Talk:Tlalpan and Daylight saving time: Difference between pages

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'''Daylight saving time''' (also called '''DST''', or '''[[summer time]]''') is the portion of the year in which a region's local time is advanced by (usually) one hour from its official standard time. It is most common in [[temperate]] regions.

Are you interested in this south-west part of Mexico City? do you want information about visiting or living here?

"Daylight saving time" is a system intended to "save" daylight, as opposed to "wasting" time (for example, by [[sleep]]ing long past sunup). The official time is adjusted forward during the spring and summer months, so that the active hours of work and school will better match the hours of daylight.

The [[Time zone#List of time zones and contained areas|list of time zones]] has information on which areas do or do not observe DST.

==Origin==

It is sometimes asserted that DST was first proposed by [[Benjamin Franklin]] in a letter to the editors of the ''[[Journal of Paris]]'' [http://webexhibits.org/daylightsaving/franklin3.html]. However, the article was humorous; Franklin was not proposing DST, but rather that people should get up and go to bed earlier.

It was first seriously proposed by [[William Willett]] in the "Waste of Daylight" [http://webexhibits.org/daylightsaving/willett.html], published in [[1907]], but he was unable to get the [[Parliament of the United Kingdom|British government]] to adopt it, despite considerable [[lobbying]].

The idea of daylight saving time was first put into practice by the [[Germany|German]] government during the [[World War I|First World War]] between [[April 30]] and [[October 1]], [[1916]]. Shortly afterward, the [[United Kingdom]] followed suit, first adopting DST between [[May 21]] and [[October 1]], [[1916]]. Then on [[March 19]], [[1918]], the [[Congress of the United States|U.S. Congress]] established several [[time zone]]s (which were already in use by [[railroad]]s and most cities since [[1883]]) and made daylight saving time official (which went into effect on [[March 31]]) for the remainder of World War I. It was observed for seven months in [[1918]] and [[1919]]. The law, however, proved so unpopular (mostly because people rose and went to bed earlier than in modern times) that the law was later [[repeal]]ed.

==Observation of DST==

Daylight saving time is generally a [[temperate]] zone practice; day lengths in the [[tropics]] do not vary enough to justify DST. [[Hawaii]], the only U.S. state in the tropics, does not observe DST. However, [[Mexico]] has adopted DST nationwide, even in its tropical regions, because of its increasing economic ties to the United States. The Mexican state of [[Sonora]] does not observe DST because it borders on the U.S. state of [[Arizona]] which also does not observe DST (except in the large [[Navajo Nation|Navajo Indian Reservation]] in northeastern Arizona). Polar or near-polar locations such as [[Iceland]] often opt out as well, as summer in these locations usually brings nearly uninterrupted daylight.

The amount of the time shift varies, but one hour is the most common. The dates of the beginning and ending of DST also vary by country. With a few exceptions, switchovers between standard time and daylight saving time generally occur in the early morning hours of a Sunday morning, presumably because doing so then causes less disruption than a change on a weekday would.

DST commonly begins in the [[Northern Hemisphere]] on either the first Sunday in April or the last Sunday in March, and ends on the last Sunday in October. In the [[Southern Hemisphere]], the beginning and ending dates are switched (thus the time difference between, e.g., the [[United Kingdom]] and [[Chile]] may be three, four, or five hours).

[[Chile]] switches to DST at 24:00 on the second Saturday in October and reverts to LST at 24:00 on the second Sunday the following March. The current law which affects the entire country was enacted in 1970, but it had observed the practice as early as 1927 when the country had been divided into two distinct time zones. In specific years the starting and ending dates have been modified for political or climactic reasons.

[[North America]] generally follows the same procedure, going by local time in each zone, each time zone switching at 02:00 LST (local standard time) to 03:00 LDT (local daylight time) on the first Sunday in April, and again from 02:00 LDT to 01:00 LST on the last Sunday in October. The Canadian province of [[Newfoundland and Labrador]] is an exception in that the time changes take place at 00:01 local standard time and 00:01 local daylight time respectively. Also, in [[1990]], they experimented with Double Daylight Time, when the clocks went ahead by two hours, instead of the usual one hour. The [[Energy Policy Act of 2005]], signed by President [[Bush, George W.|George W. Bush]] will extend DST, which might prompt neighboring countries with integrated economies and schedules (especially Canada and Mexico) to adopt these changes as well. The Canadian province of [[Ontario]] has already pledged to change its daylight savings rules to match the new US rules.

