July 4, 2026 11 min read

Daylight Saving Time: How It Works and Why It Exists

Daylight saving time (DST) is the practice of moving clocks one hour ahead in spring and one hour back in autumn, so that more of the daylight falls in the evening. In the United States the clocks go forward at 2:00 a.m. on the second Sunday in March and back at 2:00 a.m. on the first Sunday in November. In 2026 that means March 8 and November 1. Europe, the southern hemisphere and a long list of countries that have quit all do it differently, and the details matter whenever you schedule anything across borders.

What daylight saving time is

Every place has a standard time, defined as an offset from UTC. New York's is UTC-5, called Eastern Standard Time (EST). During daylight saving time the offset moves one hour closer to the east: New York becomes UTC-4, Eastern Daylight Time (EDT). Nothing about the sun changes. Sunrise and sunset simply land one hour later on the clock, which shifts an hour of light from early morning, when most people are asleep, to the evening, when they are not.

The name varies. Americans say daylight saving time (singular "saving", although "savings" is common in speech). The UK and the EU call it summer time, which is why London switches from GMT to British Summer Time and Berlin from CET to CEST. Same mechanism, different labels.

The usual memory aid is "spring forward, fall back". In spring you lose an hour of sleep; in autumn you get it back.

When the clocks change

There is no world date for the switch. Each country sets its own rule, and the three big rule families are the ones below. The 2026 dates are computed from the IANA time zone database, the data set that phones and computers use.

The US rule comes from federal law, 15 U.S.C. § 260a, as amended by the Energy Policy Act of 2005. It took effect in 2007 and stretched DST to 238 days, about 65% of the year, according to NIST. Each state may opt out entirely, but no state may move the dates.

The European rule, set by Directive 2000/84/EC, works on a single instant rather than on local 2 a.m. At 01:00 UTC on the last Sunday in March, London goes from 01:00 to 02:00, Berlin from 02:00 to 03:00 and Athens from 03:00 to 04:00, all at the same moment. That keeps the gaps between European capitals constant through the switch.

South of the equator the seasons are reversed, so the clocks are too. When New York is on daylight time in July, Sydney is on standard time (AEST, UTC+10). In October Sydney moves to AEDT (UTC+11) just as the northern hemisphere moves back.

The weeks when gaps shift

Because the US and Europe use different Sundays, the gap between them moves twice a year. In 2026, from March 8 to March 28, New York is four hours behind London instead of the usual five. From October 25 to October 31 it happens again. Australia adds its own twist: between late March and early April, Sydney and London are 10 hours apart, then 9, then 10 again in October, then 11 by November. Recurring meetings built across those weeks drift by an hour for some attendees, which is the main reason scheduling meetings across time zones should always anchor on one city's local time.

What happens to the missing and repeated hour

On the spring date, local time jumps straight from 1:59:59 to 3:00:00. In New York on March 8, 2026, the clock never shows 2:30 a.m. An alarm set for 2:30 will either fire at 3:00 or not at all, depending on the device, and a night shift that runs from 10 p.m. to 6 a.m. is only seven hours long.

In autumn the hour from 1:00 to 1:59 happens twice. In New York on November 1, 2026, 1:30 a.m. occurs first at UTC-4 (EDT) and then, an hour later, at UTC-5 (EST). The same night shift lasts nine hours. Anyone paid by the hour should be paid for nine.

This is why software stores timestamps in UTC and converts to local time only for display. A log line that says "01:30 local" on that night is ambiguous; "05:30Z" is not. If you write times for other people, add the offset or the zone abbreviation, and avoid scheduling anything between 1 a.m. and 3 a.m. on a changeover night.

Why it exists: the real history

Benjamin Franklin is often credited because of a 1784 essay about Parisians sleeping through the morning light, but he did not propose moving clocks. The modern idea came from George Vernon Hudson, a New Zealand entomologist who in 1895 presented a paper proposing a two-hour seasonal shift so that he would have more daylight after work for his insect research. Nothing came of it.

