Time in the United States
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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 for approximately the summer 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.
United States time zones 
Standard time zones in the United States are currently defined at the federal level by law 15 USC §260. 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. As of August 9, 2007, the standard time zones are defined in terms of hourly offsets from UTC. 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. From east to west, they are:
- Atlantic Standard Time (AST)
- Eastern Standard Time (EST)
- Central Standard Time (CST)
- Mountain Standard Time (MST)
- Pacific Standard Time (PST)
- Alaskan Standard Time (AKST)
- Hawaii-Aleutian Standard Time (HST)
- Samoa Standard Time (UTC−11)
- Chamorro Standard Time (UTC+10)
View the standard time zone boundaries here.
Zones used in the contiguous U.S. 
From east to west, the times zones of the contiguous United States are:
- Eastern Time Zone: (Zone R), which comprises roughly the states on the Atlantic coast and the eastern two thirds of the Ohio Valley.
- Central Time Zone: (Zone S), which comprises roughly the Gulf Coast, Mississippi Valley, and Great Plains.
- Mountain Time Zone: (Zone T), which comprises roughly the states that include the Rocky Mountains.
- Pacific Time Zone: (Zone U), which comprises roughly the states on the Pacific coast, plus Nevada.
Zones used in states beyond the contiguous U.S. 
- Alaska standard time zone: (AKST; UTC−09; Zone V), which comprises most of the state of Alaska.
- 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 
- Atlantic standard time zone: (AST; UTC−04; Zone Q), which comprises Puerto Rico and the U.S. Virgin Islands.
- Samoa standard time zone (SST; UTC−11; Zone X), which comprises American Samoa.
- 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 Wednesday on Baker and Howland islands while simultaneously being noon Thursday 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 
In Antarctica, the US research facility Palmer Station is in UTC−04, while McMurdo Station and Amundsen-Scott South Pole Station use UTC+12 in order to coordinate with their main supply base in New Zealand.
Boundaries between the zones 
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(Described from north to south along each boundary.)
Eastern/Central boundary 
- 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 on Wisconsin 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—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 and Hudspeth County from the rest of Texas
Mountain/Pacific boundary 
- follows the border between northern Idaho (west) and northwestern Montana (east)
- turns west at Idaho County line to the Salmon River (just south of Nez Perce Pass), and follows the
- follows the Salmon River west to the town of Riggins, where the Salmon River turns north. Although this puts the southern part of Idaho County in the Mountain Time Zone, the towns of Burgdorf and Warren, which are south of the Salmon River, observe Pacific time.
- turns North and follows the Salmon River to the Snake River at the Oregon border (at ). 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 seven 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 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 
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.
The Energy Policy Act specifies that continuation of daylight saving time beyond 2008 is subject to favorable evaluation of the energy savings attained. Unless the expanded DST period is rescinded, 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|
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 and Hawaii use standard time throughout the year. However:
- 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:
De facto national time 
The Eastern Time Zone is commonly used by the national media and other organizations as somewhat of a de facto national time for all of the United States. National media organizations will often report when news events happened or are scheduled to happen in Eastern Time, even if they occurred in another time zone. TV schedules, especially those that list events that are broadcast live across the country, are also almost always posted in Eastern Time. The major professional sports leagues also post all game times in Eastern time; thus, for example, the start time of a Los Angeles Dodgers–San Francisco Giants game usually is still posted by Major League Baseball and the national sports media in Eastern Time, even though the game location is covered by the Pacific Time Zone.
Broadcast times used in the U.S. 
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With four time zones in the contiguous United States, U.S. national broadcast networks and cable channels generally air at least two separate feeds to their stations and affiliates: the "east feed" that is aired simultaneously in the Eastern and Central Time Zones, and the "west feed" that is tape-delayed three hours for those in the Pacific Time Zone. This ensures that a program, for example, that airs at 8 p.m. on the east coast is also shown locally at 8 p.m. on the Pacific. Networks may also air a third feed specifically for the Mountain Time Zone, which usually airs on a one-hour delay from the east feed; otherwise those in the Mountain Time Zone get the west feed. These networks and cable channels advertise airtimes in Eastern time, sometimes also including either Central or Pacific time and until the 1980s, Mountain time, depending on whether there is a separate feed for that time zone. This has led to conventions like "tonight at 9 p.m. Eastern/8 p.m. Central", "tonight at 9/8 Central" or "tonight at 9/8c" (referring to the east feed); "tonight at 9 p.m. Eastern and Pacific" (referring to both the east feed and the three-hour delayed west feed); and "tonight at 9/8 Central and Mountain" (also including the Mountain feed). A few cable channels may not audibly refer to the Central time airtime of a program, though their promos may also visually include references to its broadcast in both the Eastern and Central time zones. So when a viewer only hears "tonight at 8," regardless of whether the promo visually includes it or not, chances are that the show they are referring to is scheduled to air at 7 p.m. in the Central time zone.
