Daylight saving time in the United States is the practice of setting the clock forward by one hour during the warmer part of the year, so that evenings have more daylight and mornings have less. Most areas of the United States observe daylight saving time (DST), the exceptions being Arizona (except for the Navajo, who do observe daylight saving time on tribal lands),  Hawaii,  and the overseas territories of American Samoa, Guam, the Northern Mariana Islands, Puerto Rico, and the United States Virgin Islands.
In general, daylight saving time is not observed in lower-latitude regions, as summer days are not much longer than winter ones as in higher latitudes, so the advantage of the change is smaller.
Daylight saving time starts on the second Sunday in March and ends on the first Sunday in November, with the time changes taking place at 2:00 a.m. local time. With a mnemonic word play referring to seasons, clocks "spring forward, fall back" —that is, in springtime the clocks are moved forward from 2:00 a.m. to 3:00 a.m. and in fall they are moved back from 2:00 a.m. to 1:00 a.m. Daylight saving time lasts for a total of 34 weeks (238 days) every year, about 65% of the entire year.
The following table lists recent past and near future starting and ending dates of daylight saving time in the United States:
|2014||March 9||November 2|
|2015||March 8||November 1|
|2016||March 13||November 6|
|2017||March 12||November 5|
|2018||March 11||November 4|
|2019||March 10||November 3|
|2020||March 8||November 1|
|2021||March 14||November 7|
|2022||March 13||November 6|
|2023||March 12||November 5|
History of DST in the United States
Benjamin Franklin proposed a form of daylight time in 1784. His essay, “An Economical Project for Diminishing the Cost of Light”, written to the editor of The Journal of Paris , observed that Parisians could save on candles by getting out of bed earlier in the morning, making use of the natural morning light instead. Franklin's suggestion seems to have been more of a joke than a real proposal, and nothing came of it. 
1916–1966: Early, inconsistent use
During World War I, in an effort to conserve fuel, Germany began observing DST on May 1, 1916. The rest of Europe soon followed. The plan was not adopted in the United States until the Standard Time Act of March 19, 1918, which confirmed the existing standard time zone system and set summer DST to begin on March 31, 1918 (reverting October 27). The idea was unpopular and Congress abolished DST after the war, overriding President Woodrow Wilson's veto.  DST became a local option and was observed in some states until World War II, when President Franklin Roosevelt instituted year-round DST, called "War Time", on February 9, 1942.  It lasted until the last Sunday (the 30th) in September 1945. After 1945 many states and cities east of the Mississippi River (and mostly north of the Ohio and Potomac rivers) adopted summer DST.
From 1945 to 1966 there was no federal law on daylight saving time, so localities could choose when it began and ended or drop it entirely. As of 1954, only California and Nevada had statewide DST west of the Mississippi, and only a few cities between Nevada and St. Louis. In the 1964 Official Railway Guide, 21 of the 48 states had no DST anywhere.
1966–1972: Federal standard established
By 1962, the transportation industry found the lack of consistency confusing enough to push for federal regulation. The result was the Uniform Time Act of 1966 (P.L. 89-387). Beginning in 1967, the act mandated standard time within the established time zones and provided for advanced time: clocks would be advanced one hour beginning at 2:00 a.m. on the last Sunday in April and turned back one hour at 2:00 a.m. on the last Sunday in October. States were allowed to exempt themselves from DST as long as the entire state did so. If a state chose to observe DST, the time changes were required to begin and end on the established dates. In 1967, Arizona and Michigan became the first states to exempt themselves from DST (Michigan would begin observing DST in 1972). In 1972, the act was amended (P.L. 92-267), allowing those states split between time zones to exempt either the entire state or that part of the state lying within a different time zone. The newly created Department of Transportation (DOT) was given power to enforce the law. As of 2014, the following states and territories are not observing DST: Arizona, Hawaii, American Samoa, Puerto Rico, and the Virgin Islands.