All countries in [[Europe]], except [[Iceland]] as already noted, observe daylight-saving time and change on the same date: moving clocks forward one hour on the last Sunday in March and back one hour on the last Sunday in October. In the West European ([[UTC]]), Central European (UTC+1), and East European (UTC+2) time zones the change is simultaneous: on both dates the clocks are changed everywhere at 01:00 UTC, i.e. from local times of 01:00/02:00/03:00 to 02:00/03:00/04:00 in March, and vice versa in October. (See also: [[European Summer Time]]). In [[Russia]], however, although the changeover dates are the same, clocks are moved forward or back at 02:00 local time in all zones. Thus in Moscow (local time = UTC+3 in winter, UTC+4 in summer), daylight-saving time commences at 05:00 UTC on the last Sunday in March, and ends at 06:00 UTC on the last Sunday in October.

With Iceland observing UTC all year round, despite being at a longitude which would indicate UTC-1, the country may be said to be on continuous DST.

[[Egypt]] switches to DST on midnight of the last Thursday of April, since the weekend holiday for most Egyptians is on Friday, thus giving workers and students a chance to adjust.

[[Israel]] adopts Daylight Saving Time on the last Friday before April 2 at 02:00, and returns to standard time at 02:00 of the Sunday of the month of [[Tishrei]] between [[Rosh Hashanah]] and [[Yom Kippur]]. Israel's Daylight Saving Time rules have changed repeatedly in recent years; there has been trouble reaching a consensus regarding [[Gregorian calendar]] end dates for DST as they are dependant on [[Jewish Holidays]], which follow the [[Lunar calendar|lunar]] [[Hebrew calendar]]. For more on this subject, see [[Israeli Daylight Savings Law]].

The [[People's Republic of China]] experimented with DST from [[1986]], but abandoned it in the [[1990s]]. The PRC now uses one universal time zone for all of the nation from [[Urumqi]] in the northwest to [[Fujian]] in the southeast; the size of the nation was a major factor why DST was not considered practical in China.

[[Australia]] has a mixed implementation of daylight saving time. During winter it has three time zones, but when daylight saving time is in effect, it has five time zones (mostly differing by 30 minutes) ranging from UTC+8 to UTC+11. Although there have been several [[referendum|referenda]] on the topic, [[Western Australia]], the [[Northern Territory]] and [[Queensland]] have not adopted the practice. As a result, the tropical regions of the country do not observe daylight saving. Interestingly, during daylight saving time, [[South Australia]] observes a time later than Queensland, despite the latter being almost entirely further east. [[Tasmania]] starts DST earlier than the rest of the country, usually at the start of October.

[[Cuba]] always starts its DST on April 1 but the end date varies.

[[Brazil]] adopted DST for the first time in [[1931]], but uninterruptedly since [[1985]] in southern states (south, southeast regions and states of [[Goiás]] and [[Mato Grosso do Sul]]). Starting and ending dates are variable: normally, Brazilian DST starts at 00:00 on an October (rarely November) Sunday and ends at 00:00 on a February Sunday.

[[Pakistan]] experimented with DST in [[2002]] going from +5:00 to +6:00. It has not used DST since then. [[India]] used DST briefly during its wars with Pakistan and China.

The Canadian province of [[Saskatchewan]] is the only part of that country (other than northeastern [[British Columbia]] and [[Southampton Island]]) that does not use DST. However, the charter of the city of [[Lloydminster, Alberta/Saskatchewan|Lloydminster]], which is bisected by the Saskatchewan–[[Alberta]] border, gives it the special exception of using DST. Lloydminster and its immediately surrounding region in Saskatchewan use DST with [[Mountain Standard Time Zone|Mountain Standard Time]], which is the time used by Alberta.