Live events that are simulcast across the country maybe either advertised in Eastern time only, or in both Eastern and Pacific (e.g. "8 p.m. Eastern/5 p.m. Pacific")
Time changes for daylight saving time may result in broadcast television stations and cable channels accommodating the time changes by altering their schedules to allow programs to continue to air at the same time year-round. Thus, advertised airtimes normally never specify if it is standard time or DST.
Network programming in Alaska and Hawaii generally follows a schedule similar to that of Central Time Zone, although primetime shows are delayed by an additional hour during DST.
tz database 
List of time zones from the tz database 
|Col||CC||Coordinates||TZ||Comments||UTC offset||UTC offset DST||Notes|
|US||+581807-1342511||America/Juneau||Alaska Time - Alaska panhandle||−09:00||−08:00|
|US||+643004-1652423||America/Nome||Alaska Time - west Alaska||−09:00||−08:00|
|US||+571035-1351807||America/Sitka||Alaska Time - southeast Alaska panhandle||−09:00||−08:00|
|US||+593249-1394338||America/Yakutat||Alaska Time - Alaska panhandle neck||−09:00||−08:00|
|US||+550737-1313435||America/Metlakatla||Metlakatla Time - Annette Island||−08:00||−08:00|
|US||+433649-1161209||America/Boise||Mountain Time - south Idaho & east Oregon||−07:00||−06:00|
|US||+332654-1120424||America/Phoenix||Mountain Standard Time - Arizona||−07:00||−07:00|
|US||+411745-0863730||America/Indiana/Knox||Central Time - Indiana - Starke County||−06:00||−05:00|
|US||+375711-0864541||America/Indiana/Tell_City||Central Time - Indiana - Perry County||−06:00||−05:00|
|US||+450628-0873651||America/Menominee||Central Time - Michigan - Dickinson, Gogebic, Iron & Menominee Counties||−06:00||−05:00|
|US||+471551-1014640||America/North_Dakota/Beulah||Central Time - North Dakota - Mercer County||−06:00||−05:00|
|US||+470659-1011757||America/North_Dakota/Center||Central Time - North Dakota - Oliver County||−06:00||−05:00|
|US||+465042-1012439||America/North_Dakota/New_Salem||Central Time - North Dakota - Morton County (except Mandan area)||−06:00||−05:00|
|US||+421953-0830245||America/Detroit||Eastern Time - Michigan - most locations||−05:00||−04:00|
|US||+394606-0860929||America/Indiana/Indianapolis||Eastern Time - Indiana - most locations||−05:00||−04:00|
|US||+382232-0862041||America/Indiana/Marengo||Eastern Time - Indiana - Crawford County||−05:00||−04:00|
|US||+382931-0871643||America/Indiana/Petersburg||Eastern Time - Indiana - Pike County||−05:00||−04:00|
|US||+384452-0850402||America/Indiana/Vevay||Eastern Time - Indiana - Switzerland County||−05:00||−04:00|
|US||+384038-0873143||America/Indiana/Vincennes||Eastern Time - Indiana - Daviess, Dubois, Knox & Martin Counties||−05:00||−04:00|
|US||+410305-0863611||America/Indiana/Winamac||Eastern Time - Indiana - Pulaski County||−05:00||−04:00|
|US||+381515-0854534||America/Kentucky/Louisville||Eastern Time - Kentucky - Louisville area||−05:00||−04:00|
|US||+364947-0845057||America/Kentucky/Monticello||Eastern Time - Kentucky - Wayne County||−05:00||−04:00|
|Col||CC||Coordinates||TZ||Comments||UTC offset||UTC offset DST||Notes|
|FM||+0725+15147||Pacific/Chuuk||Chuuk (Truk) and Yap||+10:00||+10:00|
|Col||CC||Coordinates||TZ||Comments||UTC offset||UTC offset DST||Notes|
States and counties with multiple time zones 
Some states by are covered by more than one tz database version 2012c time zone.
|State||# zones||Counties in more than one time zone|
|Alabama||1||Parts of Russell County, Chambers County, and Lee County unofficially observe Eastern Time|
|Florida||2||Gulf County is in two zones|
|Idaho||2||Idaho County is in two zones|
|Kansas||2||Note: Kearny County has no zone of its own, despite a change in the late 1980s.|
|Nebraska||2||Cherry County is in two zones|
|Nevada||2||Elko County is in two zones|
|Oregon||2||Malheur County is in two zones|
|Texas||2||parts of Culberson County are on MT|
Time representation 
See also 
- History of time in the United States
- List of time zones by U.S. state
- Daylight saving time in the United States
- Lists of time zones
- Effects of time on North American broadcasting
- Date and time notation in the United States
- Official website
- U.S. Navy time zone page
- North American Time Zone border data and images
- Standard Time Zone Boundaries 49CFR71 (also at wikisource:Code of Federal Regulations/Title 49/Subtitle A/Part 71)
- Standard Time Law 15USC260-267 (also at wikisource:United States Code/Title 15/Chapter 6/Subchapter IX)