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.

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.

United States time zones[edit]

Standard time zones in the United States are currently defined at the federal level by law 15 USC §260.[1] 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.[2] As of August 9, 2007, the standard time zones are defined in terms of hourly offsets from UTC.[3] 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[4] they are:

  1. Atlantic Time Zone
  2. Eastern Time Zone
  3. Central Time Zone
  4. Mountain Time Zone
  5. Pacific Time Zone
  6. Alaska Time Zone
  7. Hawaii–Aleutian Time Zone
  8. Samoa Time Zone (UTC−11)
  9. Chamorro Time Zone (UTC+10)

View the standard time zone boundaries here.

Standard time and daylight saving time[edit]

Time ZoneStandard TimeDaylight Saving Time
Hawaii-Aleutian Time ZoneHAST (UTC−10:00)HADT (UTC−09:00)
Aleutian Islands only
Alaska Time ZoneAKST (UTC−09:00)AKDT (UTC−08:00)
Pacific Time ZonePST (UTC−08:00)PDT (UTC−07:00)
Mountain Time ZoneMST (UTC−07:00)MDT (UTC−06:00)
Central Time ZoneCST (UTC−06:00)CDT (UTC−05:00)
Eastern Time ZoneEST (UTC−05:00)EDT (UTC−04:00)

Zones used in the contiguous U.S.[edit]

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

Zones used in states beyond the contiguous U.S.[edit]

Zones outside the states[edit]

Minor Outlying Islands[edit]

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[edit]

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[edit]

(Described from north to south along each boundary.)

Eastern/Central boundary[edit]

Marker showing the border of Wayne County, Kentucky, and the Eastern Time Zone
Time in Indiana: red areas belong to the Central Time Zone.

Central/Mountain boundary[edit]

Mountain/Pacific boundary[edit]

Daylight saving time[edit]

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:

YearStart dateEnding date
2006April 2October 29
2007March 11November 4
2008March 9November 2
2009March 8November 1
2010March 14November 7
2011March 13November 6
2012March 11November 4
2013March 10November 3
2014March 9November 2
2015March 8November 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:

De facto national time[edit]

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 DodgersSan 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.

However, the United States currently does not use any sort of national railway time like the railways in Russia; airports, railway stations and other modes of transportation follow the local time.

Broadcast times used in the U.S.[edit]

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[edit]

List of time zones from the tz database[edit]

These are the time zones for the United States and its insular areas from the tz database (version 2012c). They are initially sorted west to east and then by TZ.

