Daylight saving time

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]] Daylight saving time (DST), also known as summer time and sometimes referred to as daylight savings time, is a widely used system of adjusting the official local time forward, usually by one hour, from its official standard time for the summer months. This is intended to provide a better match between the hours of daylight and the active hours of work and school. The "saved" daylight is spent on evening activities which get more daylight, rather than being "wasted" while people sleep past dawn.

DST is most commonly used in temperate regions, due to the considerable variation in the amount of daylight versus darkness across the seasons in those regions.

Governments often tout it as an energy conservation measure, on the grounds that it allows more effective use of natural sunlight resource in summer time. Since there is less darkness in the "waking day", there is less use of electric lights. Some opponents reject this argument (see below, Criticism).

Europeans commonly refer to the system as summer time: Irish Summer Time, British Summer Time, and European Summer Time. This is reflected in the time zones names as well, e.g., Central European Time (CET) becomes Central European Summer Time (CEST).

The word "summer" in this context includes most of spring after the spring equinox and nearly all of autumn (April through October). Likewise, the word "winter" here includes part of autumn and a few weeks in spring (November through March). This varies by time zone, of course, and can change over time as well.

Origin

It is sometimes asserted that DST was first proposed by Benjamin Franklin in a letter to the editors of the Journal of Paris. Read the full text. 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", published in 1907, but he was unable to get the British government to adopt it, despite considerable lobbying.

The idea of DST was first put into practice by the German government during the First World War, between April 30 and October 1, 1916. Shortly afterward, the United Kingdom followed suit, first adopting it between May 21 and October 1, 1916. On March 19, 1918, the U.S. Congress established several time zones (which had been in use by railroads and most cities since 1883) and made DST official, effective 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 current times) that it was later repealed.

Observation of DST

DST is generally a temperate zone practice; day lengths in the tropics do not vary enough to justify DST. Hawaii and Arizona are the only U.S. states that do not observe DST.

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 DST generally occur in the early hours of a Sunday morning, because doing so then causes less disruption than a change on a weekday would.

DST commonly begins in the northern hemisphere on the last Sunday in March or the first Sunday in April, and ends on the last Sunday in October. However, beginning in 2007, the United States will begin observing DST from the second Sunday in March until the first Sunday in November. If no energy savings can be shown from the extension after the U.S. Department of Energy completes a study of impact of the change, Congress may revert back to the schedule set in 1986 under Section 110 of the Energy Policy Act of 2005. Since 2002, the European Union has fixed the last Sunday in March and the last Sunday in October as start and end dates (European Summer Time).

In the southern hemisphere, the beginning and ending dates are switched; therefore, the time difference between the United Kingdom and Chile may be four, five or six hours, depending on the time of year.

Usage and history by location

Africa

Egypt

Egypt observes DST between the last Friday in April and the last Thursday in September when the clocks are three hours ahead of Greenwich Mean Time (UTC+3). The change is at midnight (local time). Note: you must not change the date when the first 0:00 midnight occurs.

(See also 24-hour clock) Example: 25-09-2006 23:59 -> 25-09-2006 24:00 -> 25-09-2006 23:00 ... 25-09-2006 23:59 -> 26-09-2006 00:00
Wrong change: 25-09-2006 23:59 -> 25-09-2006 24:00 -> 26-09-2006 00:00-> 26-09-2006 23:00 ... 26-09-2006 23:59 -> 27-09-2006 00:00

Namibia

DST begins on the first Sunday in September, and ends on the first Sunday in April.

Asia

China

The People's Republic of China experimented with DST from 1986, but abandoned it in the 1990s. The PRC now uses one time zone for the whole country; the size of the nation was a major factor why DST was not considered practical in China.

India

India used DST briefly during its wars with Pakistan and China.

