Daylight saving time in the United States

Daylight saving time in the United States was first observed in 1918. Most areas of the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

Daylight saving time

Daylight saving time —also summer time in several countries including in British English and European official terminology —is the practice of temporarily advancing clocks during the summertime so that afternoons have more daylight and mornings have less...

, with the exceptions being the states of Arizona

Arizona

Arizona ; is a state located in the southwestern region of the United States. It is also part of the western United States and the mountain west. The capital and largest city is Phoenix...

and Hawaii

Hawaii

Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...

along with the territories of Puerto Rico

Puerto Rico

Puerto Rico , officially the Commonwealth of Puerto Rico , is an unincorporated territory of the United States, located in the northeastern Caribbean, east of the Dominican Republic and west of both the United States Virgin Islands and the British Virgin Islands.Puerto Rico comprises an...

, American Samoa

American Samoa

American Samoa is an unincorporated territory of the United States located in the South Pacific Ocean, southeast of the sovereign state of Samoa...

, Guam

Guam

Guam is an organized, unincorporated territory of the United States located in the western Pacific Ocean. It is one of five U.S. territories with an established civilian government. Guam is listed as one of 16 Non-Self-Governing Territories by the Special Committee on Decolonization of the United...

, and the Northern Mariana Islands

Northern Mariana Islands

The Northern Mariana Islands, officially the Commonwealth of the Northern Mariana Islands , is a commonwealth in political union with the United States, occupying a strategic region of the western Pacific Ocean. It consists of 15 islands about three-quarters of the way from Hawaii to the Philippines...

.

From 1987 to 2006, daylight saving time in the United States began on the first Sunday of April and ended on the last Sunday of October. The time was adjusted at 2:00 AM (0200) local time (as it still is done now).

Since 2007, daylight saving time starts on the second Sunday of March and ends on the first Sunday of November, with all time changes taking place at 2:00 AM (0200) local time. In 2011, daylight saving time began on March 13 and ended on November 6; in 2012, it will begin on March 11 and end on November 4.

1918 to 1987

During World War I, in an effort to conserve fuel, Germany began observing DST on May 1, 1916. As the war progressed, the rest of Europe adopted DST. The plan was not formally adopted in the United States until the Standard Time Act

Standard Time Act

The Standard Time Act, , also known as the Calder Act, was the first United States federal law implementing standard time and Daylight saving time in the United States. It authorized the Interstate Commerce Commission to define each time zone....

of March 19, 1918. It established both standard time zones and set summer DST to begin on March 31, 1918. The idea was unpopular, however, 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 in September 1945. The next year, many states and localities adopted summer DST.

From 1945 to 1966, there was no federal law regarding daylight saving time, so states and localities were free to choose whether to observe it, and could choose when it began and ended. By 1962, the transportation industry found the lack of nationwide consistency in time observance confusing enough to push for federal regulation. This drive resulted in the Uniform Time Act of 1966 (P.L. 89-387). 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 1968, Arizona became the first state to exempt itself from DST. 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 the power to enforce the law. Currently, the following states and territories do not observe DST: Arizona, Hawaii, American Samoa, Puerto Rico, and the Virgin Islands.

During the 1973 oil embargo by the Organization of Petroleum Exporting Countries (OPEC), 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. From the beginning, the trial was hotly debated. Those in favor pointed to the benefits of 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).

DOT, charged with 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) to evaluate the DOT report. In an April 1976 report to Congress, Review and Technical Evaluation of the DOT Daylight Saving Time Study, NBS 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 this same data was compared between 1973 and 1974 for the individual months of March and April, no significant difference was found for fatalities among school-age children in the mornings.

In 1986, the 99th Congress enacted P.L. 99-359, which amended 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.

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 the Department of Transportation (DOT). The governor or state legislature makes the request for a 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 DOT will then determine whether it meets the minimum statutory criteria before issuing a notice of proposed rulemaking, which will solicit public comment and schedule 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.

1987 to 2006

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.

The schedule through 2006 in the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

was that DST began on the first Sunday in April (April 2, 2006), and changed back to standard time on the last Sunday in October (October 29, 2006). The time is adjusted at 2:00 AM (0200) local time.

