[[Image:DaylightSaving-World-Subdivisions.png|upright=1.67|thumb|alt=World map. Europe, Russia, most of North America, parts of southern South America and southern Australia, and a few other places use DST. Most of equatorial Africa and a few other places near the equator have never used DST. The rest of the land mass is marked as formerly using DST.|Although not used by most of the world's people, daylight saving time is common in high latitudes.

]]

Daylight saving time (DST; also summer time in British English—see Terminology) is the practice 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 1895 by George Vernon Hudson, a New Zealand entomologist. Many countries have used it since then; details vary by location and change occasionally.

The practice is controversial. Adding daylight to afternoons benefits retailing, sports, and other activities that exploit sunlight after working hours, but causes problems for farming, evening entertainment and other occupations tied to the sun. Traffic fatalities are reduced when there is extra afternoon daylight; its effect on health and crime is less clear. Although an early goal of DST was to reduce evening usage of incandescent lighting, formerly a primary use of electricity, modern heating and cooling usage patterns differ greatly, and research about how DST currently affects energy use is limited and often contradictory.

DST's occasional clock shifts present other challenges. They complicate timekeeping, and can disrupt meetings, travel, billing, recordkeeping, medical devices, heavy equipment, and sleep patterns. Often, software can adjust computer clocks automatically, but this can be limited and error-prone, particularly when DST rules change.

Origin

[[Image:Clepsydra-Diagram-Fancy.jpeg|thumb|upright|alt=A water clock. A small human figurine holds a pointer to a cylinder marked by the hours. The cylinder is connected by gears to a water wheel driven by water that also floats a part that supports the figurine.|In this ancient water clock, 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. 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. 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 and some Jewish ceremonies.

[[Image:Franklin-Benjamin-LOC-head.jpeg|thumb|upright=0.7|left|alt=A seated older Benjamin Franklin from the waist up, with body facing to viewer's right but head turned toward the artist. Franklin's waistcoat is bulging a bit, his expression is inscrutable, and his hair hangs down to his shoulders.|Benjamin Franklin suggested firing cannons at sunrise to waken Parisians.]]

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. This 1784 satire proposed taxing shutters, rationing candles, and waking the public by ringing church bells and firing cannons at sunrise. Franklin did not propose DST; like ancient Rome, 18th-century Europe did not keep precise schedules. However, this soon changed as rail and communication networks came to require a standardization of time unknown in Franklin's day.

[[Image:G.V.-Hudson-Auckland-Islands-Party.jpeg|thumb|upright|left|alt=Fuzzy head-and-shoulders photo of a 40-year-old man in a cloth cap and mustache.|G.V. Hudson invented modern DST, proposing it first in 1895.]]

Modern DST was first proposed by the New Zealand entomologist George Vernon Hudson, whose shift-work job gave him leisure time to collect insects, and made him aware of the value of after-hours daylight. In 1895 he presented a paper to the Wellington Philosophical Society proposing a two-hour daylight-saving shift, and after considerable interest was expressed in Christchurch, New Zealand he followed up in an 1898 paper. Many publications incorrectly credit DST's invention to the prominent English builder and outdoorsman William Willett, who independently conceived DST in 1905 during a pre-breakfast ride, when he observed with dismay how many Londoners slept through a large part of a summer day. 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.

As described in Politics below, Willett lobbied unsuccessfully for the proposal in the UK until his death in 1915, and Germany, its World War I allies, and their occupied zones were the first European nations to use Willett's invention, starting April 30, 1916, as a way to conserve coal during wartime. 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.

How it works

[[Image:Begin CEST Transparent.png|thumb|upright=0.67|alt=Diagram of a clock showing a transition from 2:00 to 3:00.|When DST starts in central Europe, clocks advance from 02:00 CET to 03:00 CEST.]][[Image:End CEST Transparent.png|thumb|upright=0.67|alt=Diagram of a clock showing a transition from 3:00 to 2:00.|When DST ends in central Europe, clocks retreat from 03:00 CEST to 02:00 CET. Other regions switch at different times.]]