Iran

Before 1979, DST was observed in Iran. Thereafter it was abandoned until 1989, when it started on the first day of Farvardin (21-22 March) in the Iranian calendar and ended on the first day of Mehr (23-24 September). In 2006, Iran stopped observing DST.Elham: Cabinet nullifies decision on daylight saving time

Israel

Israel observes DST starting on the last Friday before April 2 and ending at 2 a.m. on the Sunday between Rosh Hashanah and Yom Kippur. Until 2005, the schedule was variable: the only requirement was that there be at least 150 days per year of DST, and was set out each year by the Ministry of the Interior.Daylight Saving Time in Middle East

In territories controlled by The Palestinian National Authority, DST ends a week earlier, which can lead to some confusion. On one occasion, terrorists were transporting a bomb that they thought was set to go off at 5 p.m. Palestinian (Daylight) Time, but which actually was set for 5 p.m. Israeli (Standard) Time, which was an hour ahead. As a result, the bomb went off, killing them (and earning them a Darwin Award). Living on Zionist Time — 1999 Darwin Awards

Kyrgyzstan

Kyrgyzstan voted to stop observing DST in 2005 by remaining on UTC+6 as Standard Time (which used to be Kyrgyzstan Summer Time) to still save energy.

Pakistan

Pakistan experimented with DST in 2002, going from +5:00 to +6:00 on the first Sunday in April at 00:01 to the first Sunday in October at 00:01. It has not used it since then.

South Korea

South Korea observed DST three times: from 1948 to 1951, from 1955 to 1960, and from 1987 to 1988. It is no longer observed now.

Taiwan

Taiwan, Republic of China implemented DST from 1945 to 1961, revoked DST from 1962 to 1973, reinstated DST from 1974 to 1975, and revoked DST from 1976 onwards.

Australasia

Australia

In Australia, daylight saving time is a state/territory-based initiative. Some states/territories implement it and some do not.

New South Wales, Victoria, Tasmania, Australian Capital Territory and South Australia apply DST. Tasmania starts DST earlier than the others, usually near the beginning of October.

Western Australia, Northern Territory and Queensland do not have DST. Queensland experimented with it for a year or two in the early 1970s, and again in the 1990s, but it was not popular and was abandoned. See also: Time in Australia and Daylight Saving by Australian Government

New Zealand

DST begins at 2 a.m. (standard time) on the first Sunday in October each year, and ends at 2 a.m. (DST) on the third Sunday in March.

New Zealand time, including DST, is used by several Antarctic bases that are supplied from New Zealand. This results in the oddity that the Amundsen-Scott South Pole Station sets its clocks an hour further ahead during the southern summer, when the sun is constantly above the horizon, than in the southern winter, when the sun is constantly below the horizon. The extreme geographic position of the base means that there is no possible adjustment of the daily activity cycle that can have any effect on the amount of sunlight received during those activities.

Europe

All countries in Europe but Iceland observe DST and change on the same date and time, starting on the last Sunday in March and ending on the last Sunday in October. Before 1996, EST ended on the last Sunday of September. In the West European (UTC), Central European (CET, 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 and British Summer Time.

Russia and the former USSR

In the USSR daylight saving time was introduced on April 1 1981 by a decision of the Council of Ministers of the USSR. In Russia it was not abandoned after the breakup of the USSR. The changeover dates in Russia are the same, as for other European countries, but clocks are moved forward or back at 02:00 standard time in all zones. Thus in Moscow (local time = UTC+3 in winter, UTC+4 in summer), DST commences at 23:00 UTC on the day before the last Sunday in March, and ends at 23:00 UTC on the day before the last Sunday in October.

Iceland

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. Polar or near-polar locations such as Iceland often opt out, as summer in these locations usually brings nearly uninterrupted daylight.

North America

North America generally follows the same procedure, with each time zone switching at 2:00 a.m. LST (local standard time) to 3:00 LDT (local daylight time) on the first Sunday in April*, and again from 2:00 a.m. LDT to 1:00 LST on the last Sunday in October*.

