Daylight Saving Time, when the clock is set one hour ahead during the summer months, is observed in about 70 countries worldwide, including some in every populated continent. Originally Benjamin Franklin's idea, Daylight Saving Time is a little bit different everywhere it is practiced and has been controversial since its introduction. Daylight Saving Time has two main purposes: to increase evening daytime hours for outdoor leisure activities and to save on energy consumption.

Benjamin Franklin first conceived the idea that would become Daylight Saving Time during a 1784 post as an American delegate in Paris. It was similar to his oft quoted maxim, "Early to bed, early to rise, makes a man healthy, wealthy and wise." Half-jokingly, Franklin suggested that Parisians shift their sleep schedules an hour back in order to save on candles in the evening. He did not suggest changing the clock, however.

William Willett of London was the first to propose an actual clock shift to move an hour of daylight from the morning to the evening in his 1907 pamphlet, "Waste of Daylight." Though his efforts resulted in a 1909 bill drafted in Parliament, his idea was not respected during his lifetime, and he died in 1915 before seeing his plan come into effect.

World War I was the catalyst for many countries adopting Daylight Saving Time, as the potential for energy savings was attractive. Germany and Austria were the first, in 1916, followed quickly by Belgium, Denmark, France, Great Britain, Italy, Luxembourg, Manitoba, Netherlands, Norway, Nova Scotia, Portugal, Sweden, Turkey, and Tasmania. Australia and Newfoundland joined in in 1917, and the United States was a relative latecomer, beginning Daylight Saving Time in the spring of 1918. However, it was so unpopular it was repealed the next year, and though some cities and states retained the practice, it would not become national law again until World War II.

Daylight Saving Time has a long history of controversy, with vehement opinions on both sides of the debate. It is notoriously unpopular among farmers, who already have to deal with darkness in the morning and whose animals do not readily adjust to the clock change. Some people enjoy the increase of daylight in the evening, while others are annoyed by the darker mornings. However, Daylight Saving Time has been shown to have a significant effect on energy savings, and though 70% of Americans rise before 7:00 a.m., the increased energy use in the morning is more than offset by the savings in the evenings.

Daylight Saving Time has also caused a lot of confusion over the years, as it has not always been applied consistently. Its use was not standardized in the United States between 1945 and the Uniform Time Act of 1966, causing significant problems for transportation, broadcasting, and other industries that relied on a standard national time. Daylight Saving Time is also not always an hour's adjustment; it has variously been a time change of 20 minutes or two hours. In modern day Russia, as in Great Britain during World War II, clocks are an hour ahead of standard time in the winter and two hours ahead in summer. Many countries, including the United States, have gone through alternate periods of observing and failing to observe Daylight Saving Time.

Since the Uniform Time Act of 1966, Daylight Saving Time has been mandated and standardized throughout the United States. States that do not wish to observe the law must pass a separate state law. States that span two time zones are allowed to observe Daylight Saving Time in one of the state's time zones and not in the other, making time uniform throughout the state during the summer months. Beginning in 2007, Daylight Saving Time in the United States is extended by three weeks in the spring and one week in the fall in order to increase energy savings, as mandated by the Energy Policy Act of 2005.