2007 to the present

By the Energy Policy Act of 2005

Energy Policy Act of 2005

The Energy Policy Act of 2005 is a bill passed by the United States Congress on July 29, 2005, and signed into law by President George W. Bush on August 8, 2005, at Sandia National Laboratories in Albuquerque, New Mexico...

, daylight saving time (DST) was extended in the United States in 2007. DST starts on the second Sunday of March and it ends on the first Sunday of November. These changes result in a DST period that is four weeks longer than in previous years. In 2008 daylight saving time ended at 2:00 AM (0200) on Sunday, November 2, and in 2009 it began at 2:00 AM (0200) on Sunday, March 8. Wyoming Senator Michael Enzi and Michigan Representative Fred Upton

Fred Upton

Frederick Stephen Upton is the U.S. Representative for , serving since 1987. He is a member of the Republican Party and Chairman of the Committee on Energy and Commerce. The district, based in Kalamazoo, stretches along the Michigan-Indiana border in the southwestern part of the state.-Early life,...

advocated the extension from October into November especially to allow children to go trick-or-treating

Trick-or-treating

Trick-or-treating or "Guising", is a customary practice for children on Halloween in many countries. Children in costumes travel from house to house in order to ask for treats such as candy with the question "Trick or treat?"...

in more daylight.

The following table lists the dates of DST under the current law through 2025, barring any further adjustments to the law:

Year	Date DST Begins	Date DST Ends
2007	March 11	November 4
2008	March 9	November 2
2009	March 8	November 1
2010	March 14	November 7
2011	March 13	November 6
2012	March 11	November 4
2013	March 10	November 3
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
2024	March 10	November 3
2025	March 9	November 2

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

National Bureau of Economic Research

The National Bureau of Economic Research is an American private nonprofit research organization "committed to undertaking and disseminating unbiased economic research among public policymakers, business professionals, and the academic community." The NBER is well known for providing start and end...

found that the 2006 DST adoption in Indiana increased energy consumption in Indiana by an average of 1%. Although energy 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.

Alaska

Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...

observes DST although there is a statewide move to abolish it. , Alaska's lieutenant governor

Lieutenant governor

A lieutenant governor or lieutenant-governor is a high officer of state, whose precise role and rank vary by jurisdiction, but is often the deputy or lieutenant to or ranking under a governor — a "second-in-command"...

Loren Leman

Loren Dwight Leman was the tenth Lieutenant Governor of Alaska, traces his family history in Alaska to a marriage in Kodiak more than 200 years ago between a Russian shipbuilder and an Alutiiq woman from Afognak...

approved a petition to collect signatures to put the initiative

Initiative

In political science, an initiative is a means by which a petition signed by a certain minimum number of registered voters can force a public vote...

measure on the ballot by 2008. 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

Time zone

A time zone is a region on Earth that has a uniform standard time for legal, commercial, and social purposes. In order for the same clock time to always correspond to the same portion of the day as the Earth rotates , different places on the Earth need to have different clock times...

is too wide and there is a large disparity between civil time

Civil time

In modern usage, civil time refers to statutory time scales designated by civilian authorities, or to local time indicated by clocks. Modern civil time is generally standard time in a time zone at a fixed offset from Coordinated Universal Time or from Greenwich Mean Time , possibly adjusted by...

and solar time

Solar time

Solar time is a reckoning of the passage of time based on the Sun's position in the sky. The fundamental unit of solar time is the day. Two types of solar time are apparent solar time and mean solar time .-Introduction:...

, with solar noon occurring as late as 3:00 PM (1500) by the clock in places like Nome

Nome, Alaska

Nome is a city in the Nome Census Area in the Unorganized Borough of the U.S. state of Alaska, located on the southern Seward Peninsula coast on Norton Sound of the Bering Sea. According to the 2010 Census, the city population was 3,598. Nome was incorporated on April 9, 1901, and was once the...

. 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.