* In 2007, the starting and ending dates for DST will change in the United States and parts of Canada (see below).

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 1988, they experimented with Double Daylight Time, when the clocks went ahead by two hours, instead of the usual one hour. Manitoba begins DST at 02:00, but switches back to standard time in October at 03:00.

Canada

In Canada, time is under provincial and territorial jurisdiction, not federal. The governments of Ontario, Manitoba, Quebec, Prince Edward Island, New Brunswick, Alberta, the Northwest Territories, British Columbia and Nova Scotia have so far pledged to change their DST rules to match the new U.S. rules. In 2007, their DST will start on the second Sunday in March, and return to standard time on the first Sunday in November. The remaining provinces and territories (Newfoundland, Yukon Territory, Nunavut, Saskatchewan where applicable) will continue to change time on the first Sunday in April and last Sunday in October unless they change their legislation.

=Saskatchewan

The province of Saskatchewan is the largest part of the country that does not observe DST. Saskatchewan is bisected by the 105° West meridian, the central meridian of the Mountain Standard Time Zone (UTC-7), yet clocks are kept at UTC-6 all year long. (This policy was implemented when the Saskatchewan Time Act was passed in 1966, to solve the problems that arose when time zones varied from town to town.) Thus, in the summer months Saskatchewan is on Mountain Daylight Time and in the winter months it is on Central Standard Time. This is equivalent to the province being on Mountain Daylight Time or Central Standard Time year-round. Officially the province is considered to be part of the Central time zone.

The charter of the city of Lloydminster, which is bisected by the Saskatchewan–Alberta boundary, gives it a special exception among areas in Saskatchewan to use DST. Lloydminster and its immediately surrounding region in Saskatchewan use the same timekeeping routine used by Alberta, DST with Mountain Standard Time.Time System in Saskatchewan Local custom in Denare Beach and Creighton, SK, is to observe DST, thereby keeping the same time as nearby Manitoba communities.

=Quebec

The eastern reaches of Quebec's North Shore, east of 63° West longitude, are in the Atlantic Time Zone, but do not observe DST (see exception, below). The effect is that in summer, their clocks match those of the rest of the province, which observes Eastern Daylight Time. In October, their clocks are rejoined by their Atlantic Standard Time neighbors. Although places east of 63° West are officially on Atlantic Time, local custom is to use Eastern Time as far east as the Natashquan River. Those communities observe DST, including all of Anticosti Island, which is bisected by the 63rd meridian.

=British Columbia

= Most of BC observes DST, but there is a large tract in east-central BC in the Mountain Time Zone that does not (most of BC is on Pacific Time). This includes Fort St. John, Charlie Lake, Taylor and Dawson Creek. The Crowsnest corridor between Creston and Yahk in the East Kootenays (southeastern BC) also keeps standard time year-round.

=Nunavut

While the rest of Nunavut observes DST, Southampton Island including Coral Harbour remain on Eastern Standard Time throughout the year.

=Ontario

Secluded in the heart of northwestern Ontario, Pickle Lake and New Osnaburgh have no use for DST. To the south, Atikokan also ignores it, using Eastern Standard Time year-round, effectively adopting Central Daylight Time in summer. Southern Ontario including Toronto, however, does observe DST.

Cuba

Since April 2004, Cuba has remained on DST. Cuba will end DST on October 29, 2006 and begin DST again on March 2007.

Guatemala

Guatemala started to use DST on April 29 2006, ending on September 30 2006.

Honduras

Honduras observed DST once in 1994 but abandoned it that same year. On May 7, 2006 it began observing DST , ending on September 3.

Mexico

Mexico adopted DST nationwide in 1996, even in its tropical regions, because of its increasing economic ties to the United States. The Mexican state of Sonora has not observed DST since 1998 because it borders on the U.S. state of Arizona, which also does not observe DST.