===In the US===

Daylight saving time was reinstated in the [[United States]] on [[February 9]], [[1942]], again as a wartime measure to conserve resources. This remained in effect until [[World War II]] began winding down and the requirement was removed on [[September 30]], [[1945]].

From [[1945]] to [[1966]], U.S. federal law did not address daylight saving time. States and localities were free to observe daylight saving time or not. This resulted in a patchwork where some areas observed DST while adjacent areas did not, and it was not unheard of to have to reset one's clock several times during a relatively short trip (e.g., bus drivers operating between [[Moundsville, West Virginia]], and [[Steubenville, Ohio]] had to reset their watches seven times over 35 miles).

The U.S. federal [[Uniform Time Act]] of 1966 mandated that daylight saving time begin nationwide on the last Sunday of [[April]] and end on the last Sunday of [[October]]. Any state that wanted to be exempt from daylight saving time could do so by passing a state law, provided that it exempt the entire state. The law was amended in [[1972]] to permit states that straddle a time zone boundary to exempt the entire area of the state lying in one time zone. The law was amended again in [[1986]] to begin daylight saving time on the first [[Sunday]] in [[April]], to take effect the following year.

In response to the [[1973 energy crisis]], daylight saving in the United States was begun earlier in both [[1974]] and [[1975]], commencing on the first [[Sunday]] in [[January]] ([[January 6]]) in the former year and the last [[Sunday]] in [[February]] ([[February 23]]) in the latter.

Starting [[March 11]], [[2007]], daylight saving time will be extended another four to five weeks, from the second Sunday of March to the first Sunday of November. The change was introduced by the [[Energy Policy Act of 2005]]; the [[U.S. House of Representatives|House]] had originally approved a motion that would have extended DST even further. Proponents claimed that the extension would save "the equivalent of" 10,000 [[barrel (unit)|barrel]]s of [[oil]] per day, but this figure was based on [[U.S. Department of Energy]] information from the [[1970s]], the accuracy and relevance of which the DoE no longer stands by. There is very little recent research on what the actual positive effects, if any, might be. [http://news.nationalgeographic.com/news/2005/07/0728_050728_daylight.html]

The extension, which puts the U.S. out of step with other countries in North America (for example [[Canada]]), was greeted by criticism from the airline industry [http://www.airlines.org/ga/files/ATA_DST_Letter.pdf], observant [[Judaism|Jews]] [http://www.beliefnet.com/story/171/story_17135_1.html], and those concerned for the safety of children traveling to school in the dark before the late [[sunrise]] [http://www.gainesville.com/apps/pbcs.dll/article?AID=/20050723/WIRE/207230308/1117/news]. Many businesses will have to reprogram or replace devices such as [[time clock]]s, [[security system]]s, and [[timed safe]]s; [[traffic light]]s must be adjusted to properly recognize [[rush hour]]. Devices that use [[atomic time]] from [[radio clock]]s will adjust automatically, but such devices are rare, and it is difficult to receive the signals inside [[building]]s. Many devices, such as [[VCR]]s, cannot be updated at all and will need to be manually adjusted twice a year.

====DST and the state of Indiana====

DST is a long-standing controversy in [[Indiana]], not only as an agricultural state, but also because the border separating the Eastern and Central [[time zone]]s divides the state. In the past, neighboring communities sometimes ended up one or even two hours apart. Being out-of-sync with neighboring states and the national changing of clocks, it is argued, has a negative economic impact on the state. It has been demonstrated that some businesses have located outside of the state once the confusion related to not changing clocks is discovered. In the current compromise, the state has three kinds of time zones:

* 77 counties, most of the state, are on ''Eastern'' Standard Time but ''do not'' use DST;

* 5 counties near [[Chicago, Illinois]] and 5 counties in the southwestern corner of the state are on ''Central'' Standard Time and ''do'' use DST; and

* 2 counties near [[Cincinnati, Ohio]] and 3 counties near [[Louisville, Kentucky]] are on ''Eastern'' Standard time but ''do'' observe DST. Their observance of DST is unofficial in this case, as a strict reading of the Uniform Time Act would not allow for this situation, but by observing DST, they remain synchronized with the greater Louisville and Cincinnati metropolitan areas.