Arizona

Arizona ; is a state located in the southwestern region of the United States. It is also part of the western United States and the mountain west. The capital and largest city is Phoenix...

observed DST in 1967 under the Uniform Time Act because the state legislature

Arizona Legislature

The Arizona Legislature is the state legislature of the U.S. state of Arizona. It is a bicameral legislature that consists of a lower house, the House of Representatives, and an upper house, the Senate. There are 60 Representatives and 30 Senators...

did not enact an exemption statute

Statute

A statute is a formal written enactment of a legislative authority that governs a state, city, or county. Typically, statutes command or prohibit something, or declare policy. The word is often used to distinguish law made by legislative bodies from case law, decided by courts, and regulations...

that year. In March 1968, the DST exemption statute

Statute

was enacted and the state of Arizona has not observed DST since 1967 (however, the large Navajo Indian Reservation

Navajo Nation

The Navajo Nation is a semi-autonomous Native American-governed territory covering , occupying all of northeastern Arizona, the southeastern portion of Utah, and northwestern New Mexico...

, which extends from Arizona into two adjacent states, does, but the Hopi Reservation

Hopi Reservation

The Hopi Reservation, or simply Hopi, is a Native American reservation for the Hopi and Arizona Tewa people, surrounded entirely by the Navajo Nation, in Navajo and Coconino counties of Arizona, USA. The site in north-eastern Arizona has a land area of 2,531.773 sq mi and as of the 2000 census had...

, which is completely contained within both the Navajo reservation and the state of Arizona, does not). This is in large part due to energy conservation: Phoenix

Phoenix, Arizona

Phoenix is the capital, and largest city, of the U.S. state of Arizona, as well as the sixth most populated city in the United States. Phoenix is home to 1,445,632 people according to the official 2010 U.S. Census Bureau data...

and Tucson

Tucson, Arizona

Tucson is a city in and the county seat of Pima County, Arizona, United States. The city is located 118 miles southeast of Phoenix and 60 miles north of the U.S.-Mexico border. The 2010 United States Census puts the city's population at 520,116 with a metropolitan area population at 1,020,200...

are hotter than any other large U.S. metropolitan area 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. Local residents remember the summer of 1967, the one year DST was observed. The State Senate Majority leader at the time owned drive-in movie theaters and was nearly bankrupted by the practice. Movies could not start until 10:00 PM (2200) at the height of summer: well past normal hours for most Arizona residents. There has never been any serious consideration of reversing the exemption.

Colorado

At the end of the 20th century, Colorado Springs Gazette

The Gazette (Colorado Springs)

The Gazette is a newspaper based in Colorado Springs, Colorado, United States. It is published daily by Irvine, California-based Freedom Communications...

columnist Ralph Routon wrote a series of columns supporting the idea of placing all of Colorado

Colorado

Colorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...

on year-round DST in order to save state residents the "aggravation of resetting their clocks every six months." The idea gathered noticeable popular support in Colorado Springs, and the attention of the state's larger newspaper

Newspaper

A newspaper is a scheduled publication containing news of current events, informative articles, diverse features and advertising. It usually is printed on relatively inexpensive, low-grade paper such as newsprint. By 2007, there were 6580 daily newspapers in the world selling 395 million copies a...

s, but when then state Senator MaryAnne Tebedo

MaryAnne Tebedo

MaryAnne Geivett Tebedo was a Colorado State Senator from Colorado Springs.From 1982 through 1988 Tebedo served as a Republican in the Colorado House of Representatives, and was elected to the Colorado State Senate in 1988. Tebedo completed her legislative career on December 31, 2001 pursuant to...

attempted to present the idea to the state legislature, her research uncovered federal laws forbidding the state-initiated extension of daylight saving time. Still determined to relieve Coloradans of the need to change their clocks, Tebedo introduced the only bill legally permitted to her: a proposal to exempt the state of Colorado from DST. The bill failed to escape committee during the 2000 legislative session.