1973–1975: Year-round experiment
During the 1973 oil embargo by the Organization of Arab Petroleum Exporting Countries (OAPEC), in an effort to conserve fuel, Congress enacted a trial period of year-round DST (P.L. 93-182), beginning January 6, 1974, and ending April 27, 1975.  The trial was hotly debated. Those in favor pointed to increased daylight hours in the winter evening: more time for recreation, reduced lighting and heating demands, reduced crime, and reduced automobile accidents. The opposition was concerned about children leaving for school in the dark. The act was amended in October 1974 (P.L. 93-434) to return to standard time for the period beginning October 27, 1974, and ending February 23, 1975, when DST resumed. When the trial ended in 1975, the country returned to observing summer DST (with the aforementioned exceptions).
1975–1986: Extension of daylight saving time
The DOT, evaluating the plan of extending DST into March, reported in 1975 that "modest overall benefits might be realized by a shift from the historic six-month DST (May through October) in areas of energy conservation, overall traffic safety and reduced violent crime." However, DOT also reported that these benefits were minimal and difficult to distinguish from seasonal variations and fluctuations in energy prices.
Congress then asked the National Bureau of Standards (NBS, today NIST) to evaluate the DOT report. Its report, "Review and Technical Evaluation of the DOT Daylight Saving Time Study" (April 1976), found no significant energy savings or differences in traffic fatalities.  It did find statistically significant evidence of increased fatalities among school-age children in the mornings during the four-month period January–April 1974 as compared with the same period (non-DST) of 1973. NBS stated that it was impossible to determine, what, if any, of this increase was due to DST. When these data were compared between 1973 and 1974 for the months of March and April, no significant difference was found in fatalities among school-age children in the mornings.
In 1986 Congress enacted P.L. 99-359, amending the Uniform Time Act by changing the beginning of DST to the first Sunday in April and having the end remain the last Sunday in October. These start and end dates were in effect from 1987 to 2006. The time was adjusted at 2:00 a.m. local time.
2005–2009: Second extension
By the Energy Policy Act of 2005, daylight saving time (DST) was extended in the United States beginning in 2007.  As from that year, DST begins on the second Sunday of March and ends on the first Sunday of November. In years when April 1 falls on Monday through Wednesday, these changes result in a DST period that is five weeks longer; in all other years the DST period is instead four weeks longer.  In 2008 daylight saving time ended at 2:00 a.m. DST (0200) (1:00 a.m. ST) on Sunday, November 2, and in 2009 it began at 2:00 a.m. (3:00 a.m. DST) on Sunday, March 8.  Wyoming Senator Michael Enzi and Michigan Representative Fred Upton advocated the extension from October into November especially to allow children to go trick-or-treating in more daylight. 
Under Section 110 of the Act, the U.S. Department of Energy was required to study the impact of the 2007 DST extension no later than nine months after the change took effect. The report, released in October 2008, reported a nationwide electricity savings of 0.03% for the year of 2007. 
An October 2008 study conducted by the University of California at Santa Barbara for the National Bureau of Economic Research found that the 2006 DST adoption in Indiana increased energy consumption in Indiana by an average of 1%. Although consumption for lighting dropped as a result of the DST adoption, consumption for heating and cooling increased by 2% to 4%. The cost to the average Indiana household of the DST adoption was determined to be $3.29 per year, for an aggregate cost of $1.7 million to $5.5 million per year. 
Procedure for modifications
Changing an area's time zone
Under the Standard Time Act of 1918, as amended by the Uniform Time Act of 1966, moving a state or an area within a state from one time zone to another requires a regulation issued by DOT. The governor or state legislature may initiate a request for the state or any part of the state; the highest elected officials in the county may make a request for that county. The standard in the statute for such decisions is the convenience of commerce in that area. The convenience of commerce is defined broadly to consider such circumstances as the shipment of goods within the community; the origin of television and radio broadcasts; the areas where most residents work, attend school, worship, or receive health care; the location of airports, railway, and bus stations; and the major elements of the community's economy.
After receiving a request for altering a time zone, DOT determines whether it meets the requirement of minimum statutory criteria before issuing a notice of proposed rulemaking, soliciting public comment and scheduling a public hearing. Usually the hearing is held in the area requesting the change so that all affected parties can be represented. After the close of the comment period, the comments are reviewed and appropriate final action taken. If the Secretary agrees that the statutory requirement has been met, the change is instituted, usually at the next changeover to or from DST.
