Daylight saving time

As of 2023, around 34 percent of the world's countries use DST, primarily in Europe and North America, with some countries observing it only in some regions. Six of the ten provinces and one of the three territories in Canada observe DST, with some regional exceptions. It is observed by four Australian states and one territory. In the United States, it is observed everywhere except the states of Hawaii and Arizona (within the latter, however, the Navajo Nation does observe it) and the territories of the US.

Historically, there is evidence that ancient societies adopted seasonal changes to their timekeeping to make better use of daylight. However, these were changes to the time divisions of the day rather than setting the whole clock forward. In a satirical letter to the editor of the Journal de Paris in 1784, Benjamin Franklin suggested that if Parisians could only wake up earlier in the summer they would economize on candle and oil usage, but he did not propose changing the clocks. In 1895, New Zealand entomologist and astronomer George Hudson made the first realistic proposal to change clocks by two hours every spring to the Wellington Philosophical Society, but this was not implemented until 1928 and in another form. In 1907, William Willett proposed the adoption of British Summer Time as a way to save energy; although seriously considered by Parliament, it was not implemented until 1916.

The first implementation of DST was by Port Arthur and Fort William (later merged into Thunder Bay), in Ontario, Canada, in 1908, but only locally, not nationally. The first nation-wide implementations were by the German and Austro-Hungarian Empires, both starting on 30 April 1916. Since then, many countries have adopted DST at various times, particularly since the 1970s energy crisis.

Industrialized societies usually follow a clock-based schedule for daily activities that do not change throughout the course of the year. The time of day that individuals begin and end work or school, and the coordination of mass transit, for example, usually remain constant year-round. In contrast, an agrarian society's daily routines for work and personal conduct are more likely governed by the length of daylight hours and by solar time, which change seasonally because of the Earth's axial tilt. North and south of the tropics, daylight lasts longer in that hemisphere's summer and is shorter in that hemisphere's winter, with the effect becoming greater the farther one moves away from the equator. DST is of little use for locations near the Equator, because these regions see only a small variation in daylight over the course of the year.

After synchronously resetting the clocks in a region to one hour ahead of standard time in spring in anticipation of longer daylight hours, individuals following a clock-based schedule will be awakened an hour earlier in the solar day than they would have been otherwise. They will begin and complete daily work routines an hour earlier; in most cases, they will have the hour of daylight that was shifted from the morning available to them after their workday activities.

The clock shift is partly motivated by practicality. At the summer solstice, in American temperate latitudes, for example, the sun rises around 04:30 standard time and sets around 19:30. Since most people are asleep at 04:30, it is seen as practical to treat 04:30 as if it were 05:30, thereby allowing people to wake closer to the sunrise and be active in the evening light, as the sun under DST sets an hour later (20:30).

Proponents of daylight saving time argue that most people prefer more daylight hours after the typical "nine to five" workday. Supporters have also argued that DST decreases energy consumption by reducing the need for lighting and heating, but the actual effect on overall energy use is heavily disputed. …

Ancient civilizations adjusted daily schedules to the sun more flexibly than DST does, often dividing daylight into 12 hours regardless of daytime, so that each daylight hour became progressively longer during spring and shorter during autumn. For example, the Romans kept time with water clocks that had different scales for different months of the year; at Rome's latitude, the third hour from sunrise (hora tertia) started 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. From the 14th century onward, equal-length civil hours supplanted unequal ones, so civil time no longer varied by season. Unequal hours are still used in a few traditional settings, such as monasteries of Mount Athos and in Jewish ceremonies.

Benjamin Franklin published the proverb "early to bed and early to rise makes a man healthy, wealthy, and wise", and published a letter in the Journal de Paris when he was an American envoy to France (1776–1785) suggesting that Parisians economize on candles by rising earlier to use morning sunlight. This 1784 satire proposed taxing window shutters, rationing candles, and waking the public by ringing church bells and firing cannons at sunrise. Despite common misconception, Franklin did not actually propose DST; 18th-century Europe did not even keep precise schedules. However, this changed as rail transport and communication networks required a standardization of clocks unknown in Franklin's day.

In 1810, the Spanish National Assembly Cortes of Cádiz issued a regulation that moved certain meeting times forward by one hour from 1 May to 30 September in recognition of seasonal changes, but it did not change the clocks. It also acknowledged that private businesses were in the practice of changing their opening hours to suit daylight conditions, but they did so of their volition.