Florida

Daylight time is less useful in Florida than in many other states because of its southern location. There is opposition to DST in Florida. In March 2008, Florida state senator Bill Posey

Bill Posey

William "Bill" Posey is the U.S. Representative for , serving since 2009. He is a member of the Republican Party. He formerly served in the Florida Senate and the Florida House of Representatives....

introduced a bill in the Florida legislature

Legislature

A legislature is a kind of deliberative assembly with the power to pass, amend, and repeal laws. The law created by a legislature is called legislation or statutory law. In addition to enacting laws, legislatures usually have exclusive authority to raise or lower taxes and adopt the budget and...

to abolish daylight time in the state and keep Florida on year-round standard time

Standard time

Standard time is the result of synchronizing clocks in different geographical locations within a time zone to the same time rather than using the local meridian as in local mean time or solar time. Historically, this helped in the process of weather forecasting and train travel. The concept...

. Because Florida is in two time zones, the Florida legislature has the option of returning all or part of the state to standard time along time zone boundaries.

Hawaii

Because of Hawaii

Hawaii

's tropical

Tropics

The tropics is a region of the Earth surrounding the Equator. It is limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately N and the Tropic of Capricorn in the southern hemisphere at S; these latitudes correspond to the axial tilt of the Earth...

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

Hawaii-Aleutian time zone

The Hawaii-Aleutian Time Zone observes Hawaii-Aleutian Standard Time , by subtracting ten hours from Coordinated Universal Time . The clock time in this zone is based on the mean solar time of the 150th meridian west of the Greenwich Observatory....

, but Oahu

Oahu

Oahu or Oahu , known as "The Gathering Place", is the third largest of the Hawaiian Islands and most populous of the islands in the U.S. state of Hawaii. The state capital Honolulu is located on the southeast coast...

, Kauai

Kauai

Kauai or Kauai, known as Tauai in the ancient Kaua'i dialect, is geologically the oldest of the main Hawaiian Islands. With an area of , it is the fourth largest of the main islands in the Hawaiian archipelago, and the 21st largest island in the United States. Known also as the "Garden Isle",...

and Niihau

Niihau

Niihau or Niihau is the seventh largest of the inhabited Hawaiian Islands in the U.S. state of Hawaii, having an area of . Niihau lies southwest of Kauai across the Kaulakahi Channel. Several intermittent playa lakes provide wetland habitats for the Hawaiian Coot, the Black-winged Stilt, and the...

are located more than 7 degrees west of the Hawaii-Aleutian time zone

Hawaii-Aleutian time zone

's meridian

Meridian (geography)

A meridian is an imaginary line on the Earth's surface from the North Pole to the South Pole that connects all locations along it with a given longitude. The position of a point along the meridian is given by its latitude. Each meridian is perpendicular to all circles of latitude...

and should, theoretically, be located in the next time zone

Time zone

to the west.

Hawaii did experiment with DST for three weeks between April 30, 1933 and May 21, 1933; there is no known official record as to why it was implemented or discontinued. Hawaii has never observed daylight saving time under the Uniform Time Act, having opted out of the Act's provisions in 1967.

Indiana

From 1970 until 2006, most of Indiana

Indiana

Indiana is a US state, admitted to the United States as the 19th on December 11, 1816. It is located in the Midwestern United States and Great Lakes Region. With 6,483,802 residents, the state is ranked 15th in population and 16th in population density. Indiana is ranked 38th in land area and is...

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 Eastern Time Zone to the Central Time Zone. One of the goals for observing DST was to get more Indiana counties observing the same timezone; formerly, 77 counties observed EST, 5 observed EST/EDT (the EDT usage being unofficial only), and 10 observed CST/CDT. At present Indiana has 12 counties observing Central Daylight Time while the remaining 80 counties observe Eastern Daylight Time.

Michigan

In 1967 the Michigan Legislature

Michigan Legislature

The Michigan Legislature is the legislative assembly of the U.S. state of Michigan. It is organized as a bicameral body consisting of the Senate, the upper house, and the House of Representatives, the lower house. Article IV of the state's Constitution, adopted in 1963, defines the role of the...