United States

The schedule for 2006 in the United States is that DST begins on the first Sunday in April (, 2006), and changes back to standard time on the last Sunday in October (October , 2006). The time is adjusted at 2 AM.

Beginning in 2007, DST will start on the second Sunday in March, and change back to standard time on the first Sunday in November. Under Section 110 of the Energy Policy Act of 2005, the U.S. Department of Energy is required to study the impact of the DST extension no later than nine months after the change takes effect. Congress has retained the right to revert back to the DST schedule set in 1986 if it cannot be shown that there are significant energy savings from an extension of DST.

DST 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. During this period, the official designation "War Time" was used for year-round DST.

From 1945 to 1966, U.S. federal law did not address DST. States and localities were free to observe DST or not, and the predominant pattern was that the states and localities that did observe DST did so from the last Sunday in April to the last Sunday in September. In the mid-1950s, many states and localities in the northeastern United States began extending DST to the last Sunday in October. The absence of federal standardization resulted in a patchwork where some areas observed DST while adjacent areas did not, and it was not unheard of to have to reset a 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 DST begin nationwide on the last Sunday in April and end on the last Sunday in October, effective in 1967. Any state that wanted to be exempt from DST could do so by passing a state law, provided that it exempted 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. On July 8, 1986, President Ronald Reagan signed the Federal Fire Prevention and Control Act of 1986 into law, which amended the starting date of DST to the first Sunday in April, to take effect the following year.

While the states retain the capability to exempt themselves from DST, they are forbidden by federal law to increase a state's time spent on DST.Ralph Routon's column, "Year-round Daylight Time is Not an Option", Gazette, The (Colorado Springs), 29 January 2000, makes note of this in passing, but unfortunately does not identify the Act responsible Only the United States Congress may take such an action, as had twice occurred prior to 2006.

In response to the 1973 energy crisis, DST in the United States began 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, DST 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 House had originally approved a motion that would have extended DST even further from the first Sunday in March to the last Sunday in November. Proponents claimed that the extension would save "the equivalent of" 10,000 barrels 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. It is also possible that the extension may not last if there are no proven energy savings from the change as Congress retains the right to revert back to the schedules set in 1986 after a study of the change is complete. One potential problem of the change is that early March sunrise times under DST would be just as late as those in early January under standard time and late October under DST. Because the difference in sunrise times would occur suddenly rather than gradually, this move could prove to be unpopular with the American public as more people are forced to awaken in darkness quite suddenly and are drowsy as a result. Extending DST past Halloween may prove to be unpopular since evening darkness is associated with the mood of Halloween. Children who wait for darkness to fall to begin trick-or-treating would have one less hour to do so before bedtime.More Daylight Savings: Energy Boon or Scheduling Snafu?

The extension was greeted with criticism by those concerned for the safety of children traveling to school in the dark before the late sunrise. In addition, the airline industry was especially concerned if DST were to be extended through to the last Sunday in November, as this is very often the Sunday after Thanksgiving. This is one of the busiest travel days at American airports, and could have resulted in much havoc among travelers who forgot that the clocks were changing that day.

If the original proposal to extend DST through the last Sunday in November had been adopted, the entire United States, with the exception of the states that exempted themselves, would have experienced the latest sunrises of the year during the month of November.

An additional issue raised by this extension is that it may require the reconfiguration of some computers. Many computers are programmed to adjust automatically for DST, but do so based on static tables stored directly on the computer itself. In order to change the dates and times at which the automatic jump to or from DST occurs, these tables must be modified, or the user must manually specify that the computer's clock should operate in DST mode. One potential issue related to this (though difficult to quantify) is the amount of labor and money that may be spent correcting errors that arise due to a failure to update computers. Certain types of information systems (those that schedule future events with reference to UTC, for example) may encounter problems arising from a lack of synchronization unless both computers and databases are carefully updated.

