Daylight saving time(DST)
Daylight saving time is a seasonal clock adjustment in which a region moves its clocks forward, usually by one hour, during part of the year.
- Common abbreviation
- DST
- Typical adjustment
- One hour forward
- Purpose
- Shift more daylight into the evening
- Other names
- Summer time, daylight time
- Defined by
- Local laws and government rules
What daylight saving time means
During the daylight saving period, clocks are advanced relative to the region's standard time. When the period ends, clocks are moved back to standard time. The exact dates, clock change, and even whether the change happens at all are determined by each jurisdiction.
For example, a location whose standard offset is UTC−05:00 may use UTC−04:00 during its daylight time. The underlying relationship to UTC changes temporarily; the location's standard time does not change its identity.
When and where it is used
Daylight saving time is used mainly in parts of North America, Europe, Oceania, and the Middle East. It is not used everywhere: many countries near the equator have little seasonal variation in daylight, and several countries that once used it have since stopped.
- In the United States and Canada, many regions advance clocks in spring and return to standard time in autumn.
- In much of Europe, the corresponding period is commonly called summer time.
- Australia and New Zealand use seasonal clock changes in some regions, but not uniformly across the whole country.
- Rules can differ between neighbouring regions, even when they use the same standard-time abbreviation.
When regions change their clocks
The table below summarizes when major regions start and end daylight saving time. Exact dates are set by local law and can change, so treat this as a general reference rather than an authoritative source for a specific year.
| Region | DST starts | DST ends | Clocks move at | Notes |
|---|---|---|---|---|
| United States & Canada (most areas) | Second Sunday in March | First Sunday in November | 2:00 a.m. local time | Arizona (except the Navajo Nation) and Hawaii do not observe DST. |
| European Union & United Kingdom | Last Sunday in March | Last Sunday in October | 1:00 a.m. UTC | The UK period is known as British Summer Time (BST). |
| Australia (NSW, VIC, SA, TAS, ACT) | First Sunday in October | First Sunday in April | 2:00 a.m. local time | Queensland, Western Australia, and the Northern Territory do not observe DST. |
| New Zealand | Last Sunday in September | First Sunday in April | 2:00 a.m. local time | — |
| Mexico | Not observed nationwide | Not observed nationwide | — | Nationwide DST ended in 2022; some northern border municipalities still follow the US schedule. |
| Most of Africa and Asia | Not observed | Not observed | — | A few countries, such as Iran and parts of the Middle East, have used their own schedules in some years. |
Why it was established
The main argument for seasonal clock changes was to move usable daylight from early morning into the evening, when more people were expected to be awake, working, shopping, or travelling. The policy was also promoted at different times as a way to reduce energy use, although the size and direction of that effect depend on local habits, climate, and energy systems.
A short history
The idea of shifting clocks to follow seasonal daylight was discussed in the nineteenth century. New Zealand entomologist George Hudson proposed a seasonal clock change in 1895, and British builder William Willett promoted a similar idea in the early twentieth century.
Germany introduced a nationwide seasonal clock change in 1916 during the First World War, partly to conserve fuel and make better use of daylight. Other countries adopted, suspended, and reintroduced the practice during wartime and later for economic or administrative reasons. Modern rules remain matters of national and regional legislation.
Related time zone definitions
The time zone abbreviations below illustrate the difference between standard and seasonal labels:
- Eastern Standard Time is commonly paired with Eastern Daylight Time.
- Central European Time is commonly paired with Central European Summer Time.
- Australian Eastern Standard Time is used as the standard-time reference in eastern Australia.