How Daylight Saving Time Works (and Who Still Uses It)
Twice a year about 70 countries move their clocks by an hour. Where the idea came from, who still does it, when the switch happens, and why many have stopped.
Daylight saving time (DST) is the practice of moving clocks forward by one hour for the warmer part of the year, so that an hour of daylight shifts from the early morning — when most people are asleep — to the evening. Nothing about the sun changes. What changes is the label we put on each moment of the day.
About 70 countries use it in some form. Most of the world’s population does not.
The mechanics: one missing hour, one repeated hour
Every DST region has two offsets from UTC: a standard offset and a daylight offset one hour ahead of it. New York is UTC−5 in winter (EST) and UTC−4 in summer (EDT). Berlin is UTC+1 in winter (CET) and UTC+2 in summer (CEST).
The switch between them creates two oddities each year:
- Spring forward. At the transition, the local clock jumps ahead. In New York the time goes from 01:59:59 straight to 03:00:00. Any local time between 02:00 and 03:00 on that day does not exist.
- Fall back. In autumn the clock is set back. In New York the hour from 01:00 to 02:00 happens twice — first in daylight time, then again in standard time. A local time of 01:30 on that day is ambiguous unless you also say which offset applies.
This is why software stores timestamps in UTC and converts to local time only for display. UTC has no gaps and no repeats.
When the clocks change
There is no global date. Each country or bloc sets its own rule. (For the next switches, see the countdowns to Europe’s clocks going back and US daylight saving time starting in 2027.)
| Region | DST begins | DST ends |
|---|---|---|
| United States and Canada | Second Sunday of March, 02:00 local | First Sunday of November, 02:00 local |
| European Union and United Kingdom | Last Sunday of March, 01:00 UTC | Last Sunday of October, 01:00 UTC |
| Australia (south-eastern states) | First Sunday of October | First Sunday of April |
| New Zealand | Last Sunday of September | First Sunday of April |
| Chile | Early September | Early April |
Two details in that table matter in practice.
First, North America and Europe switch on different dates. For two to three weeks in March and about one week in the autumn, the usual time difference between the two is off by an hour. London and New York are normally five hours apart; in those windows the gap is four.
Second, the Southern Hemisphere is reversed. Summer there runs from October to March, so Sydney moves its clocks forward just as London moves them back. The difference between London and Sydney is 11 hours in the northern winter, 9 hours in the northern summer, and briefly 10 hours in between.
Europe also does something North America does not: the whole bloc changes at the same instant, 01:00 UTC. In the United States each time zone changes at its own 02:00, so the switch rolls across the country hour by hour.
Where the idea came from
The modern proposal is usually credited to two people. George Hudson, a New Zealand entomologist who wanted more daylight after work to collect insects, suggested a two-hour seasonal shift in 1895. William Willett, a British builder, campaigned for a similar scheme from 1907 in a pamphlet called The Waste of Daylight.
Neither saw it adopted quickly. The first countries to move their clocks nationwide were Germany and Austria-Hungary, on 30 April 1916, as a wartime measure to save coal. Britain followed within weeks, and the United States in 1918.
After each world war many countries dropped the practice, then returned to it during the oil crisis of the 1970s. The United States standardised its dates with the Uniform Time Act of 1966 and extended the DST period to its current length in 2007.
Who does not use it
Most countries near the equator never adopted DST, for a simple reason: day length there barely changes over the year, so there is no spare morning daylight to move.
Several large countries tried it and stopped:
- Japan used DST from 1948 to 1951 and has not brought it back.
- China used it from 1986 to 1991.
- Russia moved to permanent summer time in 2011, then to permanent standard time in 2014.
- Turkey stayed on UTC+3 permanently from 2016.
- Brazil abolished DST in 2019.
- Mexico abolished it for most of the country in 2022, keeping it only in some municipalities along the U.S. border.
The list does not only shrink. Egypt reinstated DST in 2023 after several years without it.
Even inside countries that do observe DST, there are exceptions. In the United States, Hawaii and most of Arizona stay on standard time all year. In Australia, Queensland, the Northern Territory, and Western Australia do not change their clocks, which is why Brisbane and Sydney share a time in winter but sit an hour apart in summer.
Does it actually save energy?
The original argument was about lighting: if the sun is up later in the evening, people switch lamps on later. A century on, lighting is a much smaller share of electricity use, and heating and air conditioning are larger ones. Modern studies generally find the effect on total energy use to be small, and in some climates slightly negative.
The costs are better documented. The spring transition removes an hour of sleep for an entire population at once, and researchers have linked the following days to short-term increases in traffic accidents and heart attacks. The autumn transition is gentler but brings darker evenings, which shift when road accidents happen.
This is the background to a long-running political debate. The European Parliament voted in 2019 to end seasonal clock changes, but member states have not agreed on which time to keep, so the twice-yearly switch continues. In the United States, bills to make daylight time permanent, such as the Sunshine Protection Act, have been introduced repeatedly; the Senate passed one version in 2022, but none has become law.
Practical rules for working across DST
- Schedule in a named zone, not an offset. “15:00 Europe/London” stays correct all year. “15:00 UTC+1” is wrong for half of it.
- Re-check recurring international meetings in March, April, October, and November. A weekly call can move by an hour for one side only.
- Avoid scheduling anything between 01:00 and 03:00 local time on a transition night. That is where the missing and repeated hours live.
- Remember that abbreviations are seasonal. EST and EDT are different offsets. If you are unsure, check the time zone converter or the page for a specific abbreviation such as EST or CET.
Daylight saving time is best understood as a political layer on top of the time-zone map: a rule that says this zone uses a different offset between these two dates. The rules change more often than most people expect, which is exactly why computers carry a regularly updated time zone database rather than a fixed table of offsets.