=Arizona

= Arizona does not observe DST (however, the large Navajo Indian Reservation, which extends from Arizona into two adjacent states, does). This is in large part due to energy conservation since the temperature in and around Phoenix is hotter than any other large U.S. metropolitan area during the summer, resulting in more power usage from air conditioning units and swamp coolers in homes and businesses. An extra hour of sunlight would cause people to run their cooling systems longer, thereby using more energy.

=Colorado

= At the end of the 20th century, Colorado Springs Gazette columnist Ralph Routon wrote a series of columns suppporting the idea of placing all of Colorado on year-'round DST in order to save state residents the "aggravation of resetting their clocks every six months."Routon mentions in his original column, "Let's Make Daylight Time Year-Round" Gazette, The (Colorado Springs), 23 October 1999, several other beneficial effects, at least to himself The idea gathered noticeable popular support within Colorado Springs, and attention of the state's larger newspapers,said attention being negative, as Ed Quillen savaged the plan in his articlePlot to Eliminate the Mountain Time Zone, Denver Post, 7 November 1999, but when then state Senator MaryAnne Tebedo attempted to present the idea to the state legislature, her research uncovered Federal laws forbidding the state-initiated extension of daylight saving time. Still determined to relieve Coloradans of the need to change their clocks, Tebedo introduced the only bill legally permitted to her: a proposal to exempt the state of Colorado from DST. The bill failed to escape committee during the 2000 legislative session."Year-round Daylight Time is Not an Option" Gazette, The (Colorado Springs), 29 January, 2000: Routon mentions Tebedo's intent to introduce the bill

=Hawaii

= Because of Hawaii's tropical latitude, there is not a large variation in daylight length between winter and summer. Due to the location of Hawaiian archipelago, advancing the clock in Hawaii would have made sunrise times close to 7:00 A.M. even in June.(Most of inhabited islands are located close to the west end of the timezone, Kauai and Niihau are located in the next timezone) Hawaii did experiment with DST for three weeks between April 30, 1933 and May 21, 1933; there is no record as to why it was implemented or ultimately discontinued. [http://www.hawaiianswers.com/index.php?page=index_v2&id=25&c=19 HawaiiAnswers.com: Has Hawaii ever been on daylight saving time, even for a very short time? If yes, when? The page cites The American Atlas, 5th ed., by Thomas Shanks. It is also worth noting that at one time Hawaii Standard Time was UTC-10:30. Hawaii has never observed daylight saving time under the Uniform Time Act, having opted out of the Act's provisions in 1967. Hawaii Revised Statutes, §1-31

=Indiana

For the last several decades, most of Indiana did not observe Daylight Saving Time, but the entire state started to in April 2006.Standard Time Zone Boundary in the State of Indiana (a 139 KB PDF file)

South America

Chile

Chile switches to DST at midnight at the end of the second Saturday in October and reverts to Local Standard Time (LST) at midnight at the end of the second Saturday 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 time zones. In specific years the starting and ending dates have been modified for political or climatic reasons.

Brazil

Brazil adopted DST for the first time in 1931, and has used it continually since 1985 in southern states (south, southeast regions and states of Goiás and Mato Grosso do Sul). Starting and ending dates are variable: normally DST starts at midnight on an October (rarely November) Sunday and ends at midnight on a February or March Sunday.

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 1% while it is in effect.

Part of the reason that it is normally observed in the late spring, summer, and early autumn instead of the winter months is that the amount of energy saved by experiencing sunset one hour later would be negated by the increased need for artificial morning lighting due to a later sunrise. 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 drop in energy usage.

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. ^*]28 million in traffic costs.

Criticism of DST

DST is not universally accepted and many localities do not observe it. Opponents claim that there is not enough benefit to justify the need 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 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. Since DST exchanges morning daylight for evening daylight, late sunrises occur when DST is in effect either too far before the vernal equinox or too far after the autumnal equinox and darkness in the morning can be undesirable for early risers like schoolchildren and workers who must awaken at 6:30 a.m. or earlier.

