Daylight savings time, or summer time, is the practice of advancing clocks during summer months by one hour so that in the evening daylight is experienced one hour longer, while sacrificing normal sunrise times. Typically, regions with summer time adjust clocks forward on hour close to the start of spring and adjust them backward in the autumn to standard time.

During his time as an American envoy to France, Benjamin Franklin, publisher of the old English proverb, “Early to bed, and early to rise, makes a man healthy, wealthy and wise.” anonymously published a letter suggesting that Parisians economize on candles by rising earlier to use morning sunlight. This 1784 satire proposed taxing shutters, rationing candles and waking the public by ringing church bells and firing cannon at sunrise. Despite common misconception, Franklin did not actually propose DST: 18th century Europe did not even keep precise schedules but this soon changed as rail and communication networks came to require a standardization of time.

DST was first proposed in 1895 by the New Zealand entomologist, George Hudson, whose shift-work job gave him leisure time to collect insects, and led him to value after-hours daylight. Prominent English builder and avid golfer, William Willett, disliked cutting short his round at dusk. His solution was to advance the clock during the summer months. He lobbied for the proposal in the UK until his death in 1915. Germany and its World War I ally Austria-Hungary were the first to use DST as a way to conserve coal during wartime. Britain, most of it allies, and many European neutrals soon followed suit. William Sword Frost, mayor of Orillia, Ontario, introduced daylight savings time in the municipality during his tenure from 1911 to 1912. The United States adopted it in 1918.

The practice has received both advocacy and criticism. Putting clocks forward benefits retailing, sports and other activities that exploit sunlight after working hours, but can cause problems for farming and evening entertainment. DST clock shifts sometimes complicate timekeeping and disrupt travel, billing, record keeping, medical devices, heavy equipment and sleep patterns. Computer software can often adjust clocks automatically, but policy changes by various jurisdictions of the dates and timings of DST may be confusing. Winston Churchill argued that it enlarges “the opportunities for the pursuit of health and happiness among the millions of people who live in this country” and pundits have dubbed it “Daylight Slaving Time.”

Most of North America shifts at 2 a.m. local time, so its zones do not shift at the same time. In some cases only part of a country shifts, for example, in the U.S., Hawaii and most of Arizona do not observe DST. Clock shifts are usually scheduled near a weekend midnight to lessen disruption to weekday schedules.