In 1907 William Willett, a London builder, self-published a pamphlet called "The Waste of Daylight". He proposed moving clocks 20 minutes on each of four Sundays in April and back again in September. Bills based on his idea were introduced in Parliament from 1909 and failed, and the commentary in the tz database notes that farming interests were among the strongest opponents.

That last point kills the most popular myth. Daylight saving time was not created for farmers. Farm work runs on the sun, not the clock, so farmers had nothing to gain from moving it.

What made DST happen was war. Germany and Austria-Hungary moved their clocks forward on the night of April 30, 1916, to save coal for the war effort. The UK followed on May 21, 1916, and the United States on March 31, 1918. Most countries dropped it after the war and brought it back during the Second World War. The current US system dates from the Uniform Time Act of 1966; the dates were last changed in 2007.

Who uses it today

Fewer places than most people assume. Counting the countries and territories in the IANA database, 70 of 247 have at least one region that changes its clocks in 2026. That is under a third, and nearly half of that list is in Europe.

The pattern follows latitude. Near the equator day length barely changes across the year, so there is no summer evening light to borrow. Almost all of Africa, nearly all of Asia (Israel, Lebanon and the Palestinian territories are exceptions) and most of South America do not use DST. Tokyo, Shanghai, Mumbai and Singapore keep the same offset all year.

Even inside DST countries there are holdouts. In the US, Hawaii and Arizona stay on standard time all year, with the exception of the Navajo Nation in Arizona, which follows DST. That is why Phoenix is on the same clock as Los Angeles in summer and the same as Denver in winter. The list of states without daylight saving time also includes the US territories: Puerto Rico, Guam, the US Virgin Islands and American Samoa.

Countries that stopped changing clocks

The trend for the past fifteen years has been to quit, and every exit picks a side: permanent standard time or permanent summer time. The table lists the main changes, with dates from the tz database.

Mexico is the instructive case. Its 2022 law ended seasonal time for most of the country, so Mexico City now sits at UTC-6 all year. Municipalities along the US border kept the US dates so that Tijuana stays aligned with San Diego. One national rule, two behaviors.

In western Canada, British Columbia announced on March 2, 2026 that the spring forward on March 8 would be its last, which ends the change that had been scheduled for November 1, 2026. Alberta adopted the same approach and its law took effect in June 2026. Both provinces are now on their summer offsets permanently: Vancouver at UTC-7 and Edmonton at UTC-6. The practical effect arrives in November, when the US falls back and these provinces do not; we cover that in detail in BC and Alberta stop changing clocks.

Paraguay went the same way in 2024, staying on its summer offset. Russia and Mexico chose the opposite, permanent standard time. The arguments for each side, including the sleep research, are in our guide to permanent daylight saving time vs standard time.

In the US, permanent daylight time needs an act of Congress, because federal law only lets states opt out toward standard time. Bills to make DST permanent have been introduced several times and have not become law.

Does it save energy?

A little, and less than its supporters claimed. The best-known measurement is the US Department of Energy's 2008 report to Congress on the four extra weeks added in 2007. It found electricity savings of about 0.5% per day of extended DST, about 1.3 terawatt-hours in total. Over the whole year that is 0.03% of US electricity use. The savings came in the evening and were partly offset by slightly higher use early in the morning.

Studies from other countries point in different directions, mostly because heating and air conditioning now dwarf the lighting load that DST was meant to cut. Energy is no longer a convincing argument for DST in either direction; the live debate is about health, safety and preference for evening light.

Check any city

Every city page on this site shows the current offset, whether DST is in effect, and the date of the next clock change. London and New York are good ones to compare during the March and October gap weeks.

To plan around a switch, put two cities and a date into the time zone converter: it applies the rules for that exact day, so a call booked for March 20 shows the four-hour New York to London gap rather than the usual five. The world clock shows several cities side by side and flags which are on summer time right now. For the night before a change, the advice in how to adjust to the clock change is worth five minutes.

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