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.[citation needed]
The shift in apparent time is also motivated by practicality. In American temperate latitudes, for example, the sun rises around 04:30 at the summer solstice and sets around 19:30. Since most people are asleep at 04:30, it is seen as more practical to treat 04:30 as if it is 05:30, thereby allowing people to wake closer to the sunrise and be active in the evening light.
DST is of little use for locations near the Equator, because these regions see only a small variation in daylight in the course of the year.
The effect of daylight saving time also varies according to how far east or west the location is within its time zone, with locations farther east inside the time zone benefiting more from DST than locations farther west in the same time zone.[16] In spite of a width spanning thousands of kilometers, all of China is located within a single time zone per government mandate, minimizing any potential benefit of daylight saving time there.
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.[17] 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.[18] 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[19] and in Jewish ceremonies.[20]
Benjamin Franklin published the proverb "early to bed and early to rise makes a man healthy, wealthy, and wise",[21][22] and published a letter in the Journal de Paris during his time as an American envoy to France (1776–1785) suggesting that Parisians economize on candles by rising earlier to use morning sunlight.[23] This 1784 satire proposed taxing window shutters, rationing candles, and waking the public by ringing church bells and firing cannons at sunrise.[24] Despite common misconception, Franklin did not actually propose DST; 18th-century Europe did not even keep precise schedules. However, this changed as rail transport and communication networks required a standardization of time unknown in Franklin's day.[25]
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 volition.[26][27]
New Zealand entomologist George Hudson first proposed modern DST. His shift-work job gave him spare time to collect insects and led him to value after-hours daylight.[5] In 1895, he presented a paper to the Wellington Philosophical Society proposing a two-hour daylight-saving shift,[12] and considerable interest was expressed in Christchurch; he followed up with an 1898 paper.[28] Many publications credit the DST proposal to prominent English builder and outdoorsman William Willett,[29] who independently conceived DST in 1907 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.[30] His solution was to advance the clock during the summer, and he published the proposal two years later.[31] 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.[32] 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.[6] 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.[7][8] This was followed by Orillia, Ontario, introduced by William Sword Frost while mayor from 1911 to 1912.[33] The first states to adopt DST (German: Sommerzeit) nationally were those of the German Empire and its World War I ally Austria-Hungary commencing on 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.[34] It became common during World War II (some countries adopted double summer time), and was standardized in the US by federal law in 1966, and widely adopted in Europe from the 1970s as a result of the 1970s energy crisis. Since then, the world has seen many enactments, adjustments, and repeals.[35]
It is a common myth in the United States that DST was first implemented for the benefit of farmers.[36][37][38] In reality, farmers have been one of the strongest lobbying groups against DST since it was first implemented.[36][37][38] The factors that influence farming schedules, such as morning dew and dairy cattle's readiness to be milked, are ultimately dictated by the sun, so the time change introduces unnecessary challenges.[36][38][39]
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.[40][39] Year-round DST, or "War Time", was implemented again during World War II.[40] 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.[40][41] 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, 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.[42] A one-hour change is usual, but twenty-minute and two-hour changes have been used in the past. Notable exceptions today include Lord Howe Island with a thirty-minute change, and Troll (research station) that shifts two hours directly between CEST and GMT since 2016.[43] 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.
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.
In most countries that observe seasonal daylight saving time, the time reverts in winter to "standard time".[44][45] An exception exists in Ireland, where its winter clock has the same offset (UTC±00:00) and legal name as that in Britain (Greenwich Mean Time)—but while its summer clock also has the same offset as Britain's (UTC+01:00), its legal name is Irish Standard Time[46][47] as opposed to British Summer Time.[48]
Since 2019 Morocco observes 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 that month, its clocks are turned forward to Western European Summer Time (UTC+01:00).[49][50][51]
The time at which to change clocks differs across jurisdictions. Members of the European Union conduct a coordinated change, changing all zones at the same instant, at 01:00 Coordinated Universal Time (UTC), which means that it changes at 02:00 Central European Time (CET), equivalent to 03:00 Eastern European Time (EET). As a result, the time differences across European time zones remain constant.[52][53] North America coordination of the clock change differs, in that each jurisdiction change at 02:00 local time, which temporarily creates an imbalance with the next time zone (until it adjusts its time, one hour later, at 2 am there). For example, Mountain Time is for one hour in the spring two hours ahead of Pacific Time instead of the usual one hour ahead, and instead of one hour in the autumn, briefly zero hours ahead of Pacific Time.
The dates on which clocks change vary with location and year; consequently, the time differences between regions also vary throughout the year. For example, Central European Time is usually six hours ahead of North American Eastern Time, except for a few weeks in March and October/November, while the United Kingdom and mainland Chile could be five hours apart during the northern summer, three hours during the southern summer, and four hours for a few weeks per 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.[53] Starting in 2007, most of the United States and Canada observed DST from the second Sunday in March to the first Sunday in November, almost two-thirds of the year.[54] Moreover, the beginning and ending dates are roughly reversed between the northern and southern hemispheres because spring and autumn are displaced six months. 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.[55] In some countries time is governed by regional jurisdictions within the country such that some jurisdictions change and others do not; this is currently the case in Australia, Canada, and the United States.[56][57]
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.[58][59][60] 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.[61] 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.[62]
The concept of daylight saving has caused controversy since its early proposals.[63] 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"[64] and pundits have dubbed it "Daylight Slaving Time".[65] Retailing, sports, and tourism
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