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

Referendum

A referendum is a direct vote in which an entire electorate is asked to either accept or reject a particular proposal. This may result in the adoption of a new constitution, a constitutional amendment, a law, the recall of an elected official or simply a specific government policy. It is a form of...

was invoked. From June 14, 1967, until the last Sunday in October, 1967, Michigan

Michigan

Michigan is a U.S. state located in the Great Lakes Region of the United States of America. The name Michigan is the French form of the Ojibwa word mishigamaa, meaning "large water" or "large lake"....

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 very close vote, the exemption statute was sustained. As a result, Michigan

Michigan

did not observe DST in 1969, 1970, 1971, or 1972. In November, 1972, an initiative measure, repealing the exemption statute, was approved by the voters. Michigan

Michigan

has observed DST in 1973 and all subsequent years.

Nevada

In 2005, Nevada Assembly

Nevada Assembly

The Nevada Assembly is the lower house of the Nevada Legislature, the state legislature of the U.S. state of Nevada. The body consists of 42 members, elected to two-year terms from single-member districts. Each Assembly district contained approximately 47,400 people as of the 2000 census, although...

Bill 18 would have exempted Nevada

Nevada

Nevada is a state in the western, mountain west, and southwestern regions of the United States. With an area of and a population of about 2.7 million, it is the 7th-largest and 35th-most populous state. Over two-thirds of Nevada's people live in the Las Vegas metropolitan area, which contains its...

from daylight saving time. The bill's author, Assemblyman Bob McCleary, D-North Las Vegas, argued that because of southern Nevada's desert climate, it would reduce power usage during the peak summer months by reducing the time that people would operate their home air conditioners. The result of not observing DST, however, would place the state in an odd time configuration relative to neighboring states. Because it is on the eastern edge of the Pacific Time Zone

Pacific Time Zone

The Pacific Time Zone observes standard time by subtracting eight hours from Coordinated Universal Time . The clock time in this zone is based on the mean solar time of the 120th meridian west of the Greenwich Observatory. During daylight saving time, its time offset is UTC-7.In the United States...

, Nevada (PST) would be two hours behind Utah (MDT), its eastern neighbor, and one hour behind California (PDT), its western neighbor. In the summer, it would therefore be the same time in Nevada (PST) as it would be in the majority of Alaska (AKDT). The bill died without a vote.

Other U.S. locations

All U.S. insular territories with civilian government (American Samoa

American Samoa

American Samoa is an unincorporated territory of the United States located in the South Pacific Ocean, southeast of the sovereign state of Samoa...

, Guam

Guam

, Northern Mariana Islands

Northern Mariana Islands

, Puerto Rico

Puerto Rico

, and the Virgin Islands

United States Virgin Islands

The Virgin Islands of the United States are a group of islands in the Caribbean that are an insular area of the United States. The islands are geographically part of the Virgin Islands archipelago and are located in the Leeward Islands of the Lesser Antilles.The U.S...

), lie in the tropics and do not observe DST.

While neighboring Samoa

Samoa

Samoa , officially the Independent State of Samoa, formerly known as Western Samoa is a country encompassing the western part of the Samoan Islands in the South Pacific Ocean. It became independent from New Zealand in 1962. The two main islands of Samoa are Upolu and one of the biggest islands in...

began observing DST in September 2010, the smaller American Samoa

American Samoa

American Samoa is an unincorporated territory of the United States located in the South Pacific Ocean, southeast of the sovereign state of 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.

Time zones

Many computer operating systems (such as Linux

Linux

Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...

, UNIX

Unix

Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...

, Mac OS X

Mac OS X

Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...

, and Microsoft Windows

Microsoft Windows

Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...

) and programming languages (such as Java

Java (programming language)

Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...

, perl

Perl

Perl is a high-level, general-purpose, interpreted, dynamic programming language. Perl was originally developed by Larry Wall in 1987 as a general-purpose Unix scripting language to make report processing easier. Since then, it has undergone many changes and revisions and become widely popular...

, and most shell languages

Shell (computing)

A shell is a piece of software that provides an interface for users of an operating system which provides access to the services of a kernel. However, the term is also applied very loosely to applications and may include any software that is "built around" a particular component, such as web...