New Zealand entomologist George Hudson first proposed modern DST. His shift-work job gave him spare time to collect insects and led him to value after-hours daylight. In 1895, he presented a paper to the Wellington Philosophical Society proposing a two-hour daylight-saving shift, and considerable interest was expressed in Christchurch; he followed up with an 1898 paper. …

The relevant authorities typically schedule clock changes to occur at (or soon after) midnight and on a weekend, in order to lessen disruption to weekday schedules. A one-hour change is usual, but twenty-minute and two-hour changes have been used in the past. Notable exceptions today include Lord Howe Island with a thirty-minute change, and Troll (research station) that shifts two hours directly between GMT and CEST since 2016. In all countries that observe daylight saving time seasonally (i.e., during summer and not winter), the clock is advanced from standard time to daylight saving time in the spring, and it is turned back from daylight saving time to standard time in the autumn.

For a midnight change in spring, a digital display of local time would appear to jump from 23:59:59.9 to 01:00:00.0. For the same clock in autumn, the local time would appear to repeat the hour preceding midnight, i.e. it would jump from 23:59:59.9 to 23:00:00.0.

In most countries that observe seasonal daylight saving time, clocks revert in winter to standard time. An exception exists in Ireland, where its winter clock is offset from Irish Standard Time, having the same offset (UTC+00:00) and legal name as that in Britain (Greenwich Mean Time). In spring, Ireland returns to Irish Standard Time, which has the same offset as British Summer Time (UTC+01:00).

Between 2019 and 2026, Morocco observed daylight saving time every month but Ramadan. During the holy month, the country's civil clocks observed Western European Time (UTC+00:00, which geographically overlaps most of the nation). At the close of that month, its clocks were advanced to Western European Summer Time (UTC+01:00).

The time at which to change clocks differs across jurisdictions. Members of the European Union conduct a coordinated change, changing all zones at the same instant, at 01:00 Coordinated Universal Time (UTC), which means that it changes at 02:00 Central European Time (CET), equivalent to 03:00 Eastern European Time (EET). As a result, the time differences across European time zones remain constant. North American coordination of the clock change differs, in that each jurisdiction changes at each local clock's 02:00, which temporarily creates an imbalance with the next time zone (until it adjusts its clock, one hour later, at 2 am there). …

The concept of daylight saving has caused controversy since its early proposals. Winston Churchill argued that it enlarges "the opportunities for the pursuit of health and happiness among the millions of people who live in this country" and pundits have dubbed it "Daylight Slaving Time". Retailing, sports, and tourism interests have historically favored daylight saving, while agricultural and evening-entertainment interests (and some religious groups) have opposed it; energy crises and war prompted its initial adoption.

Willett's 1907 proposal illustrates several political issues. It attracted many supporters, including Arthur Balfour, Churchill, David Lloyd George, Ramsay MacDonald, King Edward VII (who used half-hour DST or "Sandringham time" at Sandringham), the managing director of Harrods, and the manager of the National Bank Ltd. However, the opposition proved stronger, including Prime Minister H. H. Asquith, William Christie (the Astronomer Royal), George Darwin, Napier Shaw (director of the Meteorological Office), many agricultural organizations, and theatre-owners. After many hearings, a parliamentary committee vote narrowly rejected the proposal in 1909. Willett's allies introduced similar bills every year from 1911 through 1914, to no avail. People in the US demonstrated even more skepticism; Andrew Peters introduced a DST bill to the House of Representatives in May 1909, but it soon died in committee.

Germany and its allies led the way in introducing DST during World War I on 30 April 1916, aiming to alleviate hardships due to wartime coal shortages and air-raid blackouts. The political equation changed in other countries; the United Kingdom used DST first on 21 May 1916. US retailing and manufacturing interests—led by Pittsburgh industrialist Robert Garland—soon began lobbying for DST, but railroads opposed the idea. The US' 1917 entry into the war overcame objections, and DST started in 1918.

The end of World War I brought a change in DST use. Farmers continued to dislike DST, and many countries repealed it—like Germany itself, which dropped DST from 1919 to 1939 and from 1950 to 1979. Britain proved an exception; it retained DST nationwide but adjusted transition dates over the years for several reasons, including special rules during the 1920s and 1930s to avoid clock shifts on Easter mornings. …