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 standard official time.
"Daylight saving time" is a system intended to "save" daylight, as opposed to "wasting" time (for example, by sleeping 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 list of time zones has information on which areas do or do not observe DST.
The expression daylight savings time (with the extra "s") is a common alternate form but is not considered to be proper English because "saving" is used here as a verbal adjective describing a single type of activity.
It is sometimes asserted that DST was first proposed by Benjamin Franklin in a letter to the editors of the Journal of Paris . 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, 1916 and October 1, 1916. Shortly afterward, the United Kingdom followed suit, first adopting DST between May 21, 1916 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so unpopular (mostly because people rose earlier and went to bed earlier than in modern times) that the law was later repealed.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. This resulted in a patchwork where some areas observed DST and 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 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 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. According to a report from CNN , the U.S. Congress may again pass a bill extending daylight saving time by two months, beginning on the first Sunday in March and ending on the last Sunday in November. The purpose of this bill is to address increasing energy usage, allowing the nation to conserve more energy than it would otherwise.
Rationales for DST
One of the major reasons given for observing DST in the United States 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. $28 million in traffic costs. 
Criticism of DST
DST is not universally accepted; many localities do not observe it. 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 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 AC 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.
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 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. 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 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.
DST around the world
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 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).
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.
All countries in Europe, except Iceland as already noted, observe DST and switch at the same universal time (01:00 UTC) in all five zones, going from 22:00/00:00/01:00/02:00/03:00 LST to 23:00/01:00/02:00/03:00/04:00 LDT simultaneously on the last Sunday in March, and back from 23:00/01:00/02:00/03:00/04:00 LDT to 22:00/00:00/01:00/02:00/03:00 LST on the last Sunday in October (formerly September) (for the European Union, except the overseas territories, per EU directive 2000/84/EC ; for some of Greenland: the Saturday before). As Iceland is in the UTC+0 time zone, while their longitude would indicate UTC−1, they may be said to be on DST all year round.
Israel adopts Daylight Saving Time on the last Friday of March at 02:00, and returns to standard time at midnight of the first Saturday of the month of Tishrei, between Rosh Hashanah and Yom Kippur.
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 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.
The Canadian province of Saskatchewan is the only part of that country (other than northeastern British Columbia) that does not use DST. However, the charter of the city of 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, which is the time used by Alberta.
The mnemonic "spring forward, fall backward" 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.
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.