Daylight saving time (DST; also, summer time in British English; see Terminology) is the convention of advancing clocks so that afternoons have more daylight and mornings have less. Typically clocks are adjusted forward one hour near the start of spring and are adjusted backward in autumn. Modern DST was first proposed in 1907 by the English builder William Willett. Many countries have used it since then; details vary by location and change occasionally.
The practice is controversial. Adding daylight to afternoons benefits retailing, sports, and other activities that exploit sunlight after working hours, but causes problems for farming, entertainment and other occupations tied to the sun. Extra afternoon daylight reduces traffic fatalities; its effect on health and crime is less clear. Although an early goal of DST was to reduce evening usage of incandescent lighting, formerly a primary use of electricity, modern heating and cooling usage patterns greatly differ and research about how DST currently affects energy use is limited and contradictory.
DST's clock shifts can serve as fire safety reminders, but they complicate timekeeping and can disrupt meetings, travel, billing, recordkeeping, medical devices, and heavy equipment. Many computer-based systems can adjust their clocks automatically, but this can be limited and error-prone, particularly when DST rules change.
Although not punctual in the modern sense, ancient civilizations adjusted daily schedules to the sun more flexibly than modern DST does, often dividing daylight into twelve equal hours regardless of day length, so that each daylight hour was longer during summer. For example, Roman water clocks had different scales for different months of the year: at Rome's latitude the third hour from sunrise, hora tertia, started by modern standards at 09:02 solar time and lasted 44 minutes at the winter solstice, but at the summer solstice it started at 06:58 and lasted 75 minutes. After ancient times, equal-length civil hours eventually supplanted unequal, so civil time no longer varies by season. Unequal hours are still used in a few traditional settings, such as some Mount Athos monasteries.
During his time as an American envoy to France, Benjamin Franklin, author of the proverb, "Early to bed, and early to rise, makes a man healthy, wealthy and wise", anonymously published a letter suggesting that Parisians economize on candles by rising earlier to use morning sunlight. This 1784 satire proposed taxing shutters, rationing candles, and waking the public by ringing church bells and firing cannons at sunrise. Franklin did not propose DST; like ancient Rome, 18th-century Europe did not keep accurate schedules. However, this soon changed as rail and communication networks came to require a standardization of time unknown in Franklin's day.
The prominent English builder and outdoorsman William Willett conceived DST in 1905 during a pre-breakfast ride, when he observed with dismay how many Londoners slept through the best part of a summer day. An avid golfer, he also disliked cutting short his round at dusk. His solution was to advance the clock during the summer months, a proposal he published two years later. He lobbied unsuccessfully for the proposal until his death in 1915; see Politics for more details.
Germany, its World War I allies, and their occupied zones were the first European nations to use Willett's invention, starting April 30, 1916. Britain, most of its allies, and many European neutrals soon followed suit; Russia and a few other countries waited until the next year; and the United States adopted it in 1918. Since then, the world has seen many enactments, adjustments, and repeals.
Willett's 1907 proposal argued that DST increases opportunities for outdoor leisure activities during afternoon sunlight hours. Obviously it does not change the length of the day; the longer days nearer the summer solstice in high latitudes merely offer more room to shift apparent daylight from morning to evening so that early morning daylight is not wasted. DST is commonly not observed during most of winter, because its mornings are darker: workers may have no sunlit leisure time, and children may need to leave for school in the dark.
General agreement about the day's layout confers so many advantages that a standard DST schedule usually outranks ad hoc efforts to get up earlier, even for people who personally dislike the DST schedule. The advantages of coordination are so great that many people ignore whether DST is in effect by altering their nominal work schedules to coordinate with television broadcasts or daylight.
Several studies have suggested that DST increases motor fuel consumption. U.S. gasoline demand grew an extra 1% during the newly introduced DST in March 2007.
Clock shifts correlate with decreased economic efficiency. In 2000 the daylight-saving effect implied an estimated one-day loss of $31 billion on U.S. stock exchanges. Clock shifts and DST rule changes have a direct economic cost, entailing extra work to support remote meetings, computer applications and the like. For example, a 2007 North American rule change cost an estimated $500 million to $1 billion.
In the 1970s the U.S. Law Enforcement Assistance Administration (LEAA) found a reduction of 10% to 13% in Washington, D.C.'s violent crime rate during DST. However, the LEAA did not filter out other factors, and it examined only two cities and found crime reductions only in one and only in some crime categories; the DOT decided it was "impossible to conclude with any confidence that comparable benefits would be found nationwide". Outdoor lighting has a marginal and sometimes even contradictory influence on crime and fear of crime.
In several countries, fire safety officials encourage citizens to use the two annual clock shifts as reminders to replace batteries in smoke and carbon monoxide detectors, particularly in autumn, just before the heating and candle season causes an increase in home fires. Similar twice-yearly tasks include reviewing and practicing fire escape and family disaster plans, inspecting vehicle lights, checking storage areas for hazardous materials, and reprogramming thermostats. This is not an essential function of DST, as locations without DST can instead use the first days of spring and autumn as reminders.