) 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
Eastern Time Zone Eastern Time Zone The Eastern Time Zone of the United States and Canada is a time zone that falls mostly along the east coast of North America. Its UTC time offset is −5 hrs during standard time and −4 hrs during daylight saving time...	EST (UTC-5 UTC-5 UTC−05:00 is an identifier for a time offset from UTC of −05.This offset is used in the Eastern Time Zone during standard time and in the Central Time Zone during Daylight Saving Time .... )	EDT(UTC-4 UTC-4 UTC−04-00 is an identifier for a time offset from UTC of −04.This offset is used in the Atlantic Standard Time Zone in Canada and the North American Eastern Time Zone during daylight saving time , in the Caribbean region this is considered as the Eastern Caribbean timezone... )
Central Time Zone	CST (UTC-6 UTC-6 UTC−06 is an identifier for a time offset from UTC of −06. This time is used in:-Central America:BelizeCosta RicaEl SalvadorGuatemalaHondurasNicaragua-Central Standard Time:... )	CDT (UTC-5 UTC-5 UTC−05:00 is an identifier for a time offset from UTC of −05.This offset is used in the Eastern Time Zone during standard time and in the Central Time Zone during Daylight Saving Time .... )
Mountain Time Zone Mountain Time Zone The Mountain Time Zone of North America keeps time by subtracting seven hours from Coordinated Universal Time, also known as Greenwich Mean Time, during the shortest days of autumn and winter , and by subtracting six hours during daylight saving time in the spring, summer, and early autumn...	MST (UTC-7 UTC-7 thumb\|300px\|UTC−07: Blue , Orange , Yellow , Light Blue - Sea areasUTC−07:00 is an identifier for a time offset from UTC of −07.... )	MDT (UTC-6 UTC-6 UTC−06 is an identifier for a time offset from UTC of −06. This time is used in:-Central America:BelizeCosta RicaEl SalvadorGuatemalaHondurasNicaragua-Central Standard Time:... )
Pacific Time Zone Pacific Time Zone The Pacific Time Zone observes standard time by subtracting eight hours from Coordinated Universal Time . The clock time in this zone is based on the mean solar time of the 120th meridian west of the Greenwich Observatory. During daylight saving time, its time offset is UTC-7.In the United States...	PST (UTC-8 UTC-8 UTC−08:00 is an identifier for a time offset from UTC of −08. This time is used in:-As standard time :CanadaNorthwest Territories*Tungsten and the associated Cantung MineMexico... )	PDT (UTC-7 UTC-7 thumb\|300px\|UTC−07: Blue , Orange , Yellow , Light Blue - Sea areasUTC−07:00 is an identifier for a time offset from UTC of −07.... )
Alaska Time Zone Alaska Time Zone The Alaska Time Zone observes standard time by subtracting nine hours from Coordinated Universal Time . During daylight saving time its time offset is only eight hours . The clock time in this zone is based on the mean solar time of the 135th meridian west of the Greenwich Observatory.The zone...	AKST (UTC-9 UTC-9 UTC−09 is an identifier for a time offset from UTC of −09. This time is used in:-As standard time :United States*Alaska... )	AKDT (UTC-8 UTC-8 UTC−08:00 is an identifier for a time offset from UTC of −08. This time is used in:-As standard time :CanadaNorthwest Territories*Tungsten and the associated Cantung MineMexico... )
Hawaii-Aleutian Time Zone Hawaii-Aleutian time zone The Hawaii-Aleutian Time Zone observes Hawaii-Aleutian Standard Time , by subtracting ten hours from Coordinated Universal Time . The clock time in this zone is based on the mean solar time of the 150th meridian west of the Greenwich Observatory....	HAST (UTC-10 UTC-10 UTC−10 is an identifier for a time offset from UTC of −10. This time is used in:-As standard time :Cook Islands Tokelau *French Polynesia... )	HADT (UTC-9 UTC-9 UTC−09 is an identifier for a time offset from UTC of −09. This time is used in:-As standard time :United States*Alaska... ) Aleutian Islands only

1918 to 1987

1987 to 2006

2007 to the present

Alaska

Arizona

Colorado

Florida

Hawaii

Indiana

Michigan

Nevada

Other U.S. locations

Time zones

See also