Daylight saving time là gì

     
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Daylight saving time begins again on Sunday, March 14, when most Americans will move sầu their clocks forward by an hour. On Nov. 7, 2021, daylight saving time (sometimes erroneously called daylight savings time) will over again và we"ll set our clocks bachồng by an hour in those regions that observe DST. These fall & spring clochồng changes continue a long tradition started by Benjamin Franklin lớn conserve sầu energy. 


Here"s a look at when daylight saving time starts và ends during the year, its history, why we have it now & some myths and interesting facts about the time change.

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When does daylight saving time start and end?

Historically, daylight saving time (DST) has begun in the summer months & ended right before winter, though the dates have changed over time as the U.S. government has passed new statutes, according to the U.S. Naval Observatory (USNO).

Starting in 2007, DST begins in the U.S. on the second Sunday in March, when people move their clocks forward an hour at 2 a.m. local standard time (so at 2 a.m. on that day, the clocks will then read 3 a.m. local daylight time). Daylight saving time then ends on the first Sunday in November, when clocks are moved back an hour at 2 a.m. local daylight time (so they will then read 1 a.m. local standard time).


In 2021, DST begins on March 14 and ends on Nov. 7 in the U.S., when you"ll phối the cloông xã bachồng an hour và the cycle will begin again. Daylight saving time in the U.S. will begin again on March 13, 2022, according lớn timeanddate.com. 

Why did daylight saving time start?

Benjamin Franklin takes the honor (or the blame, depending on your view of the time changes) for coming up with the idea to lớn reset clocks in the summer months as a way lớn conserve sầu energy, according to lớn David Prerau, author of "Seize the Daylight: The Curious và Contentious Story of Daylight Saving Time" (Thunder"s Mouth Press, 2005). By moving clocks forward, people could take advantage of the extra evening daylight rather than wasting energy on lighting. At the time, Franklin was ambassador to Paris & so wrote a witty letter lớn the Journal of Paris in 1784, rejoicing over his "discovery" that the sun provides light as soon as it rises.

Even so, DST didn"t officially begin until more than a century later. Germany established DST in May 1916 as a way to lớn conserve sầu fuel during World War I. The rest of Europe came onboard shortly thereafter. And in 1918, the United States adopted daylight saving time.


Though President Woodrow Wilson wanted to keep daylight saving time after WWI ended, the country was mostly rural at the time and farmers objected, partly because it would mean they lost an hour of morning light. (It"s a myth that DST was instituted khổng lồ help farmers.) And so daylight saving time was abolished until the next war brought it baông xã inlớn vogue. At the start of WWII, on Feb. 9, 1942, President Franklin Roosevelt re-established daylight saving time year-round, calling it "War Time." 

Related: 5 crazy chapters in the history of daylight saving time

After the war, a free-for-all system in which U.S. states và towns were given the choice of whether or not to lớn observe DST led to chaos. And in 1966, lớn tame such "Wild West" mayhem, Congress enacted the Uniform Time Act. That federal law meant that any state observing DST — và they didn"t have sầu to jump on the DST bandwagon — had to follow a unisize protocol throughout the state in which daylight saving time would begin on the first Sunday of April & over on the last Sunday of October.

Then, in 2007, the Energy Policy Act of 2005 went inlớn effect, expanding the length of daylight saving time to lớn the present timing.

Why bởi vì we have daylight saving time?

Fewer than 40% of the world"s countries observe daylight saving time, according to lớn timeanddate.com. However, those who vì chưng observe DST take advantage of the natural daylight in the summer evenings. That"s because the days start to lớn get longer as Earth moves from the winter season khổng lồ spring & summer, with the longest day of the year on the summer solstice. During the summer season in each hemisphere, Earth, which revolves around its axis at an angle, is tilted directly toward the sun. 

Related: Read more about the science of summer.