DST has mixed effects on health. In societies with fixed work schedules it provides more afternoon sunlight for outdoor exercise. It alters sunlight exposure; whether this is beneficial depends on one's location and daily schedule, as sunlight triggers vitamin D synthesis in the skin, but overexposure can lead to skin cancer. Sunlight strongly influences seasonal affective disorder. DST may help in depression by causing individuals to rise earlier, but some argue the reverse. The Retinitis Pigmentosa Foundation Fighting Blindness, chaired by blind sports magnate Gordon Gund, successfully lobbied in 1985 and 2005 for U.S. DST extensions, but DST can hurt night blindness sufferers.
Clock shifts disrupt sleep and reduce its efficiency. Effects on seasonal adaptation of the circadian rhythm can be severe and last for weeks. A 2008 study found that although male suicide rates rise in the weeks after the spring transition, the relationship weakened greatly after adjusting for season. The government of Kazakhstan cited health complications due to clock shifts as a reason for abolishing DST in 2005.
Some computer-based systems require downtime or restarting when clocks shift; ignoring this requirement damaged a German steel facility in 1993. Medical devices may generate adverse events that could harm patients, without being obvious to clinicians responsible for care. These problems are compounded when the DST rules themselves change, as in the Year 2007 problem. Software developers must test and perhaps modify many programs, and users must install updates and restart applications.
Some clock-shift problems could be avoided by adjusting clocks continuously or at least more gradually—for example, Willett originally suggested weekly 20-minute transitions—but this would add complexity and has never been implemented.
DST inherits and can magnify the disadvantages of standard time. For example, when reading a sundial, one must compensate for it along with time zone and natural discrepancies. Also, sun-exposure rules like "avoid the sun within two hours of noon" become less accurate when DST is in effect.
The fate of Willett's 1907 proposal illustrates several political issues involved. The proposal attracted many supporters, including Balfour, Churchill, Lloyd George, MacDonald, Edward VII (who used half-hour DST at Sandringham), the managing director of Harrods, and the manager of the National Bank. However, the opposition was stronger: it included Prime Minister Asquith, Christie (the Astronomer Royal), George Darwin, Napier Shaw (director of the Meteorological Office), many agricultural organizations, and theater owners. After many hearings the proposal was narrowly defeated in a Parliament committee vote in 1909. Willett's allies introduced similar bills every year from 1911 through 1914, to no avail. The U.S. was even more skeptical: Andrew Peters introduced a DST bill to the U.S. House in May 1909, but it soon died in committee.
World War I changed the political equation, as DST was promoted as a way to alleviate hardships from wartime coal shortages and air raid blackouts. After Germany led the way, the United Kingdom first used DST on May 21, 1916. U.S. retailing and manufacturing interests led by Pittsburgh industrialist Robert Garland soon began lobbying for DST, but were opposed by railroads. The U.S.'s 1917 entry to the war overcame objections, and DST was established in 1918.
War's end swung the pendulum back. Farmers continued to dislike DST, and many countries repealed it after the war. Britain was an exception: it retained DST nationwide but over the years adjusted transition dates for several reasons, including special rules during the 1920s and 1930s to avoid clock shifts on Easter mornings. The U.S. was more typical: Congress repealed DST after 1919. President Woodrow Wilson, like Willett an avid golfer, vetoed the repeal twice but his second veto was overridden, and only a few U.S. cities retained DST locally thereafter. Wilson's successor Warren G. Harding opposed DST as a "deception". Reasoning that people should instead get up and go to work earlier in the summer, he ordered District of Columbia federal employees to start work at 08:00 rather than 09:00 during summer 1922. Many businesses followed suit though many others did not; the experiment was not repeated.
Since Willett's day the world has seen many enactments, adjustments, and repeals of DST, with similar politics involved. The history of time in the United States includes DST during both world wars, but no standardization of peacetime DST until 1966. In the mid-1980s, Clorox (parent of Kingsford Charcoal) and 7-Eleven provided the primary funding for the Daylight Saving Time Coalition behind the 1987 extension to U.S. DST, and both Idaho senators voted for it on the basis of fast-food restaurants selling more French fries made from Idaho potatoes; in 2005, the Sporting Goods Manufacturers Association and the National Association of Convenience Stores successfully lobbied for the 2007 extension to U.S. DST. In early 2007, Western Australia continued to debate a trial use of DST and several politicians changed positions after public sentiment swung against it. In the UK the sport and leisure industry supports a proposal to observe SDST's additional hour year-round.