States
ColCCCoordinatesTZCommentsUTC offsetUTC offset DSTNotes
    US+515248-1763929America/AdakAleutian Islands−10:00−09:00
    US+211825-1575130Pacific/HonoluluHawaii−10:00−10:00
    US+611305-1495401America/AnchorageAlaska Time−09:00−08:00
    US+581807-1342511America/JuneauAlaska Time - Alaska panhandle−09:00−08:00
    US+643004-1652423America/NomeAlaska Time - west Alaska−09:00−08:00
    US+571035-1351807America/SitkaAlaska Time - southeast Alaska panhandle−09:00−08:00
    US+593249-1394338America/YakutatAlaska Time - Alaska panhandle neck−09:00−08:00
    US+340308-1181434America/Los_AngelesPacific Time−08:00−07:00
    US+550737-1313435America/MetlakatlaMetlakatla Time - Annette Island−08:00−08:00
    US+433649-1161209America/BoiseMountain Time - south Idaho & east Oregon−07:00−06:00
    US+394421-1045903America/DenverMountain Time−07:00−06:00
    US+332654-1120424America/PhoenixMountain Standard Time - Arizona−07:00−07:00
    US+415100-0873900America/ChicagoCentral Time−06:00−05:00
    US+411745-0863730America/Indiana/KnoxCentral Time - Indiana - Starke County−06:00−05:00
    US+375711-0864541America/Indiana/Tell_CityCentral Time - Indiana - Perry County−06:00−05:00
    US+450628-0873651America/MenomineeCentral Time - Michigan - Dickinson, Gogebic, Iron & Menominee Counties−06:00−05:00
    US+471551-1014640America/North_Dakota/BeulahCentral Time - North Dakota - Mercer County−06:00−05:00
    US+470659-1011757America/North_Dakota/CenterCentral Time - North Dakota - Oliver County−06:00−05:00
    US+465042-1012439America/North_Dakota/New_SalemCentral Time - North Dakota - Morton County (except Mandan area)−06:00−05:00
    US+421953-0830245America/DetroitEastern Time - Michigan - most locations−05:00−04:00
    US+394606-0860929America/Indiana/IndianapolisEastern Time - Indiana - most locations−05:00−04:00
    US+382232-0862041America/Indiana/MarengoEastern Time - Indiana - Crawford County−05:00−04:00
    US+382931-0871643America/Indiana/PetersburgEastern Time - Indiana - Pike County−05:00−04:00
    US+384452-0850402America/Indiana/VevayEastern Time - Indiana - Switzerland County−05:00−04:00
    US+384038-0873143America/Indiana/VincennesEastern Time - Indiana - Daviess, Dubois, Knox & Martin Counties−05:00−04:00
    US+410305-0863611America/Indiana/WinamacEastern Time - Indiana - Pulaski County−05:00−04:00
    US+381515-0854534America/Kentucky/LouisvilleEastern Time - Kentucky - Louisville area−05:00−04:00
    US+364947-0845057America/Kentucky/MonticelloEastern Time - Kentucky - Wayne County−05:00−04:00
    US+404251-0740023America/New_YorkEastern Time−05:00−04:00
Insular Areas
ColCCCoordinatesTZCommentsUTC offsetUTC offset DSTNotes
    UM+2813-17722Pacific/MidwayMidway Islands−11:00−11:00
    AS-1416-17042Pacific/Pago_Pago−11:00−11:00
    UM+1645-16931Pacific/JohnstonJohnston Atoll−10:00−10:00
    PR+182806-0660622America/Puerto_Rico−04:00−04:00
    VI+1821-06456America/St_Thomas−04:00−04:00
    PW+0720+13429Pacific/Palau+09:00+09:00
    FM+0725+15147Pacific/ChuukChuuk (Truk) and Yap+10:00+10:00
    GU+1328+14445Pacific/Guam+10:00+10:00
    MP+1512+14545Pacific/Saipan+10:00+10:00
    FM+0519+16259Pacific/KosraeKosrae+11:00+11:00
    FM+0658+15813Pacific/PohnpeiPohnpei (Ponape)+11:00+11:00
    MH+0905+16720Pacific/KwajaleinKwajalein+12:00+12:00
    MH+0709+17112Pacific/Majuromost locations+12:00+12:00
    UM+1917+16637Pacific/WakeWake Island+12:00+12:00
Alternate Time Zone Names – Links
ColCCCoordinatesTZCommentsUTC offsetUTC offset DSTNotes
    --Pacific/Samoa−11:00−11:00Link to Pacific/Pago Pago
    --US/Samoa−11:00−11:00Link to Pacific/Pago Pago
    --America/Atka−10:00−09:00Link to America/Adak
    --US/Aleutian−10:00−09:00Link to America/Adak
    --US/Hawaii−10:00−10:00Link to Pacific/Honolulu
    --AKST9AKDT−09:00−08:00Link to America/Anchorage
    --US/Alaska−09:00−08:00Link to America/Anchorage
    --US/Pacific−08:00−07:00Link to America/Los Angeles
    --US/Pacific-New−08:00−07:00Link to America/Los Angeles
    US+364708-1084111America/ShiprockMountain Time - Navajo−07:00−06:00Link to America/Denver
    [[ISO 3166-1:Time zone Navajo Nation is not in Template:Tz/country code|Time zone Navajo Nation is not in Template:Tz/country code]]Time zone Navajo Nation is not in Template:Tz/coordinatesNavajo NationTime zone Navajo Nation is not in {{Tz/comments}}.[[UTCTime zone Navajo Nation is not in {{Tz/utc offset}}.|Time zone Navajo Nation is not in {{Tz/utc offset}}.]][[UTCTime zone Navajo Nation is not in {{Tz/utc dst offset}}.|Time zone Navajo Nation is not in {{Tz/utc dst offset}}.]]Link to America/Denver
    --US/Arizona−07:00−07:00Link to America/Phoenix
    --US/Mountain−07:00−06:00Link to America/Denver
    --America/Knox_IN−06:00−05:00Link to America/Indiana/Knox
    --US/Central−06:00−05:00Link to America/Chicago
    --US/Indiana-Starke−06:00−05:00Link to America/Indiana/Knox
    --America/Fort_Wayne−05:00−04:00Link to America/Indiana/Indianapolis
    --America/Indianapolis−05:00−04:00Link to America/Indiana/Indianapolis
    --America/Louisville−05:00−04:00Link to America/Kentucky/Louisville
    --US/Eastern−05:00−04:00Link to America/New_York
    --US/East-Indiana−05:00−04:00Link to America/Indiana/Indianapolis
    --US/Michigan−05:00−04:00Link to America/Detroit
    --America/Virgin−04:00−04:00Link to America/St Thomas
    --Kwajalein+12:00+12:00Link to Pacific/Kwajalein
Alternate Time Zone Names
ColCCCoordinatesTZCommentsUTC offsetUTC offset DSTNotes
    [[ISO 3166-1:Time zone Hawaiian Standard Time is not in Template:Tz/country code|Time zone Hawaiian Standard Time is not in Template:Tz/country code]]Time zone Hawaiian Standard Time is not in Template:Tz/coordinatesHawaiian Standard TimeTime zone Hawaiian Standard Time is not in {{Tz/comments}}.[[UTCTime zone Hawaiian Standard Time is not in {{Tz/utc offset}}.|Time zone Hawaiian Standard Time is not in {{Tz/utc offset}}.]][[UTCTime zone Hawaiian Standard Time is not in {{Tz/utc dst offset}}.|Time zone Hawaiian Standard Time is not in {{Tz/utc dst offset}}.]]
    --PST8PDT−08:00−07:00
    [[ISO 3166-1:Time zone Mountain Standard Time is not in Template:Tz/country code|Time zone Mountain Standard Time is not in Template:Tz/country code]]Time zone Mountain Standard Time is not in Template:Tz/coordinatesMountain Standard TimeTime zone Mountain Standard Time is not in {{Tz/comments}}.[[UTCTime zone Mountain Standard Time is not in {{Tz/utc offset}}.|Time zone Mountain Standard Time is not in {{Tz/utc offset}}.]][[UTCTime zone Mountain Standard Time is not in {{Tz/utc dst offset}}.|Time zone Mountain Standard Time is not in {{Tz/utc dst offset}}.]]
    --MST7MDT−07:00−06:00
    --CST6CDT−06:00−05:00
    [[ISO 3166-1:Time zone Eastern Time Zone is not in Template:Tz/country code|Time zone Eastern Time Zone is not in Template:Tz/country code]]Time zone Eastern Time Zone is not in Template:Tz/coordinatesEastern Time ZoneTime zone Eastern Time Zone is not in {{Tz/comments}}.[[UTCTime zone Eastern Time Zone is not in {{Tz/utc offset}}.|Time zone Eastern Time Zone is not in {{Tz/utc offset}}.]][[UTCTime zone Eastern Time Zone is not in {{Tz/utc dst offset}}.|Time zone Eastern Time Zone is not in {{Tz/utc dst offset}}.]]
    --EST5EDT−05:00−04:00
US Time Zones based on tz database v2011b

States and counties with multiple time zones[edit]

Some states by are covered by more than one tz database version 2012c time zone.

State# zonesCounties in more than one time zone
Alabama1Parts of Russell County, