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Daylight saving time (DST), also summer time in British English, is the convention of advancing clocks so that afternoons have more daylight and mornings have less. Typically clocks are adjusted forward one hour near the start of spring and are adjusted backward in autumn; the ancients lengthened summer hours instead. Presaged by a 1784 satire, modern DST was first proposed in 1907 by William Willett, and 1916 saw its first widespread use as a wartime measure aimed at conserving coal. Many countries have used DST since then; details vary by location and change occasionally.

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Adding daylight to afternoons benefits retailing, sports, and other activities that exploit sunlight after working hours, but causes problems for farmers and other workers whose hours depend on the sun. The extra afternoon daylight also cuts overall traffic fatalities; its effect on health and crime is less clear. DST can save electricity by reducing the need for artificial evening lighting, but it can also boomerang by boosting peak demand, increasing overall electricity costs.

DST's clock shifts complicate timekeeping and can disrupt meetings, travel, billing, medical devices, and heavy equipment. Many computer-based systems can adjust their clocks automatically when DST starts and ends, but their DST settings can be limited and error-prone, particularly when DST rules change. The clock shifts also can serve as twice-yearly reminders to replace smoke alarm batteries and review fire escape plans.

Origin

Though not punctual in the modern sense, the ancients adjusted daily schedules to the sun more flexibly than modern DST does. For example, ancient Rome divided daylight into twelve hours regardless of day length, so Roman hours were longer in summer and Roman water clocks had different scales for different months of the year. At Rome's latitude the third hour from sunrise, hora tertia, started in modern terms at 09:02 solar time and lasted 44 minutes at the winter solstice, but at the summer solstice it started at 06:58 and lasted 75 minutes. After ancient times equal-length hours eventually supplanted unequal, so civil time no longer varied by season.

While an American envoy to France, Benjamin Franklin anonymously published a letter in 1784 suggesting that Parisians economize by arising earlier to use morning sunlight, thereby burning fewer candles in the evening. Franklin's mild satire proposed taxing shutters, rationing candles, and waking the public by ringing church bells or firing cannons at sunrise, all in the spirit of his earlier proverb "Early to bed and early to rise / Makes a man healthy, wealthy, and wise." Franklin did not propose shifting clocks; like ancient Rome, 18th-century Europe did not keep accurate schedules. However, this soon changed as rail and communication networks came to require a standardization of time unknown in Franklin's day.

In 1905 the English builder and outdoorsman William Willett invented DST during a pre-breakfast horseback ride where he was dismayed by how many Londoners slept through the best part of a summer day. An avid golfer, he also disliked cutting short his round at dusk. Two years later he published a proposal for DST which attracted many supporters, including Balfour, Churchill, Lloyd George, and MacDonald. Edward VII also favored DST and had already been using it at Sandringham. However, Prime Minister Asquith opposed the proposal and after many hearings it was narrowly defeated in a Parliament committee vote in 1909. Willett's allies introduced DST bills every year from 1911 through 1914, to no avail.

World War I changed the political equation, as DST was promoted as a way to alleviate hardships from wartime coal shortages and air raid blackouts. Germany, its allies, and their occupied zones were the first European countries to use DST, starting April 30, 1916. Most belligerents and many European neutrals soon followed suit—for example, the United Kingdom first observed DST on May 21, 1916—but Russia and a few other countries waited until the next year. Australia and Canada followed fitfully, some locations observing DST as early as 1916. However, the measure proved unpopular among farmers, and many countries repealed DST after the war. For example, in 1918 the United States established DST from the last Sunday in March to the last Sunday in October, but Congress repealed DST after 1919; Woodrow Wilson, another avid golfer, vetoed the repeal twice but his second veto was overridden. Since then the world has seen many enactments, adjustments, and repeals of DST, with similar politics involved.