There is also a question whether the decrease in lighting costs justifies the increase in summertime air conditioning costs. While many 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.

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.

Many also point to the fact that the actual time change causes confusion and ambiguity. For example, during a North American time change, a fall night during which clocks are reset from 2 a.m. DST to 1 a.m. Standard Time, times between 1 a.m. and 2 a.m. will occur twice, causing confusion in transport schedules, payment systems, etc. On a more trivial note, this also means that people born during one of those two hours have no way to know which one it really was, unless someone such as a parent bothers to make a note of it; birth certificates rarely keep track of this. It is difficult to imagine anyone caring about this, but at least one aristocrat, Lord Balfour of the UK, has lodged a farfetched objection: "Supposing some unfortunate Lady was confined with twins and the first child was born 10 minutes before 3 o'clock British Summer Time. ... the time of birth of the two children would be reversed. ... Such an alteration might conceivably affect the property and titles in that House."The Epoch Times "Daylight Saving Time Change Upcoming" Lord Balfour quote. "Supposing some unfortunate Lady was confined with twins..."

Daylight savings time also causes much confusion with international business, people who commute across time zones, especially those time zones that do not observe DST, and computer networks that span multiple time zones.

Most windows based computers reset their internal clock to DST accordingly. Many Linux based computers set their internal clocks to "Zulu Time" (the time at the prime meridian) and have a conpensation factor depending on your time zone and daylight savings time status.

Some societies like India have a system of calculating one's Horoscope which critically depends on the time of birth. Such oriental systems will be thrown out of gear by the ambiguity in time.

Some studies do show that changing the clock increases the traffic accident rate.Ferguson, S.A. et al. (1995) Daylight saving time and motor vehicle crashes: the reduction in pedestrian and vehicle occupant fatalities. American Journal of Public Health 85, 92–95. Following the spring shift to DST, when one hour of sleep is lost, there is a measurable increase in the number of traffic accidents that result in fatalities.

DST is particularly unpopular among people working in agriculture because they must rise with the sun regardless of what the clock says, and thus the people are placed out of synchronization with the rest of the community, including school times, broadcast schedules, and the like.

Other critics suggest that DST is, at its heart, government paternalism and that people rise in the morning as a matter of choice because many people enjoy night-time hours and their jobs do not require them to make the most of daylight. Different people start their day at different times (office workers start their day later than factory workers, who start their day later than farm workers), regardless of daylight saving time.

Mnemonic

The mnemonic "spring forward, fall back" (also "spring ahead, fall back") tells us how to reset clocks when the time changes. This uses the word "fall" to mean "autumn," a usage that is very common in American and Canadian English, though no longer so in British 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 detectors. For example, the 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 fires.

Computer systems

Most modern computer operating systems include the capability to automatically change the local time when daylight saving starts and finishes. See the Time zone article for general information on time zones and computer systems. Israel, until a few years ago, observed DST on different dates each year, and as its new system relies on the Jewish lunar calendar, most computers do not handle Israel Summer Time (IST).

Microsoft Windows

The time zone database in most Windows-based computer systems stores only a single start and end rule for each zone, and daylight saving information is stored in the registry key HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones\, under the TZI registry value. (In Windows XP and Windows 2003, timezone information is stored in the registry key HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\TimeZoneInformation\). For example, DST ends on the last Sunday in October, regardless of year. When the rule changes (e.g. Australian DST ending one week later than usual in 2006, or DST being extended in the United States starting in 2007), an update needs to be applied. In the case of a single-year anomaly, a new time zone is created and used. Before the following year, the time zone will have to be switched back to the original. For permanent rule changes, the rule definition for the time zone can be changed without requiring a new time zone to be set up.

