Daylight saving time

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Daylight saving time (also called DST) is the U.S. term for a system intended to "save" daylight (the British observe summer time, and likewise the Europeans). The official time is adjusted forward, (usually) one hour from its official standard time, remaining that way for the duration of the spring and summer months. This is intended to provide a better match between the hours of daylight and the active hours of work and school. DST is most commonly used in temperate regions, due to the considerable variation in the amount of daylight versus darkness through the seasons in those regions.

Note that the term commonly used in the United States, Daylight Savings Time, is technically incorrect: the correct name as provided by the act which inaugurated it in the United States is Daylight Saving Time.



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 daylight saving time 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 DST between May 21 and October 1, 1916. Then on March 19, 1918, the U.S. Congress established several time zones (which were already in use by railroads 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proved so unpopular (mostly because people rose and went to bed earlier than in current times) that the law was later repealed.

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.

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).

Usage and history by location

South America

Chile switches to DST at 24:00 on the second Saturday in October and reverts to Local Standard Time (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 climatic reasons.

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.

North America

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 1988, 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 George W. Bush, will extend DST and 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.


In Canada, time is under provincial, not federal, jurisdiction. The province of Saskatchewan is the only part of that country (other than northeastern British Columbia and Southampton Island) that does not use DST, that is, it does not change times in spring and fall. Saskatchewan is bisected by the 105th meridian, the central meridian of the Mountain Standard Time Zone (GMT -7), yet clocks are kept at GMT minus six hours 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 in sync with Mountain Daylight Time and in the winter months it is in sync with Central Standard Time. Observationally, this is equivalent to the province being on Mountain Daylight Time year-round, though 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 border, 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. See this document produced by Saskatchewan Government Relations for further details on Saskatchewan's time policies.

United States

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 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 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. (See this article, for example.)

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 and those concerned for the safety of children traveling to school in the dark before the late sunrise (see this article for example).

An additional issue raised by this extension is that it requires reconfiguration of virtually every computer in the United States. Most computers are programmed to adjust automatically for DST, but they 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, which requires some sort of manual intervention by a human being in the great majority of cases. A two-minute procedure for updating a computer, multiplied by a hundred million computers, represents nearly 1700 years of full-time labor. More 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) are almost guaranteed to encounter serious desynchronization problems unless both computers and databases are carefully updated—in some cases by hand.


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 zones 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 out of state due to the time-related confusion. Prior to October 30, 2005, 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, most of the state is in the Eastern time zone; however, its time zone is currently under federal review, as discussed in this article. 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. On October 25, 2005, the U.S. Department of Transportation, which is overseeing the proposed time-zone boundary, published a tenative plan [1], which is as follows:

  • 77 counties, most of the state, will be on Eastern Standard Time and will use DST.
  • The 5 counties near Chicago, Illinois and 5 counties in the southwestern corner of the state that were originally in Central Standard Time remain in Central Standard Time and will use DST.
  • Five additional counties, two near Chicago (St. Joseph and Starke) and three in the southwestern corner (Knox, Pike, and Perry) will be moved to Central Standard Time and will use DST.

The plan, though, is not final, and local hearings will be held in November to hear the residents' opinions on the subject.


Most of Arizona does not observe DST. However, the large Navajo Indian Reservation within it does.


Hawaii does not observe DST.


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


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.


All countries in Europe, except Iceland as noted below, 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 winter time in all zones. Thus in Moscow (local time = UTC+3 in winter, UTC+4 in summer), daylight-saving time 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.


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.



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.


Iran uses the Persian calendar. Thus, DST in Iran starts on the first day of Farvardin (around 21-22 March) and ends on the first day of Mehr (around 22 September).



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.


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 Hebrew calendar. For more on this subject, see Israeli Daylight Savings Law.


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.


In Australia, daylight savings 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 daylight savings time. Tasmania starts DST earlier than the others, usually near the beginning of October.

Western Australia, Northern Territory and Queensland do not.

See the Australian time zones article or this site for maps and further information on standard and daylight savings time in Australia.

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 while 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. $28 million in traffic costs. (Stats from this article).

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,[2] 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.

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.

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.

For example, during a North American time change, an autumn night where clocks are reset from 3 AM summer to 2 AM winter time, times between 2AM and 3AM will occur twice, causing confusion in transport schedules, payment systems, etc.

Some studies do show that changing the clock increases the traffic accident rate. Following the spring shift to Daylight Saving Time (when one hour of sleep is lost) there is a measurable increase in the number of traffic accidents that result in fatalities. Some suspect that the change in lighting conditions confuses drivers in their regular commutes. Others speculate that it relates more to the amount of sleep a person gets. Still others suspect that people hurry because they have looked at an incorrect clock and discover that they are late. And a few even suggest that the traffic accidents are caused by people trying to reset the clocks in their cars while driving.

Some campaigners in 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 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.


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. Another common mnemonic of equal meaning is "spring ahead, fall behind."

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.

The name

In the standard form of the name, "daylight saving" is a compound adjective (part of which is a participle) that modifies "time." A common variant is daylight savings time. Although this alternate form is frequently heard in speech, it is nonstandard and appears rarely in edited writing. Most compound adjectives are joined with a hyphen, but "daylight-saving time," too, is nonstandard. Nevertheless, the form "daylight savings time" appears without remark as to its nonstandardness in some dictionaries, including The American Heritage Dictionary.


  1. ^  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.

See also


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