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 that day has 23 hours, whereas in autumn the clock jumps backward from 02:00 DST to 01:00 standard time, repeating that hour, and that day has 25 hours. A digital display of local time does not read 02:00 exactly at the shift, 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. 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. 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, in 2008 most DST-observing areas shifted clocks forward on October 5 but Western Australia shifted on October 26.

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. 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. 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 now that an energy-consumption study has been done.

[[Image:Time-differences-brz-us.png||thumb|left|alt=Time graph. The horizontal axis shows dates in 2008. The vertical axis shows the UTC offsets of eastern Brazil and eastern U.S. The difference between the two starts at 3 hours, then goes to 2 hours on February 17 at 24:00 Brazil eastern time, then goes to 1 hour on March 9 at 02:00 U.S. eastern time.|In early 2008 central Brazil was one, two, or three hours ahead of eastern U.S., depending on the date.]]

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

[[Image:2007-02-20 time zones.svg|thumb|upright=1.67|alt=Map of the time zone boundaries of the world. Generally the borders run north-south and there are about 24 zones, but there are many exceptions where the borders follow national boundaries and a few half-hour or quarter-hour zones exist.|Time zones often lie west of their idealized boundaries, resulting in year-round DST.]]

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 longitude, slightly west of center of the idealized Mountain Time Zone , but the time in Saskatchewan is Central Standard Time year-round, so Saskatoon is always about 67 minutes ahead of mean solar time. Conversely, northeast India and a few other areas skew time zones eastward, in effect observing negative DST. 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.

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 longitude, which is just west of center of the idealized Samoa Time Zone , but Nome observes Alaska Time with DST, so it is slightly more than two hours ahead of the sun in winter and three in summer. Double daylight saving time has been used on occasion; for example, Britain used it during World War II.

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. Only a minority of the world's population uses DST because Asia and Africa generally do not observe it.

Benefits and drawbacks

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

Willett's 1907 proposal argued that DST increases opportunities for outdoor leisure activities during afternoon sunlight hours. The longer days nearer the summer solstice in high latitudes offer more room to shift daylight from morning to evening so that early morning daylight is not wasted. 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.

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. 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.

Energy use

DST's potential to save energy comes primarily from its effects on residential lighting, which consumes about 3.5% of electricity in the U.S. and Canada. 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. Although energy conservation remains an important goal, energy usage patterns have greatly changed since then, and recent research is limited and reports contradictory results. Electricity use is greatly affected by geography, climate, and economics, making it hard to generalize from single studies.

• 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, but the National Bureau of Standards (NBS) reviewed the DOT study in 1976 and found no significant savings.
• 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.
• In Western Australia during summer 2006–07, DST increased electricity consumption during hotter days and decreased it during cooler days, with consumption rising 0.6% overall.
• Although a 2007 study estimated that introducing DST to Japan would reduce household lighting energy consumption, 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. DST's effect on lighting energy use is noticeable mainly in residences.
• A 2007 study found that the earlier start to DST that year had little or no effect on electricity consumption in California.
• A 2007 study estimated that winter daylight saving would prevent a 2% increase in average daily electricity consumption in Great Britain.
• 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.
• The U.S. Dept. of Energy (DOE) concluded in a 2008 report that the 2007 U.S. extension of DST saved 0.5% of electricity usage during the extended period.

Several studies have suggested that DST increases motor fuel consumption. The 2008 DOE report found no significant increase in motor gasoline consumption due to the 2007 U.S. extension of DST.

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. 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. A 1999 study estimated that DST increases the revenue of the European Union's leisure sector by about 3%. 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. DST also hurts prime-time broadcast ratings and drive-in and other theaters.

Changing clocks and DST rules has 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. Although it has been argued that clock shifts correlate with decreased economic efficiency, and that in 2000 the daylight-saving effect implied an estimated one-day loss of $31 billion on U.S. stock exchanges, the estimated numbers depend on the methodology and the results have been disputed.

