Daylight saving time

Daylight saving time (DST), called summer time in Britain and the European Union, is the practice of setting clocks one hour ahead of standard time for part of the year, so that more daylight falls in the evening and less in the morning.

What it does

The U.S. National Institute of Standards and Technology (NIST) describes the change as moving one hour of daylight from the morning to the evening when clocks go forward, and moving it back again when they return to standard time. Standard time is the legal time of a time zone for the rest of the year. The familiar memory aid "spring forward, fall back" appears in NIST's own explanation.

The change does not add daylight; it shifts the hours at which people do things against the sun. On the day clocks go forward, one clock hour is skipped and the day is 23 hours long; on the day they go back, one hour is repeated and the day is 25 hours long. NIST gives the U.S. example: in spring, at 2 a.m. the local time skips ahead to 3 a.m., and in autumn, at 2 a.m. the local time becomes 1 a.m. and that hour is repeated.

Origins

Early ideas

Benjamin Franklin is often credited with the idea, but the Franklin Institute in Philadelphia states plainly that daylight saving time is "one thing that Franklin did not invent". His 1784 piece "An Economical Project", published in the Journal de Paris, was a satirical essay arguing that Parisians could save large sums on candles by using sunlight instead. The same institute credits George Hudson, a New Zealand entomologist, with proposing the idea in 1895.

William Willett and the Daylight Saving Bill

In Britain the campaign is associated with William Willett, who published and circulated a pamphlet, The Waste of Daylight. Its nineteenth edition, dated March 1914, sets out the proposal: "If we will reduce the length of one Sunday, in the Spring, by 60 minutes, a loss of which no one would be conscious, we shall have 60 minutes more daylight after 6 o'clock, on each succeeding day, until the Autumn." In that edition Willett proposed that at 2 a.m. on the third Sunday in April standard time should advance 60 minutes, and recede 60 minutes on the third Sunday in September, giving "one Sunday in April 23 hours long, and one Sunday in September 25 hours long."

The pamphlet records that a Daylight Saving Bill was first introduced in the House of Commons by Robert Pearce in 1908 and referred to a Select Committee, which examined 41 witnesses. The committee's reported findings included that the measure would "reduce the industrial, commercial, and domestic expenditure, on artificial light" and "benefit the physique, general health, and welfare of all classes of the community." A second bill, introduced by T. W. Dobson in 1909, passed its second reading by 130 votes to 94. The pamphlet also notes that copies of it had been circulated in France and Germany.

Wartime adoption

The measure was first adopted in wartime. A 2018 European Commission proposal summarises the history: summertime arrangements were first adopted by Germany and France during the First World War "to conserve coal, in particular the one consumed for lighting purposes", and Britain, most of its allies and many neutral European countries soon followed. Many European countries abandoned it after the two world wars ended.

In the United States, NIST records that DST was formally introduced in 1918. The federal statute that set up the country's standard time zones is the Act of March 19, 1918, commonly known as the Calder Act.

Modern rules

United States

The present federal framework is the Uniform Time Act of 1966, Public Law 89-387, enacted 13 April 1966. As amended, it provides that from 2 a.m. on the second Sunday of March to 2 a.m. on the first Sunday of November, the standard time of each zone "shall be advanced one hour."

The dates have moved several times. A 1986 amendment changed the start from the last Sunday of April to the first Sunday of April. The Energy Policy Act of 2005 moved the start to the second Sunday of March and the end from the last Sunday of October to the first Sunday of November, taking effect on 1 March 2007. NIST notes that this lengthened DST by about a month, to 238 days, or about 65% of the year, and that the change was made "in the interest of reducing energy consumption." The statute's notes record that Congress kept the right to revert to the 2005 schedule once a Department of Energy study was complete.

During the 1970s energy crisis, the Emergency Daylight Saving Time Energy Conservation Act of 1973 extended daylight saving time; it took effect at 2 a.m. on the fourth Sunday after 15 December 1973 and ended at 2 a.m. on the last Sunday of April 1975.