On [[April 29]], [[2005]], the Indiana legislature voted to begin observing daylight-saving time in 2006. Currently, the state is in the Eastern time zone; however, its time zone is currently under federal review[http://www.indystar.com/apps/pbcs.dll/article?AID=/20050429/NEWS02/504290510]. There was further controversy after this passed, as some people that supported it initially had thought that the time would fall back an extra hour in winter instead of going ahead an extra hour in summer.

==Rationales for DST==

One of the major reasons given for observing DST is energy conservation. Theoretically, the amount of residential electricity needed in the evening hours is dependent both on when the sun sets and when people go to bed. Because people tend to observe the same bedtime year-round, by artificially moving sunset one hour later, the amount of energy used is theoretically reduced. [[United States Department of Transportation]] studies showed that DST reduces the country's electricity usage by one percent during each day DST is in effect.

Part of the reason that it is normally observed in the late spring, summer, and early autumn is because during the winter months the amount of energy saved by moving sunset one hour later is negated by the increased need for morning lighting by moving sunrise by the same amount. During the summer most people would wake up after the sun rises, regardless of whether daylight saving time is in effect or not, so there is no increased need for morning lighting to offset the afternoon energy savings.

Another perceived benefit of DST is increased opportunities for outdoor activities. Most people plan outdoor activities during the increased hours of sunlight. Other benefits cited include prevention of traffic injuries (by allowing more people to return home from work or school in daylight), and crime reduction (by reducing people's risk of being targets of crimes that are more common in dark areas).

When the U.S. went on extended DST in [[1974]] and [[1975]] in response to the [[1973 energy crisis]], Department of Transportation studies found that observing DST in March and April saved 10,000 barrels of oil a day, and prevented about 2,000 traffic injuries and 50 fatalities saving about U.S. [[USD|$]]28 million in traffic costs. [http://www.boulder.nist.gov/timefreq/general/daylightsavingtime.html]

==Criticism of DST==

DST is not universally accepted; many localities do not observe it. Opponents claim that there is not enough benefit to justify needing to adjust clocks twice every year. The disruption in sleep patterns associated with setting clocks either forward or backward correlates with a spike in the number of severe auto accidents, as well as emotional trauma and lost productivity as sleep-disrupted workers adjust to the schedule change. It is also noted that much effort is spent reminding everyone twice a year of the change, and thousands are inconvenienced by showing up at the wrong time when they forget.

There is also a question whether the savings in lighting costs justifies the increase in summertime [[air conditioning]] costs. While most people use more sunlight under DST, most people also experience more heat, which prompts many people to turn on the air conditioner during the warmer afternoon hours. When air conditioning was not widely available, the change did save energy; however, air conditioning is much more widespread now than it was several decades ago. Air conditioning often uses more energy than artificial lighting. It was for this reason that Arizona rejected DST and opted to stay on standard time all year. <!-- Do studies exist showing comparisons? It'd be good to back this speculation up with science. -->

It is also speculated that one of the benefits, more afternoon sun, would also actually increase energy consumption as people get into their cars to enjoy more time for shopping and the like.

No formal studies have been performed, but an enormous amount of time has been spent by software developers to deal with the fact that 2400 hours past 2pm is not necessarily 2pm 100 days later.

Some campaigners in [[United Kingdom|Britain]] would like the country to stay on [[British Summer Time]] (BST) all year round, or in other words, adopt [[Central European Time]] and abolish BST. Alternatively, some would like Britain to adopt Central European Time and jump forward another hour during the summer (adopting a Single/Double Summer Time from Britain's perspective). This would make winter evenings longer, thereby reducing traffic accidents and cases of [[seasonal affective disorder]]. Opponents point to the longer hours of darkness on winter mornings, especially in [[Scotland]], which might well cause an increase in road accidents. It has even been suggested that Scotland should be placed on a different [[time zone]] from the rest of the UK, which, unlikely though it may sound, would be possible as the [[British Parliament|UK Parliament]] could legislate to put the UK forward an hour, and then the [[Scottish Parliament]] could put Scotland back onto GMT.