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As Earth orbits the sun, it also spins around its own imaginary axis. Because it revolves around this axis at an angle, different parts of our planet experience the sun"s direct rays at different times of the year, leading khổng lồ the seasons. (Image credit: BlueRingMedia / Shutterstock.com)

Regions farthest away from the equator & closer lớn the poles get the most benefit from the DST cloông chồng change, because there is a more dramatic change in sunlight throughout the seasons.

Research has also suggested that with more daylight in the evenings, there are fewer traffic accidents, as there are fewer cars on the road when it"s dark outside. More daylight also could mean more outdoor exercise (or exercise at all) for full-time workers.

The nominal reason for daylight saving time has long been khổng lồ save sầu energy. The time change was first instituted in the U.S. during World War I, and then reinstituted again during WW II, as a part of the war effort. During the Arab oil embargo, when Arab members of the Organization of Petroleum Exporting Countries (OPEC) stopped selling petroleum khổng lồ the United States, Congress even enacted a trial period of year-round daylight saving time in an attempt khổng lồ save sầu energy. 

But the evidence for energy savings is sllặng. Brighter evenings may save on electric lighting, said Stanton Hadley, a senior researcher at Oak Ridge National Laboratory who helped prepare a report khổng lồ Congress on extended daylight saving time in 2007. But lights have sầu become increasingly efficient, Hadley said, so lighting is responsible for a smaller chunk of total energy consumption than it was a few decades ago. Heating and cooling probably matter more, and some places may need air-conditioning for the longer, hotter evenings of summer daylight saving time.

Hadley & his colleagues found that the four weeks of extra daylight saving time that went inlớn effect in the United States in 2007 did save sầu some energy, about half of a percent of what would have otherwise been used on each of those days. However, Hadley said, the effect of the entire months-long stretch of daylight saving could very well have the opposite effect. 

A 1998 study in Indiana before & after implementation of daylight saving time in some counties found a small increase in residential energy usage. Temporary changes in Australia"s daylight saving timing for the summer Olympics of 2000 also failed khổng lồ save sầu any energy, a 2007 study found.

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Part of the trouble with estimating the effect of daylight saving time on energy consumption is that there are so few changes khổng lồ the policy, making before-and-after comparisons tricky, Hadley told Live Science. The 2007 extension of daylight saving time allowed for a before-and-after comparison of only a few weeks" time. The changes in Indiamãng cầu and Australia were geographically limited.

Ultimately, Hadley said, the energy question probably isn"t the real reason the United States sticks with daylight saving time, anyway.

"In the vast scheme of things, the energy saving is not the big driver," he said. "It"s people wanting lớn take advantage of that light time in the evening." 

What places observe sầu daylight saving time?

U.S. daylight saving time

Most of the United States và Canadomain authority observe DST on the same dates with a few exceptions. Hawaii và Arizomãng cầu are the two U.S. states that don"t observe sầu daylight saving time, though Navajo Nation, in northeastern Arizomãng cầu, does follow DST, according to NASA.

And, every year there are bills put forth to lớn get rid of DST in various states, as not everyone is keen on turning their clocks forward an hour. In 2018, Florida"s Senate và House passed legislation called the Sunshine Protection Act (a PDF of the legislation) that would ask the U.S. Congress khổng lồ exempt the state from the federal 1966 Unisize Time Act. If approved, Florida would remain in DST year-round. In order lớn allow Florida"s year-round DST, however, the U.S. Congress would have to lớn amkết thúc the Unisize Time Act (15 U.S.C. s. 260a) khổng lồ authorize states this allowance, according to The Thủ đô New York Times. Congress has yet to lớn approve the legislation, the South Floridomain authority Sun Sentinel reported. Fifteen other states have sầu made similar moves with laws, voter initiatives and resolutions. These states include: Arkansas, Alabama, California, Delaware, Georgia, Idaho, Louisiana, Maine, Ohio, Oregon, South Carolina, Tennessee, Utah, Washington, and Wyoming, according lớn a statement from the office of U.S. Sen. Marco Rubio (R–Fla.).

