Daylight saving time explained

What daylight saving time is

Daylight saving time (DST), called summer time in the UK and Europe, means setting clocks forward, usually by one hour, for part of the year. The aim is to move daylight from the early morning, when many people are asleep, to the evening, when they are awake. Clocks go forward in spring and back in autumn. In the northern hemisphere that means roughly March to October or November. In the southern hemisphere it is roughly October to April.

The effect depends on latitude. Near the equator, day length barely changes through the year, so there is little point, and almost no tropical country uses DST. At high latitudes summer days are long anyway. The countries that use it are mostly in the mid-latitudes of Europe, North America, and parts of South America and Oceania.

A short history

Benjamin Franklin is often credited with the idea, but his 1784 letter to a Paris newspaper was satire about lazy Parisians wasting candles. It did not propose changing clocks. The modern proposal came from George Vernon Hudson, a New Zealand entomologist, in 1895, and independently from William Willett, a London builder, who campaigned from 1907 for lighter summer evenings.

The First World War made it happen. Germany and Austria-Hungary introduced summer time on 30 April 1916 to save coal, and the UK followed within weeks. The United States adopted DST in 1918, together with its standard time zones, but repealed the national DST rule the following year. For decades after that, US practice was a patchwork set by states and cities, until the Uniform Time Act of 1966 standardised it. The current US dates date from 2007.

Many countries adopted DST during the energy crises of the 1970s. The European Community began harmonising its members' dates in the 1980s. Since 1996, EU countries and the UK have changed their clocks at the same instant: 01:00 UTC on the last Sundays of March and October. That rule is now set by an EU directive adopted in 2000.

Dates for 2026 and 2027

All dates are from the IANA time zone database (release 2026c). For Egypt, the autumn change happens at midnight at the end of the Thursday shown. Each city page on this site gives the exact local time of each change.

Why the dates differ

Each country, or in some cases each state or province, sets its own rules. That is why the gap between two cities can change several times a year. London and New York are 5 hours apart, except for one week in autumn and two or three weeks in spring when they are 4 hours apart. London and Sydney swing between 9 and 11 hours, because their clocks move in opposite directions. Our time difference pages list the exact dates for popular city pairs.

Countries that have stopped

A notable trend of the past decade is countries abandoning seasonal clock changes:

• Russia stayed on summer time all year from 2011, then moved to permanent standard time in 2014.
• Turkey stayed on UTC+3 all year from 2016.
• Brazil abolished DST in 2019.
• Mexico abolished it in most of the country in 2022, keeping it only in some areas on the US border.
• British Columbia moved to permanent UTC−7 in March 2026, and Alberta to permanent UTC−6 in June 2026. Both effectively stayed on their summer offset.
• Morocco returned to permanent UTC+0 in September 2026, ending a system in which it used UTC+1 for most of the year and UTC+0 during Ramadan.

Some countries have gone the other way. Egypt brought DST back in 2023 after several years without it.

In the EU, the European Commission proposed ending seasonal changes in 2018, and the European Parliament supported it in 2019. Member states have not agreed which time each country would keep, so the change has not happened. In the US, the Sunshine Protection Act, which would make daylight saving time permanent, passed the Senate in 2022 but did not become law.

The arguments

Supporters of DST point to lighter evenings for leisure, shopping and outdoor activity in summer. The original energy-saving argument is weaker today, and studies of its effect on electricity use have reached mixed conclusions.

Critics point to the disruption of the change itself. Losing an hour of sleep in spring is unpleasant, and some studies have linked the days after the spring change to small increases in accidents and health problems. Sleep researchers' organisations, including the American Academy of Sleep Medicine, have argued that if clocks are to stop changing, permanent standard time fits the body clock better than permanent summer time. Others prefer permanent summer time for its lighter evenings, which is the choice British Columbia and Alberta made. There is no consensus, which is one reason reform has stalled in so many places.

Practical tips around the changes

• Check meetings with other countries in the weeks around each change. That is when remembered time differences are most likely to be wrong.
• Remember that on the spring change night one hour does not exist, and on the autumn night one hour happens twice. Overnight timetables, alarms and automated jobs can behave oddly.
• In software, store times in UTC with an IANA zone name such as Europe/London, not a fixed offset. And keep your time zone data updated: the 2026 changes in Canada and Morocco reached devices only through updates.
• Use the time zone converter with the actual date. It applies the rules in force on that day.

Sources

• IANA time zone database and release notes: iana.org/time-zones
• GOV.UK, When do the clocks change?: gov.uk
• NIST, Daylight saving time: nist.gov
• European Commission, summertime arrangements: ec.europa.eu