What is daylight saving time?

What is daylight saving time?

Daylight saving time is a public-policy practice of advancing a country or region's clocks by one hour for a defined summer period each year, then returning them to standard time for the rest of the year. The European Union codifies the same definition in its summer-time directive, which describes the summer-time period as "the period of the year during which clocks are put forward by 60 minutes compared with the rest of the year."¹

DST applies to local civil time, not to UTC. UTC has no DST and no seasonal shifts; instead, each time zone is encoded in the IANA Time Zone Database — the operational reference for every region's history of offsets and DST rules — as a sequence of UTC offsets and transition rules.¹¹² When a zone springs forward, its UTC offset increases by one hour for the duration of the summer-time period, then decreases again at the autumn transition.

Naming conventions vary. North America says daylight saving time (singular "saving"); the European Union and most of the Commonwealth say summer time. Time-zone abbreviations follow the same split: EDT, CDT, MDT, and PDT in the United States; CEST, BST, and EEST in Europe; NZDT in New Zealand; AEDT in south-eastern Australia.²¹²

When does daylight saving time start and end?

In the United States, DST runs from 02:00 local time on the second Sunday of March to 02:00 local time on the first Sunday of November. These dates were set by the Energy Policy Act of 2005 and have been in effect since March 2007.⁵⁶ In 2026 that means DST is in effect from 8 March to 1 November — about 238 days, or roughly 65% of the year, in DST and the remaining 127 days on standard time.⁶

In the European Union, a 2001 directive harmonises summer time across all member states. Clocks in every EU country move forward at 01:00 GMT on the last Sunday of March and fall back at 01:00 GMT on the last Sunday of October.¹ (The directive uses Greenwich Mean Time, not UTC, in its operative text. The two are equivalent for scheduling clock transitions, but the directive's drafting predates UTC's now-universal use in legal documents.) The United Kingdom keeps the same dates under the Summer Time Act 1972 and has retained them since Brexit.¹³

In the southern hemisphere the seasons are reversed, so DST runs on the opposite calendar. New Zealand DST currently runs from the last Sunday of September to the first Sunday of April; south-eastern Australia (New South Wales, Victoria, Tasmania, South Australia, and the Australian Capital Territory) runs from the first Sunday of October to the first Sunday of April.¹¹

What happens at the spring-forward and fall-back transitions?

Spring-forward and fall-back are the two transitions that bookend the summer-time period each year. At the spring transition the local clock skips an hour; at the autumn transition it repeats one. Both leave UTC and the actual elapsed time unchanged — the wall clock is the only thing that moves.

The United States National Institute of Standards and Technology describes the mechanic in plain terms. At the start of DST, "at 2 a.m. the local time skips ahead to 3 a.m. so there is one less hour in that day." At the end of DST, "at 2 a.m. the local time becomes 1 a.m. and that hour is repeated, so there is an extra hour in that day."⁶

The two mechanics produce different problems for software and scheduling. The skipped-forward hour creates non-existent local times — in the United States, no real instant is labelled 02:30 on the second Sunday of March, because the clock jumps directly from 02:00 to 03:00. The repeated hour creates ambiguous local times — 01:30 on the first Sunday of November labels two real instants an hour apart. Older programming-language conventions for distinguishing the two cannot resolve every case unambiguously,² which is why modern timestamp formats and time-zone-aware libraries always carry an explicit UTC offset rather than relying on local labels alone.

Where did daylight saving time come from?

The first published proposal of seasonal clock-shifting came from George Vernon Hudson, a New Zealand entomologist working shift hours at the General Post Office in Wellington. In a paper read to the Wellington Philosophical Society in 1895, Hudson proposed advancing clocks by two hours each summer to give workers more daylight after work.¹⁴ His proposal was politely received but not acted upon.

A decade later, the London builder William Willett published The Waste of Daylight (1907), which proposed advancing clocks by 80 minutes in four 20-minute steps on consecutive Sundays in April and reversing the same way in September.¹⁵ The pamphlet went through nineteen editions in English and other languages between 1907 and 1914, and underpinned a series of summer-time bills repeatedly debated in the UK Parliament from 1908 onward.¹² Willett died in March 1915, a year before the policy he had spent eight years promoting was adopted.

DST was first put into effect as a wartime fuel-conservation measure during the First World War, beginning with Germany in spring 1916.³ The United Kingdom followed within weeks: the Summer Time Act 1916 received royal assent on 17 May 1916, and the first British summer time ran from 21 May 1916 to 1 October 1916.⁴ The United States adopted both standard time and DST simultaneously in March 1918 under the Standard Time Act, with spring-forward on the last Sunday of March and fall-back on the last Sunday of October.⁵

US DST was repealed in 1919 over a presidential veto, and reverted to a patchwork of local options through the inter-war period. Both world wars triggered nationwide year-round DST as energy-conservation measures: the United States ran "War Time" from 1942 to 1945, and a brief year-round-DST experiment ran from January 1974 to October 1975 in response to the oil crisis.⁵³ The current federal regime dates to the Uniform Time Act of 1966, which reinstituted national DST from the last Sunday of April to the last Sunday of October, and to the Energy Policy Act of 2005, which extended DST to its present dates effective March 2007.⁵⁶

How is daylight saving time decided in the United States?

