Daylight saving time

Daylight saving time (DST) is the practice of advancing clocks during part of the year, usually by one hour, so that a greater share of daylight occurs during the evening according to local clock time. Regions using DST later return their clocks to standard time. The change does not alter the total amount of daylight. It changes the relationship between clock time and the natural times of sunrise and sunset.

Daylight saving time is mainly used in Europe and North America. Other regions remain on standard time throughout the year, particularly where the length of daylight changes little between seasons.

History

The idea of changing clock time to correspond more closely with daylight was discussed during the 18th century. New Zealand entomologist George Vernon Hudson presented a formal proposal in 1895.

Daylight saving time was first adopted on a broad scale during the First World War. European governments introduced seasonal clock changes in 1916 to reduce the use of artificial lighting and conserve fuel. Some countries discontinued the practice after the war.

Seasonal clock changes were reintroduced in parts of the world during the Second World War. Further adoption followed during the energy crises of the 1970s. Some countries later established regular annual schedules, while others returned permanently to standard time.

Operation

In a region that observes daylight saving time, clocks are moved forward before the part of the year with longer evening daylight. They are moved back when the region returns to standard time. In the Northern Hemisphere, these changes usually occur during spring and autumn.

The forward transition causes an interval of local clock time to be skipped. When clocks are moved back, an interval occurs twice. The exact transition dates and clock times are determined by each jurisdiction. Changes are generally scheduled during the night to reduce disruption.

During daylight saving time, a region normally uses a time zone offset that is one hour ahead of its standard offset from Coordinated Universal Time. Sunrise and sunset therefore occur at later clock times, although their astronomical timing is unchanged.

The phrases “spring forward” and “fall back” are commonly used in North America to describe the two transitions.

Global use

Most European countries observe daylight saving time and coordinate their transitions on common dates. The system is also used across most of the United States and Canada, although some jurisdictions remain on standard time throughout the year.

Daylight saving time is uncommon near the equator because sunrise and sunset times vary little between seasons. Most countries in Asia do not currently use seasonal clock changes. The practice is also uncommon in Africa. Most South American countries remain on standard time, although individual countries have used DST during earlier periods.

Governments periodically consider replacing seasonal clock changes with permanent standard time or permanent daylight saving time. Such proposals affect the legal time used throughout the year rather than the amount of available daylight.

Effects

The effect of daylight saving time on energy use depends on climate and patterns of electricity consumption. Reduced demand for evening lighting may be offset by increased use of heating or air conditioning. Studies conducted in different regions have therefore produced different results.

Later evening daylight can provide more time for outdoor recreation after ordinary working or school hours. It may also affect commercial activity by changing when people travel or spend time outside their homes.

The transitions between standard time and daylight saving time can temporarily disrupt sleep. They may also complicate transport schedules and electronic timekeeping. Systems that record local time must distinguish between the skipped interval during the forward transition and the repeated interval when clocks are moved back.

See also

• Coordinated Universal Time
• Greenwich Mean Time
• Time zone