In a typical case where a one-hour shift occurs at 02:00 local time, in spring the clock jumps forward from 02:00 standard time to 03:00 DST and the day has 23 hours, whereas in autumn the clock jumps backward from 02:00 DST to 01:00 standard time, repeating that hour, and the day has 25 hours. A digital display of local time does not read 02:00 exactly, but instead jumps from 01:59:59.9 either forward to 03:00:00.0 or backward to 01:00:00.0. In this example, a location observing UTC+10 during standard time is at UTC+11 during DST; conversely, a location at UTC−10 during standard time is at UTC−9 during DST.
Clock shifts are usually scheduled near a weekend midnight to lessen disruption to weekday schedules. A one-hour shift is customary, but Australia's Lord Howe Island uses a half-hour shift. Twenty-minute and two-hour shifts have been used in the past.
Coordination strategies differ when adjacent time zones shift clocks. The European Union shifts all at once, at 01:00 UTC; for example, Eastern European Time is always one hour ahead of Central European Time. Most of North America shifts at 02:00 local time, so its zones do not shift at the same time; for example, Mountain Time can be temporarily either zero or two hours ahead of Pacific Time. Australian districts go even further and do not always agree on start and end dates; for example, to start DST in 2006 Tasmania shifted clocks forward on October 1, Western Australia on December 3, and the remaining DST-observing areas on October 29.
Start and end dates vary with location and year. Since 1996 European Summer Time has been observed from the last Sunday in March to the last Sunday in October; previously the rules were not uniform across the European Union. Starting in 2007, most of the United States and Canada observe DST from the second Sunday in March to the first Sunday in November, almost two-thirds of the year. The 2007 U.S. change was part of the Energy Policy Act of 2005; previously, from 1987 through 2006, the start and end dates were the first Sunday in April and the last Sunday in October, and Congress retains the right to go back to the previous dates once an energy-consumption study is done.
Beginning and ending dates are the reverse in the southern hemisphere. For example, mainland Chile observes DST from the second Saturday in October to the second Saturday in March, with transitions at 24:00 local time. The time difference between the United Kingdom and mainland Chile may therefore be three, four, or five hours, depending on the time of year.
Western China, Iceland, and other areas skew time zones westward, in effect observing DST year-round without complications from clock shifts. For example, Saskatoon, Saskatchewan, is at longitude, slightly west of center of the idealized Mountain Time Zone , but the time in Saskatchewan is Central Standard Time year-round, so Saskatoon is always about 67 minutes ahead of mean solar time. Conversely, northeast India and a few other areas skew time zones eastward, in effect observing negative DST. The United Kingdom and Ireland experimented with year-round DST from 1968 to 1971 but abandoned it because of its unpopularity, particularly in northern regions.
Western France, Spain, and other areas skew time zones and shift clocks, in effect observing DST in winter with an extra hour in summer. For example, Nome, Alaska, is at longitude, which is just west of center of the idealized Samoa Time Zone , but Nome observes Alaska Time with DST, so it is slightly more than two hours ahead of the sun in winter and three in summer.
DST is generally not observed near the equator, where sunrise times do not vary enough to justify it. Some countries observe it only in some regions; for example, southern Brazil observes it while equatorial Brazil does not. Only a minority of the world's population uses DST because Asia and Africa generally do not observe it.
Time zone names typically change when DST is observed. American English replaces standard with daylight: for example, Pacific Standard Time (PST) becomes Pacific Daylight Time (PDT). British English uses summer: for example, Greenwich Mean Time (GMT) becomes British Summer Time (BST). Abbreviations do not always change: for example, many (though not all) Australians say that Eastern Standard Time (EST) becomes Eastern Summer Time (also EST).
The American English mnemonic "spring forward, fall back" (also "spring ahead …", "spring up …", and "… fall behind") helps people remember which direction to shift clocks. Much of North America now advances clocks before the vernal equinox, so the mnemonic disagrees with the astronomical definition of spring, but a proposed substitute "March forward … works only in the northern hemisphere, and is less robust against future rule changes.
Many computer-based systems can shift their clocks automatically when DST starts and finishes, based on their time zone settings. Two implementations in wide use today are zoneinfo and Microsoft Windows. Some applications standardize on UTC to avoid problems with clock shifts and time zone differences.
Older or stripped-down systems may support only the TZ values required by POSIX, which specify at most one start and end rule explicitly in the value. For example,
TZ='EST5EDT,M3.2.0/02:00,M11.1.0/02:00' specifies time for eastern North America starting in 2007. TZ must be changed whenever DST rules change, and the new TZ value applies to all years, mishandling some older time stamps.
These limitations have caused problems. For example, before 2005, DST in Israel varied each year and was skipped some years. Windows 95 used rules correct for 1995 only, causing problems in later years. In Windows 98 Microsoft gave up and marked Israel as not having DST, forcing Israeli users to shift their computer clocks manually twice a year. The 2005 Israeli Daylight Saving Law established predictable rules but Windows zone files cannot represent the rules' dates in a year-independent way. Partial workarounds, which mishandle older time stamps, include manually switching zone files every year and a Microsoft tool that switches zones automatically.