The United States Department of Transportation has statutory authority over US time zones and the dates of DST under the Uniform Time Act of 1966. States may opt out of DST and stay on permanent standard time, but they cannot opt into permanent DST without an act of Congress. Hawaii has therefore never observed DST under the modern federal regime, and Arizona observed it in 1967 only — the state legislature repealed observance the following year, effective 21 March 1968. The Navajo Nation within Arizona continues to observe DST to stay aligned with the rest of its territory in Utah and New Mexico.⁵⁶ American Samoa, Guam, the Northern Mariana Islands, Puerto Rico, and the US Virgin Islands also do not observe DST.³

Pressure to make DST permanent has been a perennial item on the US legislative agenda. The Sunshine Protection Act of 2021, sponsored by Senator Marco Rubio, passed the Senate by unanimous consent on 15 March 2022, and would have made DST "the new, permanent standard time, effective November 5, 2023."¹⁶¹⁷ The bill was held at the desk in the House on 16 March 2022 and died there at the end of the 117th Congress without a floor vote.¹⁷

The Sunshine Protection Act has been reintroduced in the 119th Congress (2025–2026) by Senator Rick Scott and Representative Vern Buchanan. Both bills carry the same text, with the date-certain stripped: "This bill makes daylight saving time the new, permanent standard time. States with areas exempt from daylight saving time may choose the standard time for those areas."¹⁸¹⁹ At least twenty US states have passed laws or resolutions that would adopt year-round DST contingent on Congress acting; none can take effect until federal law is changed.²⁰

The medical consensus, however, points the other way. The American Academy of Sleep Medicine's 2024 position statement concludes that the United States should adopt permanent standard time rather than permanent DST.²¹ As of mid-2026 the status of US DST law is unchanged: the Energy Policy Act of 2005 dates remain in force.

How does the European Union handle daylight saving time?

The European Union harmonised summer time across all member states under a 2001 directive in force since 2002. The directive sets a single transition pattern for the entire bloc — last Sunday of March, last Sunday of October, both at 01:00 GMT — and applies indefinitely.¹ The United Kingdom keeps the same dates under the Summer Time Act 1972 and has retained them since Brexit.¹³

In September 2018 the European Commission proposed repealing the directive. Each member state would have chosen whether to remain on permanent summer time or permanent winter time. The European Parliament voted 410-to-192 in support of abolition on 26 March 2019.²²⁷

The proposal has been stalled in the Council of the European Union — the body where member-state governments must agree on legislation — since 2019. There has been no further progress, and the Council has not adopted a position. Discussions resumed under the Polish presidency in the first half of 2025, with the Commission undertaking further analysis; the most recent plenary debate on the file was on 23 October 2025.⁷ Until the Council acts, the directive remains in force and EU clocks continue to change twice a year.

Which countries don't observe daylight saving time?

The countries that do not observe DST fall into three groups: those that have never observed it under modern rules, those that abolished it recently, and the sub-national jurisdictions of countries that mostly do.

The "never observed" group covers most of Africa, almost all of Asia (China, Japan, India, Indonesia, Saudi Arabia, and the Gulf states), most of South America, and Iceland in northern Europe. The IANA Time Zone Database is the operational source of truth for which zones carry no DST rules, encoding each zone as a sequence of offsets and any seasonal transitions that apply to it.¹¹

Recent abolitions have come in waves. Russia stopped switching seasonally on 26 October 2014, reverting to permanent standard time after a brief experiment with permanent summer time imposed by a 2011 law.⁸¹² Mexico abolished nationwide DST in 2022 with a new federal time-zone law that took effect on 30 October. The law preserves a northern-border exception zone — the state of Baja California, plus 21 named municipalities along the US border in Coahuila, Nuevo León, and Tamaulipas — that continues to observe US-aligned DST to keep cross-border commerce on synchronised clocks.⁹ Iran abolished DST starting from the Iranian year 1402, which began on 21 March 2023, after observing it since 1977.¹⁰ Other countries that have abandoned DST in the past two decades include Armenia (2012) and Azerbaijan (2016).¹⁰

Does daylight saving time save energy or improve health?

The most rigorous official study of DST's energy effect is the US Department of Energy's 2008 report to Congress on the 2007 extension of DST.²³ The study found a small reduction in US electricity consumption — about 0.03% over the year, or roughly 1.3 terawatt-hours, worth 80 to 100 million dollars in avoided electricity cost. Morning consumption increased while evening consumption decreased; the net savings depended on season and region. Other studies have found similar or smaller effects.

The medical evidence on DST is more decisive than the energy evidence. The American Academy of Sleep Medicine's 2024 position statement concludes that the United States should eliminate seasonal time changes in favour of permanent standard time — not permanent DST.²¹ The cited evidence for acute effects in the days after the spring-forward transition includes increased rates of heart attacks, strokes, atrial fibrillation, motor-vehicle fatalities (up to about 6%), and emergency-room admissions. The chronic effects of permanent DST — where the body's circadian clock is held an hour out of step with the sun for half the year — include elevated risk of obesity, metabolic syndrome, and cardiovascular disease. The position statement is endorsed by twenty professional organisations, including the American Medical Association and the National Safety Council.²¹

The mismatch between the medical consensus, which favours permanent standard time, and the dominant US legislative proposals, which favour permanent DST, is unresolved. Both options end seasonal clock changes; they make different choices about which half of the year's evenings get the extra hour of daylight.

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