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%, but the 1976 NBS review of the DOT study found no differences in traffic fatalities. In 1995 the Insurance Institute for Highway Safety estimated a reduction of 1.2%, including a 5% reduction in crashes fatal to pedestrians. Others have found similar reductions. 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. It is not clear whether sleep disruption contributes to fatal accidents immediately after the spring clock shifts. A correlation between clock shifts and traffic accidents has been observed in North America and the UK but not in Finland or Sweden. If this effect exists, it is far smaller than the overall reduction in traffic fatalities. A 2009 U.S. study found that on Mondays after the switch to DST, workers sleep an average of 40 minutes less, and are injured at work more often and more severely.

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". Outdoor lighting has a marginal and sometimes even contradictory influence on crime and fear of crime.

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, particularly 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, reprogramming thermostats, and seasonal vaccinations. Locations without DST can instead use the first days of spring and autumn as reminders.

Health

[[Image:Greenwich GB DaylightChart.png|upright=1.67|thumb|alt=Graph of sunrise and sunset times for 2007. The horizontal axis is the date; the vertical axis is the times of sunset and sunrise. There is a bulge in the center during summer, when sunrise is early and sunset late. There are step functions in spring and fall, when DST starts and stops.|Clock shifts affecting apparent sunrise and sunset times at Greenwich in 2007.]]

DST has mixed effects on health. In societies with fixed work schedules it provides more afternoon sunlight for outdoor exercise. 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, but some argue the reverse. The Retinitis Pigmentosa Foundation Fighting Blindness, chaired by blind sports magnate Gordon Gund, successfully lobbied in 1985 and 2005 for U.S. DST extensions, but DST can hurt night blindness sufferers.

Clock shifts disrupt sleep and reduce its efficiency. Effects on seasonal adaptation of the circadian rhythm can be severe and last for weeks. A 2008 study found that although male suicide rates rise in the weeks after the spring transition, the relationship weakened greatly after adjusting for season. A 2008 Swedish study found that heart attacks were significantly more common the first three weekdays after the spring transition, and significantly less common the first weekday after the autumn transition. The government of Kazakhstan cited health complications due to clock shifts as a reason for abolishing DST in 2005.

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. As more devices contain clocks, more time is spent changing them. 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. During an autumn transition from 02:00 to 01:00, a clock reads times from 01:00:00.0 through 01:59:59.9 twice, possibly leading to confusion.

[[Image:Willett memorial.JPG|thumb|upright|alt=A standing stone in a grassy field surrounded by trees. The stone contains a vertical sundial centered on 1 o'clock, and is inscribed "HORAS NON NUMERO NISI ÆSTIVAS" and "SUMMER TIME ACT 1925".|The William Willett Memorial Sundial is always on DST.]]

Some computer-based systems require downtime or restarting when clocks shift; ignoring this requirement damaged a German steel facility in 1993. Medical devices may generate adverse events that could harm patients, without being obvious to clinicians responsible for care. 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. Consumers must update devices such as programmable thermostats with the correct DST rules, or manually adjust the devices' clocks.

Some clock-shift problems could be avoided by adjusting clocks continuously or at least more gradually—for example, Willett at first suggested weekly 20-minute transitions—but this would add complexity and has never been implemented.

DST inherits and can magnify the disadvantages of standard time. For example, when reading a sundial, one must compensate for it along with time zone and natural discrepancies. Also, sun-exposure rules like avoiding the sun within two hours of noon become less accurate when DST is in effect.

Politics

Daylight saving has caused controversy since it began.See Further reading for full citations to these books:

• 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". 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", and wags have dubbed it "Daylight Slaving Time". 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 energy crisis and war.

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. The U.S. was even more skeptical: Andrew Peters introduced a DST bill to the U.S. House of Representatives in May 1909, but it soon died in committee.

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

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. 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.

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. 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. Only a few U.S. cities retained DST locally thereafter, including New York so that its financial exchanges could maintain an hour of arbitrage trading with London, and Chicago and Cleveland to keep pace with New York. Wilson's successor 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.

