The short answer
Daylight saving time (DST) is the practice of setting clocks one hour ahead of standard time during the warmer part of the year. In spring the clocks go forward, and in autumn they go back to standard time. In the United Kingdom and much of Europe the same idea is called summer time.
The goal is to line up waking hours with daylight. In summer the sun rises very early in mid and high latitudes, often before most people get up. Shifting the clock moves one hour of that early light to the evening, when more people are awake and active.
How the change looks on the clock
On the spring change date, a typical clock skips an hour: in the United States, 2:00 a.m. becomes 3:00 a.m. instantly, so that night is one hour shorter. On the autumn date, 2:00 a.m. becomes 1:00 a.m. again, and the hour from 1:00 to 2:00 happens twice. The popular memory aid is spring forward fall back.
During DST a time zone’s offset from Coordinated Universal Time (UTC) grows by one hour. New York moves from UTC−5 to UTC−4, and London moves from UTC+0 to UTC+1. For a closer look at the mechanics, see how daylight saving time works.
Where the idea came from
The idea is often linked to Benjamin Franklin, but his 1784 essay was a joke about Parisians sleeping through sunny mornings, and he did not propose changing clocks. Modern proposals came later:
- George Vernon Hudson, a New Zealand entomologist, proposed a seasonal two-hour shift in 1895.
- William Willett, a London builder, campaigned for summer time from 1907 in a pamphlet called “The Waste of Daylight”.
- Germany and Austria-Hungary were the first countries to adopt it nationally, in 1916, to save coal during World War I. The United Kingdom and many other countries followed within weeks or months.
After the war many countries dropped the practice, then brought it back during World War II and again during the energy crises of the 1970s.
Who uses it today
Daylight saving time is mostly used in North America, Europe and parts of the southern hemisphere, such as south-east Australia, New Zealand, Chile and Paraguay. In the southern hemisphere summer runs from about October to April, so clocks go forward around the time they go back in the north.
Many countries do not use it at all. Most of Asia and Africa keep the same time all year, and places near the equator gain little from it because the length of the day hardly changes. In the United States, Hawaii and most of Arizona stay on standard time all year.
You can check whether DST is in effect now, and when the next change is, on the daylight saving time page.
The arguments for and against
Supporters point to brighter evenings for shopping, sport and outdoor activity, and some studies have linked lighter evenings with fewer road accidents. The original goal was to save energy, but modern studies disagree on how much, if anything, is saved today, since lighting is a smaller share of energy use than it was in 1916.
Critics point to the disruption of changing clocks twice a year. The spring change costs an hour of sleep, and some research has found short-term rises in health problems and accidents in the days that follow. Darker winter-like mornings during DST are another complaint, especially in the west of a time zone.
The debate is far from settled. The European Parliament voted in 2019 to end seasonal clock changes, but the plan has not been put into effect. In the United States, several states have passed laws asking for permanent DST, which would need a change in federal law to take effect.
Key terms
| Term | Meaning |
|---|---|
| Standard time | The time a zone keeps outside the DST period |
| Daylight time / summer time | Standard time plus one hour |
| Spring forward | Clocks move ahead one hour |
| Fall back | Clocks move back one hour |
| UTC offset | The difference between local time and UTC |
To compare time zones that do and do not change their clocks, use the time zone converter.