States may opt out but may not opt in to permanent DST. Under the 1966 Act a state lying within one time zone may exempt itself by law, but only if the entire state then keeps standard time, and a state in more than one zone may exempt the part lying within one zone. NIST lists the places that do not observe DST as Hawaii, American Samoa, Guam, Puerto Rico, the Virgin Islands and Arizona, except the Navajo Indian Reservation, which does observe it. DST and time zones are regulated by the U.S. Department of Transportation, not by NIST.

For 2026, NIST gives the period as 8 March at 2 a.m. to 1 November at 2 a.m., local time.

European Union and United Kingdom

In the European Union, Directive 2000/84/EC of 19 January 2001 sets a common schedule. It defines the "summer-time period" as the period during which clocks are put forward by 60 minutes, and provides that from 2002 it begins at 1.00 a.m. Greenwich Mean Time on the last Sunday in March and ends at 1.00 a.m. GMT on the last Sunday in October in every member state. Because the change is fixed in GMT, it happens at the same moment across the EU's time zones. The directive's stated reason is that a common date and time are important for the functioning of the internal market.

In Great Britain, the Summer Time Act 1972 provides that during summer time the time for general purposes is one hour in advance of Greenwich mean time, from 1 a.m. GMT on the last Sunday in March to 1 a.m. GMT on the last Sunday in October, the same moments as in the EU.

The European Commission's 2018 history notes that the UK and Ireland abolished summertime arrangements in 1968 and switched back in 1972; that modern arrangements began with Italy (1966) and Greece (1971); and that between 1976 and 1981 ten member states introduced them, mostly to align with their neighbours.

Elsewhere

According to the same Commission document, summertime arrangements were observed in about 60 countries as of 2018, including in North America and Oceania, while a number of countries had chosen not to use them or had abolished them: Iceland, China (from 1991), Russia (from 2011), Belarus (from 2011) and Turkey (from 2016).

Effects and evidence

Energy

Energy saving has long been a stated reason for DST, and it was the stated purpose of the 2005 U.S. extension, but measured savings are small. The U.S. Department of Energy's report to Congress of October 2008, required by the 2005 Act, found that the extended DST of 2007 saved about 1.3 terawatt-hours of electricity, 0.5% per day of extended DST, or 0.03% of electricity consumption over the year, against a national total of 3,900 TWh. Savings came mostly over three to five hours in the evening, with small increases in early-morning use, and were slightly smaller in some southern regions, possibly because of increased air conditioning.

The European Commission's 2018 review reached a similar view, stating that research indicates the overall energy savings effect of summer-time is marginal, though results vary by location. It cited the Italian grid operator Terna's 2016 estimate of about 580 GWh a year (around 0.2% of Italy's annual electricity consumption), and noted that newer lighting technology reduces the potential for savings.

Health and safety

The evidence on health is disputed. The European Commission described findings that the human biological rhythm adjusts less well than previously thought to the spring change and that the autumn change poses fewer problems, while also noting possible benefits from more outdoor leisure; it concluded that the evidence on overall health impacts "remains inconclusive", as does the evidence on road accidents.

The American Academy of Sleep Medicine has taken a firm position. In a statement released on 27 August 2020, accompanying a position statement in the Journal of Clinical Sleep Medicine, it said that "Public health and safety would benefit from eliminating daylight saving time" and favoured permanent standard time, citing evidence of increased risks of motor vehicle accidents, cardiovascular events and mood disturbances after the spring change. An earlier AASM health advisory, adopted on 3 March 2016, said that "the sudden change in clock time has an untoward effect on sleep duration and quality."

Proposals to end the clock change

There are two ways to stop changing the clocks: stay on standard time all year, or stay on summer time all year. Both have been proposed.

European Union. The European Commission held a public consultation from 4 July to 16 August 2018 that drew about 4.6 million replies, over 99% from citizens; 84% favoured ending the twice-yearly change and 16% wanted to keep it. Respondents most often cited human health, followed by the lack of energy saving. On 12 September 2018 the Commission proposed a directive to end seasonal changes while letting each member state choose whether to stay permanently on its summer time or its "winter time". The European Parliament adopted its first-reading position on 26 March 2019. As of November 2026, the European Parliament's Legislative Observatory lists the procedure as awaiting the Council's first-reading position, so Directive 2000/84/EC still governs the schedule.

United States. The Sunshine Protection Act of 2021 (S. 623), a bill "to make daylight saving time permanent", passed the Senate on 15 March 2022. It would have repealed the seasonal period in the Uniform Time Act and advanced every zone's standard time by one hour, from 5 November 2023, while letting states that had exempted themselves keep their existing standard time. The seasonal change remained in force in 2026, as NIST's published dates for that year show. Federal law as it stands lets states choose permanent standard time but not permanent DST, which is why proposals for year-round DST require an act of Congress.

Supporters of year-round DST point to lighter evenings, the original rationale of Willett's campaign. Supporters of year-round standard time, including the AASM, argue that it most closely matches the body's circadian sleep-wake cycle, and that permanent DST would mean more darkness in the morning.

Timekeeping and computers

Computers handle the change themselves. NIST explains that its time services broadcast Coordinated Universal Time (UTC), and that corrections for local time zone and daylight saving time are provided by a computer's operating system. The 2007 U.S. rule change therefore required software updates, and NIST warns that some older self-setting clocks with the old rule built in will switch on the wrong date. The European Commission likewise noted that ending seasonal changes would carry transition costs, since IT systems would have to be reprogrammed and transport timetables adjusted.

Sources

• National Institute of Standards and Technology, "Daylight Saving Time Rules", Time and Frequency Division (updated 9 February 2026, read November 2026). nist.gov/pml/time-and-frequency-division/popular-links/daylight-sav…
• 15 U.S. Code § 260a, "Advancement of time or changeover dates", with statutory notes, Legal Information Institute, Cornell Law School (read November 2026). law.cornell.edu/uscode/text/15/260a
• S. 623 (117th Congress), Sunshine Protection Act of 2021, as engrossed in the Senate, 15 March 2022, U.S. Government Publishing Office. govinfo.gov/content/pkg/BILLS-117s623es/html/BILLS-117s623es.htm
• Directive 2000/84/EC of the European Parliament and of the Council of 19 January 2001 on summer-time arrangements, EUR-Lex. eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:32000L0084
• European Commission, Proposal for a Directive discontinuing seasonal changes of time and repealing Directive 2000/84/EC, COM(2018) 639 final, 12 September 2018, EUR-Lex. eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:52018PC0639
• European Parliament, Legislative Observatory, procedure file 2018/0332(COD), "Seasonal changes of time" (read November 2026). oeil.europarl.europa.eu/oeil/en/procedure-file?reference=2018/0332%…
• Summer Time Act 1972, section 1, legislation.gov.uk (read November 2026). legislation.gov.uk/ukpga/1972/6/section/1
• William Willett, The Waste of Daylight, nineteenth edition (London, March 1914), digitised from the Royal College of Surgeons of England, Internet Archive. archive.org/details/b22443800
• The Franklin Institute, "Did Ben Franklin Invent Daylight Saving Time?" (read November 2026). fi.edu/benjamin-franklin/daylight-savings-time
• U.S. Department of Energy, Impact of Extended Daylight Saving Time on National Energy Consumption: Report to Congress, Energy Policy Act of 2005, Section 110, October 2008. www1.eere.energy.gov/analysis/pdfs/epact_sec_110_edst_report_to_con…
• American Academy of Sleep Medicine, "American Academy of Sleep Medicine calls for elimination of daylight saving time", 27 August 2020. aasm.org/american-academy-of-sleep-medicine-calls-for-elimination-o…
• American Academy of Sleep Medicine, "Daylight Saving Time Health Advisory", adopted 3 March 2016. aasm.org/advocacy/position-statements/daylight-saving-time-health-a…