Time in the United States

Time in the United States, by law, is divided into nine standard time zones covering the states and its possessions, with most of the United States observing daylight saving time (DST) for approximately the spring, summer, and fall months. The time zone boundaries and DST observance are regulated by the Department of Transportation. Official and highly precise timekeeping services (clocks) are provided by two federal agencies: the National Institute of Standards and Technology (NIST) (an agency of the Department of Commerce); and its military counterpart, the United States Naval Observatory (USNO). The clocks run by these services are kept synchronized with each other as well as with those of other international timekeeping organizations.

It is the combination of the time zone and daylight saving rules, along with the timekeeping services, which determines the legal civil time for any U.S. location at any moment.

Map of U.S. time zones between April 2, 2006, and March 11, 2007. The current situation is different only in that several Indiana counties are now in the eastern time zone instead of the central time zone.

History

Prior to the adoption of four standard time zones for the continental United States, many towns and cities set clocks based on sunsets and sunrises at their locations and by observing the Sun's zenith at noon (mean solar time). Dawn and dusk occur at different times but time differences between distant locations were barely noticeable prior to the 19th century because of long travel times and the lack of long-distance instant communications prior to the development of the telegraph.

The use of local solar time became increasingly awkward as railways and telecommunications improved.^[1] American railroads maintained many different time zones during the late 1800s. Each train station set its own clock making it difficult to coordinate train schedules and confusing passengers. Time calculation became a serious problem for people travelling by train (sometimes hundreds of miles in a day), according to the Library of Congress. Every city in the United States used a different time standard so there were more than 300 local sun times to choose from. Time zones were therefore a compromise, relaxing the complex geographic dependence while still allowing local time to be approximate with mean solar time. Railroad managers tried to address the problem by establishing 100 railroad time zones, but this was only a partial solution to the problem.^[1]

Operators of the new railroad lines needed a new time plan that would offer a uniform train schedule for departures and arrivals. Four standard time zones for the continental United States were introduced at noon on November 18, 1883, when the telegraph lines transmitted time signals to all major cities.^[2]

In October 1884, The International Meridian Conference at Washington DC adopted a proposal which stated that the prime meridian for longitude and timekeeping should be one that passes through the centre of the transit instrument at the Greenwich Observatory in the United Kingdom (UK). The conference therefore established the Greenwich Meridian as the prime meridian and Greenwich Mean Time (GMT) as the world's time standard. The US time-zone system grew from this, in which all zones referred back to GMT on the prime meridian.^[1]

From GMT to UTC

GMT was superseded as the international civil time standard by Coordinated Universal Time (UTC) in 1960, when the International Radio Consultative Committee formalized the concept of Coordinated Universal Time, abbreviated as UTC. It is, within about 1 second, mean solar time at 0°.^[3] It does not observe daylight saving time. It is one of several closely related successors to Greenwich Mean Time (GMT). For most purposes, UTC is considered interchangeable with GMT, but GMT is no longer precisely defined by the scientific community.

United States time zones

Standard time zones in the United States are currently defined at the federal level by law 15 USC §260.^[4] The federal law also establishes the transition dates and times at which daylight saving time occurs, if observed. It is ultimately the authority of the Secretary of Transportation, in coordination with the states, to determine which regions will observe which of the standard time zones and if they will observe daylight saving time.^[5] As of August 9, 2007, the standard time zones are defined in terms of hourly offsets from UTC.^[6] Prior to this they were based upon the mean solar time at several meridians 15° apart west of Greenwich (GMT).

Only the full-time zone names listed below are official; abbreviations are by common use conventions, and duplicated elsewhere in the world for different time zones.

The United States uses nine standard time zones. As defined by US law^[7] they are:

View the standard time zone boundaries here.

Standard time and daylight saving time

Time Zone	Standard Time	Daylight Time
Hawaii–Aleutian Time Zone	HAST (UTC−10:00)	HADT (UTC−09:00) Aleutian Islands only
Alaska Time Zone	AKST (UTC−09:00)	AKDT (UTC−08:00)
Pacific Time Zone	PST (UTC−08:00)	PDT (UTC−07:00)
Mountain Time Zone	MST (UTC−07:00)	MDT (UTC−06:00)
Central Time Zone	CST (UTC−06:00)	CDT (UTC−05:00)
Eastern Time Zone	EST (UTC−05:00)	EDT (UTC−04:00)

Zones used in the contiguous U.S.

From east to west, the times zones of the contiguous United States are:

The eastern standard time zone: (Zone R), which comprises roughly the states on the Atlantic coast and the eastern two thirds of the Ohio Valley.
The central standard time zone: (Zone S), which comprises roughly the Gulf Coast, Mississippi Valley, and Great Plains.
The mountain standard time zone: (Zone T), which comprises roughly the states that include the Rocky Mountains.
The Pacific standard time zone: (Zone U), which comprises roughly the states on the Pacific coast, plus Nevada and the Idaho panhandle.