Moving an area on or off DST
Under the Uniform Time Act, moving an area on or off DST is accomplished through legal action at the state level. Some states require legislation while others require executive action such as a governor's executive order. Information on procedures required in a specific state may be obtained from that state's legislature or governor's office. Although it may exempt itself, if a state decides to observe DST, the dates of observance must comply with federal legislation.
Local DST observance
Alaska observes DST. Due to its high latitude, Alaska has nearly round-the-clock daylight during summer, and DST is seen by some Alaskans as unnecessary and a nuisance. Another issue is that the Alaskan mainland's single time zone, which approximates solar time in the capital, Juneau, leads to a large disparity between civil time and solar time for much of the state, with solar noon occurring as late as 2:00 p.m. in the population centers of Anchorage and Fairbanks, and as late as 3:00 p.m. in places such as Nome. In Fairbanks, for example, sunset occurs well after civil midnight in the summer. Others argue that ending daylight saving time will place Alaska as much as five hours from Eastern Daylight Time, making coordination of travel and phone conversations more difficult.
In March 2015 the Alaska Senate passed a bill, introduced by state senator Anna MacKinnon, to end daylight saving time. The Alaska House formed a sub-committee that was due to look into the issue during the 2016 legislative session. 
Arizona observed DST in 1967 under the Uniform Time Act because the state legislature did not enact an exemption statute that year. In March 1968 the DST exemption statute was enacted and the state of Arizona has not observed DST since 1967. This is in large part due to an effort to conserve energy. Phoenix and Tucson are among the hottest U.S. metropolitan areas during the summer, resulting in more power usage from air conditioning units and evaporative coolers in homes and businesses. An extra hour of sunlight while people are active would cause people to run their cooling systems longer, thereby using more energy. 
Reservations in Arizona
The Navajo Indian Reservation extends into Utah and New Mexico and does observe daylight saving time.  The Hopi Reservation is entirely within the state of Arizona and is an enclave within the Navajo Indian Reservation, but does not observe DST.
California voters passed Proposition 12  in 1949, approving DST in the state.  In February 2017, California Assembly member Kansen Chu introduced A.B. 807,  which, if enacted and approved by voters, would enable the legislature to eliminate DST in the state, or, if federal law allows, to adopt DST for the entire year.. 
Daylight time is less useful in Florida than in many other states because its southern location leads to less variation in day length between winter and summer. Without DST, Miami, for instance, would experience similar sunrise and sunset times throughout the year to cities such as Honolulu or Hong Kong, both of which have abandoned DST for decades. There is opposition to DST in Florida.  State senator Bill Posey introduced a bill in March 2008 to abolish daylight time in the state and keep Florida on year-round standard time.  Because Florida straddles two time zones, the Florida legislature has the option of returning all or part of the state to standard time along time zone boundaries (i.e. placing the portion of the state in the Eastern time zone on standard time, while leaving the portion in the Central time zone on DST, so that the whole state had a common time).
Because of Hawaii's tropical latitude, there is not a large variation in daylight length between winter and summer. Advancing the clock in Hawaii would make sunrise times close to 7:00 a.m. even in June.  Most of the inhabited islands are located close to the west end of the Hawaii-Aleutian time zone, but Oahu, Kauai, and Niihau are located more than 7 degrees west of the Hawaii-Aleutian time zone's meridian and should, theoretically, be located in the next time zone to the west. (Until about 1946 Hawaiian standard time was based on longitude 157.5 degrees west rather than 150 degrees.)
On April 26, 1933, the Territorial Legislature enacted a bill placing Hawaii on daylight saving time from the last Sunday in April (April 30 in that year) to the last Sunday in September, but the law was repealed three weeks later on May 21, 1933.  During World War II between February 9, 1942 and September 30, 1945, Hawaiian Standard Time was advanced one hour to so-called "Hawaiian War Time," effectively placing the territory on year-round daylight saving time.
