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



The idea of aligning waking hours to daylight hours to conserve candles was first proposed in 1784 by U.S. polymath Benjamin Franklin. In a satirical letter to the editor of The Journal of Paris, Franklin suggested that waking up earlier in the summer would economize on candle usage; and calculated considerable savings.[1][2] In 1895, New Zealand entomologist and astronomer George Hudson proposed the idea of changing clocks by two hours every spring to the Wellington Philosophical Society.[3] In 1907, British resident William Willett presented the idea as a way to save energy. After some serious consideration, it was not implemented.[4]

Industrialized societies usually follow a clock-based schedule for daily activities that do not change throughout the course of the year. The time of day that individuals begin and end work or school, and the coordination of mass transit, for example, usually remain constant year-round. In contrast, an agrarian society's daily routines for work and personal conduct are more likely governed by the length of daylight hours[8][9] and by solar time, which change seasonally because of the Earth's axial tilt. North and south of the tropics, daylight lasts longer in summer and shorter in winter, with the effect becoming greater the farther one moves away from the equator.

Proponents of daylight saving time argue that most people prefer a greater increase in daylight hours after the typical "nine to five" workday.[14][15] Supporters have also argued that DST decreases energy consumption by reducing the need for lighting and heating, but the actual effect on overall energy use is heavily disputed.

Ancient civilizations adjusted daily schedules to the sun more flexibly than DST does, often dividing daylight into 12 hours regardless of daytime, so that each daylight hour became progressively longer during spring and shorter during autumn.[19] For example, the Romans kept time with water clocks that had different scales for different months of the year; at Rome's latitude, the third hour from sunrise (hora tertia) started at 09:02 solar time and lasted 44 minutes at the winter solstice, but at the summer solstice it started at 06:58 and lasted 75 minutes.[20] From the 14th century onward, equal-length civil hours supplanted unequal ones, so civil time no longer varied by season. Unequal hours are still used in a few traditional settings, such as monasteries of Mount Athos[21] and in Jewish ceremonies.[22]

In 1810, the Spanish National Assembly Cortes of Cádiz issued a regulation that moved certain meeting times forward by one hour from 1 May to 30 September in recognition of seasonal changes, but it did not change the clocks. It also acknowledged that private businesses were in the practice of changing their opening hours to suit daylight conditions, but they did so of their own volition.[28][29]

New Zealand entomologist George Hudson first proposed modern DST. His shift-work job gave him leisure time to collect insects and led him to value after-hours daylight.[3] In 1895, he presented a paper to the Wellington Philosophical Society proposing a two-hour daylight-saving shift,[10] and considerable interest was expressed in Christchurch; he followed up with an 1898 paper.[30] Many publications credit the DST proposal to prominent English builder and outdoorsman William Willett,[31] who independently conceived DST in 1905 during a pre-breakfast ride when he observed how many Londoners slept through a large part of a summer day.[15] Willett also was an avid golfer who disliked cutting short his round at dusk.[32] His solution was to advance the clock during the summer months, and he published the proposal two years later.[33] Liberal Party member of parliament Robert Pearce took up the proposal, introducing the first Daylight Saving Bill to the British House of Commons on 12 February 1908.[34] A select committee was set up to examine the issue, but Pearce's bill did not become law and several other bills failed in the following years.[4] Willett lobbied for the proposal in the UK until his death in 1915.