Since Willett's day the world has seen many enactments, adjustments, and repeals of DST, with similar politics involved. The history of time in the United States includes DST during both world wars, but no standardization of peacetime DST until 1966. 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 based on the premise that during DST fast-food restaurants sell more French fries, which are made from Idaho potatoes; in 2005, the Sporting Goods Manufacturers Association and the National Association of Convenience Stores successfully lobbied for the 2007 extension to U.S. DST. After a three-year trial, 55% of Western Australians voted against DST in 2009, with urban areas split evenly and rural areas strongly opposed; it was the fourth consecutive referendum since 1975 to reject DST. In the UK the sport and leisure industry supports a proposal to observe SDST's additional hour year-round.

Terminology

Although daylight saving time is considered to be correct, daylight savings time is commonly used. The form daylight saving time uses the present participle saving as an adjective, as in labor saving device; the first two words are sometimes hyphenated, as in daylight-saving time. The common variants daylight savings time and daylight savings use savings by analogy to savings account. Daylight time is also common. Willett's 1907 proposal used the term daylight saving, but by 1911 the term summer time replaced daylight saving time in draft legislation in Britain, and continental Europe uses similar phrases, such as Sommerzeit in Germany and l'heure d'été in France.

The name of local time typically changes when DST is observed. American English replaces standard with daylight: for example, Pacific Standard Time (PST) becomes Pacific Daylight Time (PDT). British English typically inserts 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 now advances clocks before the vernal equinox, so the mnemonic disagrees with the astronomical definition of spring, but a proposed substitute "March forward ..." works only in the northern hemisphere, and is less robust against future rule changes.

Computing

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

Many computer-based systems can adjust automatically when DST starts and finishes, based on their time zone settings. However, changes to DST rules causes problems in existing installations. For example, the 2007 change to DST rules in North America required many computer systems to be upgraded, with the greatest impact on email and calendaring programs like Microsoft Outlook; the upgrades consumed a significant effort by corporate information technologists. Some applications standardize on UTC to avoid problems with clock shifts and time zone differences.

Two computer-based 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; HP's "tztab" database is similar but incompatible. When temporal authorities change DST rules, zoneinfo updates are installed as part of ordinary system maintenance. In Unix-like systems the TZ environment variable specifies the location name, as in TZ='America/New_York'.

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.

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. 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.

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 using the Jewish calendar 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 and a Microsoft tool that switches zones automatically.

Microsoft Windows keeps the system real-time clock in local time. This causes several problems, including compatibility when multi booting with operating systems that set the clock to UTC, and double-adjusting the clock when multi booting different Windows versions, such as with a rescue boot disk. In 2008 Microsoft hinted that future versions of Windows will partially support a Windows registry entry RealTimeIsUniversal that had been introduced many years earlier, when Windows NT supported RISC machines with UTC clocks, but had not been maintained since.

References

[[Image:Franklin-Benjamin-Journal-de-Paris-1784.jpg|thumb|upright|alt=Yellowed magazine cover containing mostly print that is too small to read. Near the top is "JOURNAL DE PARIS."|Franklin's 1784 letter about daylight had neither title nor byline.]][[Image:Waste-of-Daylight-19-cover.jpg|thumb|upright|alt=Pamphlet cover showing a large British flag in red, white, and dark blue, with the large title "THE WASTE OF DAYLIGHT", an unreadable subtitle, and "WILLIAM WILLETT" near the bottom.|William Willett's pamphlet promoting DST went through nineteen editions.]]

1. Its first publication was in the journal's "Économie" section. The revised English version (retrieved on 2009-02-13) is commonly called "An Economical Project", a title that is not Franklin's; see
2. Willett pamphlet: * *
3. An earlier version is in:
4. Lay summary – New York Times (2008-03-08).
5. Effect on those whose hours are set by the sun: * *
6. Data supporting Coren's half of this exchange are in:
7. Clock shifts and accidents: * * * *
8. Clock shifts as safety reminders: * * * *
9. DST and circadian rhythm: * *
10. Daylight saving time and its variants: * * * *

Further reading

• The British version, focusing on the UK, is

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