' uses the ] ''saving'' as an adjective, as in ''labor saving device''; the first two words are sometimes hyphenated, as in ''daylight-saving time''. ''Daylight savings time'', ''daylight savings'', and ''daylight time'' are common variants, the ''savings'' by analogy to '']''.<ref>''Daylight saving time'' and its variants:
*{{cite journal|author=Richard A. Meade|title=Language change in this century|journal=English Journal|year=1978|pages=27–30|volume=67|issue=9|doi=10.2307/815124}}
*{{cite book |chapter= daylight-saving time |chapterurl=http://www.bartleby.com/61/65/D0046500.html |editor= Joseph P. Pickett et al. |title= The American Heritage Dictionary of the English Language |edition= 4th ed. (2000) |publisher= Houghton Mifflin |location=Boston |isbn=0-395-82517-2 |quote= variant forms: or ''daylight-savings time'' |accessdate=2007-05-26}}
*{{cite web|title=daylight saving time|url=http://www.m-w.com/dictionary/daylight+saving+time|work=Merriam-Webster's Online Dictionary|accessdate=2007-03-08|quote=called also ''daylight saving'', ''daylight savings'', ''daylight savings time'', ''daylight time''}}
*{{cite web|title=15 U.S.C. § 260a notes|url=http://www.law.cornell.edu/uscode/html/uscode15/usc_sec_15_00000260---a000-notes.html|accessdate=2007-05-09|quote=Congressional Findings; Expansion of Daylight Saving Time}}</ref> Willett's original proposal used the term ''daylight saving'', but by 1911 the term ''summer time'' replaced ''daylight saving time'' in draft legislation in Britain.<ref>{{cite book |author=Prerau |title= Seize the Daylight |pages=22}}</ref>

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

The American English ] "] forward, ] back" (also "spring ahead&nbsp;…", "spring up&nbsp;…", and "… fall behind") helps people remember which direction to shift clocks. Much of North America now advances clocks before the ], so the mnemonic disagrees with the astronomical definition of spring, but a proposed substitute "March forward&nbsp;…"<ref>{{cite news |author= Brent Hopkins |title= Spring ahead? that's so last year. On Sunday, we will all&nbsp;... march forward. Is this a good thing? that all depends |url=http://www.thefreelibrary.com/SPRING+AHEAD%3f+THAT%27S+SO+LAST+YEAR.+ON+SUNDAY%2c+WE+WILL+ALL+...+MARCH...-a0160458551 |work= Los Angeles Daily News |accessdate=2008-11-01 |date=2007-03-10}}</ref> works only in the ], 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 ]. Two implementations in wide use today are zoneinfo and Microsoft Windows. Some applications standardize on UTC to avoid problems with clock shifts and time zone differences.<ref>{{cite conference |author= A. Gut, L. Miclea, Sz. Enyedi, M. Abrudean, I. Hoka |title= Database globalization in enterprise applications |booktitle= 2006 IEEE International Conference on Automation, Quality and Testing, Robotics |year=2006 |pages=356–359}}</ref>

===Zoneinfo===
The ] 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 ] operating systems, ], and ];<ref>{{cite web |author= Paul Eggert |coauthors= Arthur David Olson |url=http://www.twinsun.com/tz/tz-link.htm |title= Sources for time zone and daylight saving time data |date=2007-12-26 |accessdate=2007-12-31}}</ref> ]'s "tztab" database is similar but incompatible.<ref>{{cite web |publisher= Hewlett-Packard Co. |work= HP-UX Reference: HP-UX 11i Version 3 |url=http://docs.hp.com/en/B2355-60130/tztab.4.html |accessdate=2007-05-16 |title=tztab(4) |year=2007}}</ref> When temporal authorities change DST rules, zoneinfo updates are installed as part of ordinary system maintenance. In Unix-like systems the TZ ] specifies the ], as in <code>TZ='America/New_York'</code>.

Older or stripped-down systems may support only the TZ values required by ], which specify at most one start and end rule explicitly in the value. For example, <code>TZ='EST5EDT,M3.2.0/02:00,M11.1.0/02:00'</code> 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>{{cite web|url=http://www.unix.org/version3/|accessdate=2007-05-16|publisher=The Open Group|work=IEEE Std 1003.1-2004|year=2004|url=http://www.opengroup.org/susv3/basedefs/xbd_chap08.html#tag_08_03|title=Other environment variables}}</ref>

===Microsoft Windows===
As with zoneinfo, a user of ] 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>{{cite web |url=http://support.microsoft.com/gp/cp_dst |title= Daylight saving time help and support center |publisher= Microsoft Corp. |date=2007-09-14 |accessdate=2007-10-24}}</ref> ] 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>{{cite web |url=http://msdn2.microsoft.com/vstudio/bb264729 |title= Visual Studio and daylight saving time change |publisher= Microsoft Corp. |year=2007 |accessdate=2007-03-22}}</ref>

These limitations have caused problems. For example, before 2005, DST in ] varied each year and was skipped some years. ] used rules correct for 1995 only, causing problems in later years. In ] Microsoft gave up and marked Israel as not having DST, forcing Israeli users to shift their computer clocks manually twice a year. The 2005 ] 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>{{cite web |url=http://lingnu.com/support.html#timezone |title= Windows daylight savings timezones for Israel |publisher= Lingnu Open Source Consulting |date=2005-04-20 |accessdate=2007-03-22}}</ref> and a Microsoft tool that switches zones automatically.<ref>{{cite web |url=http://www.microsoft.com/israel/support/tips/other/DLSTIssues.mspx |title= Microsoft support entry regarding daylight saving time in Israel |publisher= Microsoft Corp. |language=Hebrew |year=2007 |accessdate=2007-03-22}}</ref>

==References==
]'s 1784 letter about daylight had neither title nor ].<ref name=Franklin/>]]
]'s pamphlet promoting DST went through nineteen editions.<ref name=Willett/>]]

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==Further reading==
* {{cite book |author= Michael Downing |title= Spring Forward: The Annual Madness of Daylight Saving Time |publisher= Shoemaker & Hoard |year=2005 |isbn=1-59376-053-1}}
* {{cite book |author= David Prerau |title= Seize the Daylight: The Curious and Contentious Story of Daylight Saving Time |publisher= Thunder’s Mouth Press |year=2005 |isbn=1-56025-655-9}} The British version, focusing on the UK, is {{cite book |title= Saving the Daylight: Why We Put the Clocks Forward|publisher=Granta Books|isbn=1-86207-796-7}}

==External links==
{{Spoken Misplaced Pages|Daylight_saving_time.ogg|2008-05-20}}
* - general history and anecdotes
* - North American viewpoint
* - European viewpoint
* - technical resources

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Revision as of 05:11, 2 November 2008