DST is particularly unpopular among people working in [[agriculture]] because the animals do not observe it, and thus the people are placed out of synchronization with the rest of the community, including school times, broadcast schedules, and the like.

Canadian author [[Robertson Davies]], in ''[[The Diary of Samuel Marchbanks]]'' ([[1947]]), wrote the following: "I object to being told that I am saving daylight when my reason tells me that I am doing nothing of the kind ... At the back of the Daylight Saving scheme, I detect the bony, blue-fingered hand of Puritanism, eager to push people into bed earlier, and get them up earlier, to make them healthy, wealthy, and wise in spite of themselves."

==Mnemonic==

The [[mnemonic]] "spring forward, fall back", tells us how to reset clocks when the time changes, regardless of hemisphere (although it has to be remembered that spring and autumn occur during different months in the northern and southern hemispheres). This uses the word "fall" to mean "[[autumn]]"; while this usage has died out in [[British English]], it is still very common in [[North American English]].

==Associated practices==

Fire safety officials in Australia, Canada, New Zealand, and the United States encourage citizens to use the two annual time changes as a reminder to check the batteries in home and office fire alarms and [[smoke detector]]s. For example, the [http://www.cfa.vic.gov.au/ Country Fire Authority] of Victoria in Australia has been running a program called "Change Your Clock, Change Your Smoke Alarm Battery" for several years. This is especially important in autumn, just before the [[heating]] season causes an increase in home [[fire]]s.

==The name==

The expression ''daylight saving'''s''' time'' (with the extra "s") is a common alternate form but is sometimes considered to be improper English because "saving" is used here as a [[Participle|verbal adjective]] describing a single type of activity. Because "daylight saving" is a [[compound adjective]], the proper form would actually be ''daylight-saving time'', with the [[hyphen]].

==See also==

*[[British Summer Time]]

*[[European Summer Time]]

*[[Eastern Daylight Time]]

*[[Central Daylight Time]]

==Books==

*[http://seizethedaylight.com ''Seize the Daylight: The Curious and Contentious Story of Daylight Saving Time'' by David Prerau] (Thunder’s Mouth Press; $23.00; ISBN 1-56025-655-9)

*[http://www.shoemakerhoard.com/catalog/spring.html ''Spring Forward'' by Michael Downing] (Shoemaker &amp; Hoard; $23.00; ISBN 1-59376-053-1)

==External links==

*[http://www.seizethedaylight.com/dst/ A Brief History of DST]

*[http://webexhibits.org/daylightsaving/ Straightforward discussion of DST]

*[http://www.standardtime.com/ End Daylight Saving Time] (history and analysis of the practice)

*[http://europa.eu.int/eur-lex/pri/en/oj/dat/2001/l_031/l_03120010202en00210022.pdf EU directive 2000/84/EC]

*[http://nationalatlas.gov/articles/boundaries/a_savingtime.html Saving Time, Saving Energy] - United States DST schedule (including new rules starting in [[2007]]), explanation, history

*[http://www.timeanddate.com/time/aboutdst.html DST changeover times throughout the world]

*[http://www.twinsun.com/tz/tz-link.htm Sources for daylight saving time data, including histories and predictions]

*[http://www.merlyn.demon.co.uk/uksumtim.htm Summer Time] by JR Stockton. With future changeover dates EU (until 2007 final, from then extrapolating)

*[http://www.worldtimeserver.com/ World Time Server]

<!-- *[http://sandbox.xerox.com/stewart/tzconvert.cgi Time Zone Converter] Link is down as of 4 November 2004 -->

*[http://www.mcmaster.ca/inabis98/occupational/coren0164/two.html Sleep deficit and accidents]

*[http://www.nast.tk National Association of Standard Time]

*[http://wwp.greenwichmeantime.com/ Greenwich Mean Time: World Time]

*[http://photosydney.blogspot.com/2004/12/daylight-saving.html PhotoSydney: Daylight Saving] examines daylight saving in Australia.

*[http://pcdsh01.on.br/tzedit.zip Time Zone Editor for Windows] (download .zip)

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