From 1970 until 2006, most of Indiana in the Eastern Time Zone did not observe daylight saving time, but the entire state started to do so in April 2006 after eight counties in western Indiana were shifted from the Central Time Zone to the Eastern Time Zone.  One goal for observing DST was to get more Indiana counties observing the same time zone; formerly, 77 counties observed EST, 5 observed EST/EDT (the EDT usage being unofficial only), and 10 observed CST/CDT. As of 2016, Indiana has 12 counties observing Central Daylight Time while the remaining 80 counties observe Eastern Daylight Time. Those counties observing CST are in two groups: one near or in the Chicago metropolitan area, and the other around Evansville in the southwest corner of the state.
In 1967 the Michigan Legislature adopted a statute, Act 6 of the Public Acts of 1967, exempting the state from the observance of DST. The exemption statute was suspended on June 14, 1967, however, when the referendum was invoked. From June 14, 1967 until the last Sunday in October, 1967, Michigan observed DST, and did so in 1968 as well. The exemption statute was submitted to the voters at the General Election held in November 1968, and, in a close vote, the exemption statute was sustained. As a result, Michigan did not observe DST in 1969, 1970, 1971, or 1972. (Radio station CKLW in Windsor, Ontario, just across the river from Detroit, was popular on both sides of the border and would often give both times on the air, e.g. "It's 4:30 in Detroit and 5:30 in Windsor!") In November 1972, an initiative measure, repealing the exemption statute, was approved by the voters. Michigan again observed DST in 1973, and has continued to do so since then.
In 2015 the Nevada Senate passed Nevada Assembly Joint Resolution 4, which would make it so that Nevada remains on Daylight Saving Time throughout the year. This would mean that Nevada is on the same time as Arizona all year, but would be an hour ahead of California in the winter.  The United States Congress must now approve the change.
In 2015, Rep. Elizabeth Scott ( R - Monroe) filed a House bill to end DST in Washington, and a companion bill was filed in the Senate.  She told a House committee that the semiannual time switches are not only inconvenient but lead to health problems and accidents due to lost sleep.  She added that the bill to drop daylight saving time would reduce heart attacks, car wrecks and work accidents found to increase with the sleep-schedule disruptions.  A different Senate Bill would petition the federal government for year-round DST. 
Other U.S. locations
While neighboring Samoa began observing DST in September 2010, the smaller American Samoa cannot legally follow because of the DST observation period mandated by the Uniform Time Act. This period is actually wintertime in this Southern Hemisphere territory.
Many computer operating systems (such as Microsoft Windows, Linux, UNIX, and macOS) and programming languages (such as Java, perl, and most shell languages) allow a local time zone setting in the format of (standard time zone abbreviation)(UTC hour difference)(daylight saving time zone abbreviation) . This allows programs and programming languages that must do calculations based on local time to more easily calculate differences between local time and UTC, as well as knowing whether calculations should be changed during daylight saving time. For example, a time zone setting of EST5EDT indicates that local time on the computer is 5 hours behind UTC and should be adjusted for daylight saving time.
|Time Zone||Standard Time||Daylight Saving Time||TZ|
|Atlantic Time Zone||AST ( UTC-4)||ADT ( UTC-3)||AST4ADT|
|Eastern Time Zone||EST ( UTC-5)||EDT ( UTC-4)||EST5EDT|
|Central Time Zone||CST ( UTC-6)||CDT ( UTC-5)||CST6CDT|
|Mountain Time Zone||MST ( UTC-7)||MDT ( UTC-6)||MST7MDT|
|Pacific Time Zone||PST ( UTC-8)||PDT ( UTC-7)||PST8PDT|
|Alaska Time Zone||AKST ( UTC-9)||AKDT ( UTC-8)||AKST9AKDT|
|Hawaii–Aleutian Time Zone||HAST ( UTC-10)|| HADT ( UTC-9) |
Aleutian Islands only
- Daylight saving time
- Daylight saving time by country
- CHU distributes official time in Canada
- History of time in the United States includes a list of historical daylight saving dates back to 1918
Official civil time distribution
Quasi-governmental time distribution systems