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Daylight saving time

Daylight saving time (also called DST, or Summer Time) is the local time a region is designated for a portion of the year, usually an hour forward from its standard official time.

It is a system intended to "save" daylight (as opposed to wasting it by, say, sleeping while the sun shines). 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.

Locations that observe or do not observe DST are listed on the list of time zones. The expression daylight savings time (with the extra "s") is a common alternate form, but considered incorrect English by some.



It is sometimes asserted that DST was first proposed by Benjamin Franklin in a letter to the editors of the Journal of Paris [1] . 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" [2] , published in 1907, but he was unable to get the British government to adopt it despite considerable lobbying.

Canadian railroad engineer Sir Sandford Fleming invented and proposed Standard Time, which first divided the world into one-hour time zones, in 1878. It was not widely adopted by the railways until 1883, and even then it was not supported by any governmental body. However, it relieved the problem of scheduling train stops at separate stations set to their own time based on the local positioning of the sun, and it soon became widely accepted by railroads, freight clients, and passengers.


The idea of daylight saving time was first put into practice by the German government during the First World War between April 30, 1916 and October 1, 1916. Shortly afterwards, the United Kingdom followed suit, first adopting DST between the 21st of May, 1916 and the 1st of October, 1916. Then on March 19, 1918 the U.S. Congress established several time zones (which were already in use by railroads 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so unpopular (mostly because people rose earlier and went to bed earlier than in modern times) that the law was later repealed.

Daylight saving time was reinstated in the United States on February 9, 1942, again as a wartime measure to conserve resources, this time in order to fight World War II. This remained in effect until the war began winding down and the requirement was removed on September 30, 1945. From 1945 to 1966, there was no federal law about daylight saving time. States and localities were free to observe daylight saving time or not. The 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 exempts 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. In response to the 1973 energy crisis, daylight saving was begun earlier in both 1974 and 1975, commencing on the first Sunday in January in the former year and the last Sunday in February in the latter. The law was amended again in 1986 to begin daylight saving time on the first Sunday in April, to take effect the following year.

Criticism of DST

DST is not universally accepted; many localities do not observe it. Nevertheless, proponents claim that DST helps more than it hurts. The primary claim is that it reduces energy consumption. Opponents claim that there's not enough benefit to justify needing to adjust clocks twice per year. The disruption in sleep patterns associated with setting clocks forward, and thereby "losing" an hour, correlates with a spike in the number of severe auto accidents, as well as emotional trauma and lost productivity as tired workers adjust to the schedule change.

There is also a question whether the savings in lighting costs (people just home from work don't turn on the electric lights because there is enough sunlight through the windows) justifies the increase in summertime air conditioning costs (people home from work DO turn up the air conditioning during the late-afternoon peak load times, because it's still warm outside). 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.

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.

DST is a long-standing controversy in Indiana, not only as an agricultural state, but also because the meridian separating the eastern and central time zones divides the state. In the past, neighboring communities sometimes ended up one or even two hours apart. 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 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 use DST.

DST around the world

For fairly obvious reasons, DST is 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 U.S. The Mexican state of Sonora does not observe DST because it borders on a U.S. state that also does not observe DST (Arizona).

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, but it commonly begins in the Northern Hemisphere at 2:00 AM on either the first Sunday in April or the last Sunday in March, and ends at 2:00 AM 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 UK and Chile may be 3, 4 or 5 hours).

North America generally follows the same procedure, going by local time in each zone, each time zone switching at 2am LST (local standard time) to 3am LDT on the first Sunday in April, and again from 2am LDT to 1am LST on the last Sunday in October.

All countries in Europe, except Iceland, observe DST and switch at the same universal time (1:00 UTC) in all five zones, going from 10pm/0/1/2/3am LST to 11pm/1/2/3/4am LDT simultaneously on the last Sunday in March, and back from 11pm/1/2/3/4am LDT to 10pm/0/1/2/3am LST on the last Sunday in October (formerly September) (for the European Union, except the overseas territories, per EU directive 2000/84/EC [3]
 ; for some of Greenland: the Saturday before).

The People's Republic of China experimented with DST from the late 1980s, but abandoned it in the 1990s. The PRC 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 did not stick well with China.

Australia has a mixed implementation of Daylight Saving. During winter it has three time zones, but during Daylight Saving it has five time zones (mostly differing by 30 minutes) ranging from UTC+8 to UTC+11. Although there have been several referenda on the topic, Western Australia, the Northern Territory and Queensland have not adopted the practice. 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 12:00AM of a October (rarely November) Sunday and ends at 12:00AM of a February Sunday. This variation happens because starting and ending DST on a Sunday is better than any other day (at least, that is the belief of Brazilian Government).

The Canadian province of Saskatchewan is the only part of that country which does not follow DST.


The mnemonic "spring forward, fall back" tells us how to reset clocks when the time changes, regardless of hemisphere. This uses the word "fall" to mean "autumn"; while this usage has died out in British English, it is still very common in American English.

Associated practices

Fire safety officials in 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.

See also

External links

  • EU directive 2000/84/EC
  • Straightforward discussion of DST
  • Daylight saving time, its history and why we use it
  • DST changeover times throughout the world
  • - with future changeover dates EU (until 2007 final, from then extrapolating)
  • World Time Server
  • Sleep deficit and accidents
  • Daylight saving humor
  • National Association of Standard Time
  • Greenwich Mean Time: World Time

Last updated: 02-07-2005 18:31:58
Last updated: 05-03-2005 09:00:33