Port Arthur, Ontario, Canada, was the first city in the world to enact DST, on 1 July 1908.[5][6] This was followed by Orillia, Ontario, introduced by William Sword Frost while mayor from 1911 to 1912.[35] The first states to adopt DST (German: Sommerzeit) nationally were those of the German Empire and its World War I ally Austria-Hungary commencing 30 April 1916, as a way to conserve coal during wartime. Britain, most of its allies, and many European neutrals soon followed. Russia and a few other countries waited until the next year, and the United States adopted daylight saving in 1918. Most jurisdictions abandoned DST in the years after the war ended in 1918, with exceptions including Canada, the United Kingdom, France, Ireland, and the United States.[36] It became common during World War II (some countries adopted double summer time), and was widely adopted in America and Europe from the 1970s as a result of the 1970s energy crisis. Since then, the world has seen many enactments, adjustments, and repeals.[37]

DST was first implemented in the US with the Standard Time Act of 1918, a wartime measure for seven months during World War I in the interest of adding more daylight hours to conserve energy resources.[42][41] Year-round DST, or "War Time", was implemented again during World War II.[42] After the war, local jurisdictions were free to choose if and when to observe DST until the Uniform Time Act which standardized DST in 1966.[42][43] Permanent daylight saving time was enacted for the winter of 1974, but there were complaints of children going to school in the dark and working people commuting and starting their work day in pitch darkness during the winter months, and it was repealed a year later.

The relevant authorities usually schedule clock changes to occur at (or soon after) midnight, and on a weekend, in order to lessen disruption to weekday schedules.[44] A one-hour change is usual, but twenty-minute and two-hour changes have been used in the past. In all countries that observe daylight saving time seasonally (i.e. during summer and not winter), the clock is advanced from standard time to daylight saving time in the spring, and they are turned back from daylight saving time to standard time in the autumn. The practice, therefore, reduces the number of civil hours in the day of the springtime change, and it increases the number of civil hours in the day of the autumnal change. For a midnight change in spring, a digital display of local time would appear to jump from 23:59:59.9 to 01:00:00.0. For the same clock in autumn, the local time would appear to repeat the hour preceding midnight, i.e. it would jump from 23:59:59.9 to 23:00:00.0.

While most countries that change clocks for daylight saving time observe standard time in winter and DST in summer, Morocco observes (since 2019) daylight saving time every month but Ramadan. During the holy month (the date of which is determined by the lunar calendar and thus moves annually with regard to the Gregorian calendar), the country's civil clocks observe Western European Time (UTC+00:00, which geographically overlaps most of the nation). At the close of this month, its clocks are turned forward to Western European Summer Time (UTC+01:00), where they remain until the return of the holy month the following year.[50][51][52]

From year to year, the dates on which to change clock may also move for political or social reasons. The Uniform Time Act of 1966 formalized the United States' period of daylight saving time observation as lasting six months (it was previously declared locally); this period was extended to seven months in 1986, and then to eight months in 2005.[59][60][61] The 2005 extension was motivated in part by lobbyists from the candy industry, seeking to increase profits by including Halloween (31 October) within the daylight saving time period.[62] In recent history, Australian state jurisdictions not only changed at different local times but sometimes on different dates. For example, in 2008 most states there that observed daylight saving time changed clocks forward on 5 October, but Western Australia changed on 26 October.[63]

The concept of daylight saving has caused controversy since its early proposals.[64] 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"[65] and pundits have dubbed it "Daylight Slaving Time".[66] Retailing, sports, and tourism interests have historically favored daylight saving, while agricultural and evening-entertainment interests (and some religious groups[67][68][69][70]) have opposed it; energy crises and war prompted its initial adoption.[71]

Since Germany's adoption of DST in 1916, the world has seen many enactments, adjustments, and repeals of DST, with similar politics involved.[81] The history of time in the United States features DST during both world wars, but no standardization of peacetime DST until 1966.[82][83] St. Paul and Minneapolis, Minnesota, kept different times for two weeks in May 1965: the capital city decided to switch to daylight saving time, while Minneapolis opted to follow the later date set by state law.[84][85] In the mid-1980s, Clorox and 7-Eleven provided the primary funding for the Daylight Saving Time Coalition behind the 1987 extension to U.S. DST. Both senators from Idaho, Larry Craig and Mike Crapo, voted for it based on the premise that fast-food restaurants sell more French fries (made from Idaho potatoes) during DST.[86] 041b061a72


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