One of the problems of this approach is that software that uses time zone information will get incorrect results if referring to a year with rules that are different from those currently in the database. A good example is the Lotus Notes calendar system, which stores event times in UTC. Events created with calendar dates near DST start and end dates can have their local time interpretation changed after the time zone database is updated (i.e. after an operating system update is applied). Another issue was highlighted when the Australian government changed daylight saving time to end on April 2 instead of March 26, because of the 2006 Commonwealth Games. Microsoft did not modify the start and end rule for the time zones affected, but instead added new timezones with the words "(Commonwealth Games)" Microsoft Download: Australian Daylight Savings Changes for Microsoft Products for the Year 2006; & MS-KB article 909915, "Microsoft products do not reflect Australian daylight saving time changes for the year 2006" - February 27, 2006 which caused various issues with many software applications, including Microsoft OutlookAussie Microsoft calendar mess. Office Watch, volume 11 no. 5, 25 January 2006. and several accounting packages. Workarounds for the issue were to use the Microsoft utility timezone to modify the start and finish of each affected timezone, then either reboot the computer or go into Date and Time in the Control Panel, click on the Time Zone tab and click on OK to force Windows to refresh its daylight saving time information.

Unix

Unix systems (including Linux and Mac OS X) mostly use the Zoneinfo utility which allows a single time zone to have multiple DST rules to handle changes from year to year. As soon as a rule change is announced, it can be safely added to the system. All the standard library routines which calculate times access this database, so software that queries whether a particular date will have DST in effect (for the time zone of the process) will get the correct answers as long as the time zone rule is correct for the year in question.

Java

Java uses a similar database to Unix, so rules for multiple years (not just the current year) can be represented. This database is separate from the underlying operating system time zone database, so it must also be updated when DST rules change.

The name

In the normative form of the name, "daylight saving" is a compound adjective that modifies "time." A common variant is daylight savings time, which is frequently heard in speech and appears in some dictionaries. "daylight saving time." Merriam-Webster Online Dictionary, accessed July 13, 2006. "called also daylight saving, daylight savings, daylight savings time, daylight time" daylight-saving time. The American Heritage Dictionary of the English Language, 4th ed., accessed July 13, 2006. "Variant Forms: or daylight-savings time"

Hour interchange

When DST begins, the day loses one hour (hour interchange of -1). At this date, a full hour is skipped and does not exist either before or after the transition, so this date includes only 23 hours.
When DST ends, the day gains one hour (hour interchange of +1). At this date, the same hour occurs twice, first in daylight (summer) time, and then in standard (winter) time, so this date includes 25 hours.
Note that some areas apply an offset of only one half-hour for their DST, such as Lord Howe Island: at the date of transition to DST, the legal day counts only 23 hours and 30 minutes, and at the date of transition back to standard time, the legal day counts 24 hours and 30 minutes.

Notes

Books

Prerau, David. Seize the Daylight: The Curious and Contentious Story of Daylight Saving Time (Thunder’s Mouth Press; ISBN 1-56025-655-9)
Prerau, David. Saving the Daylight: Why We Put the Clocks Forward (Granta Books; ISBN 1-86207-796-7) — The Story of Summer Time/Daylight Saving time with a focus on the U.K.
Downing, Michael. Spring Forward: The Annual Madness of Daylight Saving Time (Shoemaker & Hoard; ISBN 1-59376-053-1)

External links

A Brief History of DST
Congressional Research Service (CRS) Reports regarding DST
DST by WebExhibits
EU directive 2000/84/EC
Saving Time, Saving Energy - United States DST schedule (including new rules starting in 2007), explanation, history
Sources for daylight saving time data, including histories and predictions
Summer Time by JR Stockton. With future changeover dates EU (until 2007 final, from then extrapolating)
Sleep deficit and accidents
National Association of Standard Time
PhotoSydney: Daylight Saving examines daylight saving in Australia.
www.TimeAndDate.com - Time calculation services

Time zones | Benjamin Franklin

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