Zones used in states beyond the contiguous U.S.

The Alaska standard time zone: (AKST; UTC−09; Zone V), which comprises most of the state of Alaska.
The Hawaii-Aleutian standard time zone: (or unofficially Hawaii Standard Time: HST) (HAST; UTC−10; zone W), which includes Hawaii and most of the length of the Aleutian Islands chain (west of 169°30′W).

Zones outside the states

The Atlantic standard time zone: (AST; UTC−04; Zone Q), which comprises Puerto Rico and the U.S. Virgin Islands.
The Samoa standard time zone (SST; UTC−11; Zone X), which comprises American Samoa.
The Chamorro standard time zone: (ChST; UTC+10; Zone K), which comprises Guam and the Northern Mariana Islands.

Minor Outlying Islands

Some United States Minor Outlying Islands are outside the time zones defined by 15 U.S.C. §260 and exist in waters defined by Nautical time. In practice, military crews may simply use Zulu time (UTC±0) when on these islands. Baker Island and Howland Island are in UTC−12, while Wake Island is in UTC+12. Because they exist on opposite sides of the International Date Line, it can, for example, be noon Thursday on Baker and Howland islands while simultaneously being noon Friday on Wake Island. Other outlying islands include Jarvis Island, Midway Atoll, Palmyra Atoll, and Kingman Reef (UTC−11); Johnston Atoll (UTC−10); and Navassa Island, Bajo Nuevo Bank, and Serranilla Bank (UTC−05).

Antarctic research stations

Boundaries between the zones

(Described from north to south along each boundary.)

Eastern/Central boundary

Marker showing the border of Wayne County, Kentucky, and the Eastern Time Zone

Time in Indiana: red areas belong to the Central Time Zone.

roughly follows the border between Wisconsin (to the south and west) and the Upper Peninsula of Michigan (to the north and east); the Upper Peninsula counties that border Wisconsin (namely Gogebic, Iron, Dickinson, and Menominee counties) observe CT, all other counties in the U.P. observe ET
follows Lake Michigan
divides a small portion of Northwestern Indiana near Chicago from the rest of the state
follows the border between Illinois (west) and Indiana (east)
divides a small portion of Southwestern Indiana from the rest of the state
divides Kentucky in half roughly along a line that is west of Louisville, Kentucky running from northwest to southeast.
divides the region legally defined as East Tennessee, except for four counties adjoining Middle Tennessee, from the rest of Tennessee.
follows the border between Alabama (west) and Georgia (east)
divides the Florida Panhandle along the Apalachicola River and Intracoastal Waterway just west of Tallahassee, Florida.

Central/Mountain boundary

divides the southwest portion of North Dakota from the rest of the state
divides South Dakota roughly in half
divides the western third of Nebraska from the rest of the state
divides a very small portion of extreme western Kansas bordering Colorado (Greeley, Hamilton, Sherman, and Wallace counties) from the rest of the state (three other counties which border Colorado—Cheyenne, Morton, and Stanton counties—observe CST)
follows the border between New Mexico (west) and Oklahoma (east)
follows the border between New Mexico (west) and Texas (east)
divides El Paso County, Hudspeth County, and a portion of northern Culberson County that includes eastern Guadalupe Mountains National Park from the rest of Texas

Mountain/Pacific boundary

follows the border between northern Idaho (to the west) and northwestern Montana (to the east)
turns west at 45°33.46′N 114°33.89′W / 45.55767°N 114.56483°W / 45.55767; -114.56483 (just south of Nez Perce Pass), and follows the Idaho County line to the Salmon River
follows the Salmon River west to the town of Riggins, where the Salmon River turns north. This puts almost all of northern Idaho in the Pacific time zone, except for the small loop described next.
turns north and follows the Salmon River to the Snake River at the Oregon border (at 45°51.3′N 116°47.5′W / 45.8550°N 116.7917°W / 45.8550; -116.7917). Interestingly, this loop to the north creates a curious situation where one can enter a more-westerly time zone by traveling east over one of the bridges across this portion of the Salmon River.
turns south and follows the Snake River between Oregon (west) and Idaho (east) to the northern border of Malheur County, Oregon
turns west and follows the northern border of Malheur County, Oregon to its western border, where it turns south
follows the western border of Malheur County to latitude 42.45° (42°27′ N), where it turns east, and returns to the Oregon/Idaho border
turns south and follows the border between Oregon (west) and Idaho (east)
turns east and follows the border between Idaho (north) and Nevada (south) along the 42nd parallel north to longitude 114.041726 W.
turns south and follows the border between Nevada (west) and Utah (east), except for following the west city limit line of West Wendover dividing it from the rest of Nevada, and putting it in the mountain time zone. Jackpot, Nevada, just south of the 42nd parallel and some 25 miles (40 km) west of the time zone south turn, also observes Mountain Time, on an unofficial basis.
follows the border between Nevada (west) and Arizona (east)
follows the border between California (west) and Arizona (east), as defined by the Colorado river, to the border between the U.S. and Mexico.

