Daylight Saving Time Calculator

Daylight saving transition dates are not astronomical events; they are legal rules expressed as an nth-weekday-of-month formula. Under the United States rule in force since 2007, clocks advance at 02:00 local time on the second Sunday of March and return at 02:00 local time on the first Sunday of November, giving a DST period of about 238 days and a standard-time period of about 127. The European Union rule is defined against UTC rather than local time: the change occurs at 01:00 UTC on the last Sunday of March and the last Sunday of October, so all member states switch at the same instant, and the summer-time period runs about 210 to 217 days. The Australian states that observe it - New South Wales, Victoria, South Australia, Tasmania, and the ACT - shift on the first Sunday of October and the first Sunday of April. Two consequences matter more than the dates. The spring transition deletes a local clock hour, so 02:30 does not exist on that date; the autumn transition repeats an hour, so a local timestamp inside it is ambiguous between two distinct UTC instants - the standard argument for storing instants in UTC and applying a zone only for display. The second consequence is coverage. Arizona outside the Navajo Nation, Hawaii, and Puerto Rico stay on standard time year round, as do Queensland, Western Australia, and the Northern Territory in Australia, most of Asia and Africa, and Russia since its final change in 2014. Any specific location should be resolved through the IANA time zone database rather than a country-level rule.

The idea was published by George Vernon Hudson, a Wellington entomologist who proposed a two-hour seasonal shift in a paper to the Wellington Philosophical Society in 1895. The London builder William Willett promoted it independently and far more loudly, publishing the pamphlet The Waste of Daylight in 1907 and campaigning until his death in 1915. Neither argument succeeded on its merits; wartime fuel economy did. Germany and Austria-Hungary introduced summer time on 30 April 1916, Britain followed within weeks with the Summer Time Act of 1916, and the United States adopted it with the Standard Time Act of 1918, which also gave the country its legal time zones. Postwar US practice was chaotic, with cities and states choosing their own dates, until the Uniform Time Act of 1966 imposed a federal schedule from the last Sunday in April to the last Sunday in October while allowing states to opt out entirely. Congress then repeatedly moved the boundaries: a year-round trial under the Emergency Daylight Saving Time Energy Conservation Act of 1973, effective from January 1974, was abandoned within months after complaints about dark winter mornings, a 1986 amendment moved the start to the first Sunday in April, and the Energy Policy Act of 2005 set the present schedule from 2007. Europe harmonized separately, the arrangement being made open-ended by Directive 2000/84/EC. The European Parliament voted in March 2019 to abolish seasonal clock changes, but the Council has not agreed the proposal, and the twice-yearly change continues.