In the fall of 2018, California voted in favor of Proposition 7 that would attempt to repeal the annual clock changes. Next, the state legislature needs to vote on the proposition, followed by the Congress, according to lớn an article on Vox. However, none of that happened, because the federal government didn"t approve the time change, San Diego"s CBS8 reported.

Canada daylight saving time

Nine of Canada"s 10 provinces observe daylight saving time. The provinces & territories in Canada that stay on standard time all year include: Some regions of the province of British Columbia, parts of Saskatchewan, northwest Ontario và east Quebec, according lớn timeanddate.com. Meanwhile, Yukon made DST permanent in 2020. The locations in British Columbia that don"t use DST include: Chetwynd, Creston, Dawson Creek, Fort Nelson & Fort St. John; in Saskatchewan, only Creighton and Denare Beach observe DST, according to lớn timeanddate.com.

Europe daylight saving time

Most of Europe currently observes daylight saving time, which begins at 1 a.m. GMT on the last Sunday in March — that"s March 28, 2021, when Europeans move their clocks ahead one hour at 1 a.m. GMT. Daylight saving time ends (winter time) at 1 a.m. GMT on the last Sunday in October, or Oct. 31, 2021, when clocks are moved back an hour. 

Most European countries observe DST, with the exception of Russia, Icel& và Belarus, according to timeanddate.com. In the United Kingdom, DST is called British Summer Time (BST).

DST is called Central European Summer Time (CEST) in: Austria, France, Germany, Italy, Hungary, Norway, Pol&, Spain & Switzerland. Daylight saving starts at 2 a.m. local time for these countries, when clocks are moved ahead an hour to lớn 3 a.m. The same 2 a.m. clock change is followed for Bulgaria, Estonia, Finland, Greece, Latvia, Lithuania and Romania, which điện thoại tư vấn DST Eastern European Summer Time (EEST).

During summers in Irelvà, DST is called Irish Standard Time (IST) và it begins at 1 a.m. local time, when clocks are moved ahead an hour to 2 a.m. The same cloông chồng change occurs in the Canary Islands, the Faroe Islands & Portugal, which điện thoại tư vấn DST Western European Summer Time (WEST).However, even the European Union may propose an kết thúc to clock changes, as a recent poll found that 84% of 4.6 million people surveyed said they wanted khổng lồ nix them, the Wall Street Journal reported. If the lawmakers & thành viên states agree, the EU members could decide khổng lồ keep the EU in summer time or winter time, according lớn the WSJ.

Southern Hemisphere DST

The DST-observing countries in the Southern Hemisphere — in nước Australia, New Zealvà, South America và southern Africa — mix their clocks forward an hour sometime during September through November and move them baông chồng khổng lồ standard time during the March-April timeframe.

nước Australia, being such a big country (the sixth-largest in the world), doesn"t follow DST uniformly: New South Wales, Victoria, South Australia, Tasmania và the Australian Capital Territory follow daylight saving, while Queensland, the Northern Territory (Western Australia) do not, according to lớn the Australian government. Clocks in the observing areas spring forward an hour at 2 a.m. local time on the first Sunday in October —which is Oct. 3, 2021 — and push back an hour at 3 a.m. local daylight time on the first Sunday in April — or April 4, 2021.

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Daylight saving time myths

Before the Unikhung Time Act was passed in the United States, there was a period in which any place could or could not observe sầu DST, leading to lớn chaos. For instance, if one took a 35-mile bus ride from Moundsville, West Virginia, to lớn Steubenville, Ohio, he or she would pass through no fewer than seven time changes, according to Prerau củ. At some point, Minneapolis and St. Paul were on different clocks.The fact that the time changes at 2 a.m. at least in the U.S., may have khổng lồ vì chưng with practicality. For instance, it"s late enough that most people are trang chính from outings & setting the cloông chồng bachồng an hour won"t switch the date khổng lồ "yesterday." In addition, it"s early enough not lớn affect early shift workers & early churchgoers, according to the WebExhibits, an online museum.

Editor"s note: This article was updated on March 11, 2021. 


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