Daylight saving time

Main article: Daylight saving time in the United States

The Energy Policy Act of 2005 extended daylight saving time (DST) for an additional month beginning in 2007. The start of DST now occurs on the second Sunday in March and ends on the first Sunday in November.

Clocks will be set ahead one hour at 2 a.m. on the following start dates and set back one hour at 2 a.m. on these ending dates:

Year	Start date	Ending date
2006	April 2	October 29
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

In response to the Uniform Time Act of 1966, each state of the US has officially chosen to apply one of two rules over its entire territory:

Most use the standard time for their zone (or zones, where a state is divided between two zones), except for using daylight saving time during the summer months. Originally this ran from the last Sunday in April until the last Sunday in October. Two subsequent amendments, in 1986 and 2005, have shifted these days so that daylight saving time now runs from the second Sunday in March until the first Sunday in November.
Arizona time zones
Arizona and Hawaii use standard time throughout the year. However:
- The Navajo Nation observes DST throughout its entire territory, including the portion that lies in Arizona. But the Hopi Nation, which is entirely surrounded by the Navajo Nation and is entirely in Arizona, does not observe DST. (See map inset right.)
In 2005, Indiana passed legislation which took effect on April 2, 2006, that placed the entire state on daylight saving time (see Time in Indiana). Before then, Indiana officially used standard time year-round, with the following exceptions:
- The portions of Indiana that were on Central Time observed daylight saving time.
- Also, some Indiana counties near Cincinnati and Louisville were on Eastern Time, but did (unofficially) observe DST.

Time representation

Further information: Date and time notation in the United States

Further information: Speaking clock § United States

There exist more than one hundred clock towers in the United States; see Category:Clock towers in the United States.

Popular culture references

There have been numerous mentions of time zones in films, TV shows, and books, including the following:

In a 1963 episode of The Twilight Zone, "Of Late I Think of Cliffordville", Mr. Feathersmith is transported from a jetliner in c. 1963 to a train in 1910 after changing his watch to Central Time from Eastern Time after the plane crosses the Indiana border into Illinois.
In the 2002 film Serving Sara, Joe Tyler realizes that he can collect 1 million dollars by showing that his business rival, Tony, forgot to change his watch from Eastern Time to Central Time when Tony served the eponymous Sara Moore.
TV's "The West Wing" made reference to adjacent counties being in different time zones during an episode taking place during a campaign stop tour.

References

1 2 3 http://www.timeanddate.com/time/time-zones-history.html
↑ What is Greenwich Mean Time (GMT) http://wwp.greenwichmeantime.com/info/gmt.htm
↑ Bernard Guinot, "Solar time, legal time, time in use" Metrologica, August 2011 (volume 48, issue 4), pages 181–185.
↑ 15 USC §260.
↑ 15 USC 260–262
↑ H.R.2272 — America COMPETES Act (Sec. 3013)
↑ Standard Time Zone Boundaries 49CFR71

External links

Time in the Americas

North America South America

Sovereign states	Antigua and Barbuda Argentina Bahamas Barbados Belize Bolivia Brazil Canada Chile Colombia Costa Rica Cuba Dominica Dominican Republic Ecuador El Salvador Grenada Guatemala Guyana Haiti Honduras Jamaica Mexico Nicaragua Panama Paraguay Peru Saint Kitts and Nevis Saint Lucia Saint Vincent and the Grenadines Suriname Trinidad and Tobago United States Uruguay Venezuela

Dependencies and other territories	Anguilla Aruba Bermuda Bonaire British Virgin Islands Cayman Islands Curaçao Falkland Islands French Guiana Greenland Guadeloupe Martinique Montserrat Puerto Rico Saint Barthélemy Saint Martin Saint Pierre and Miquelon Saba Sint Eustatius Sint Maarten South Georgia and the South Sandwich Islands Turks and Caicos Islands US Virgin Islands

Time in the United States

States	Alabama Alaska Arizona Arkansas California Colorado Connecticut Delaware Florida Georgia Hawaii Idaho Illinois Indiana Iowa Kansas Kentucky Louisiana Maine Maryland Massachusetts Michigan Minnesota Mississippi Missouri Montana Nebraska Nevada New Hampshire New Jersey New Mexico New York North Carolina North Dakota Ohio Oklahoma Oregon Pennsylvania Rhode Island South Carolina South Dakota Tennessee Texas Utah Vermont Virginia Washington West Virginia Wisconsin Wyoming

Federal district	Washington, D.C.

Insular areas	American Samoa Guam Northern Mariana Islands Puerto Rico U.S. Virgin Islands

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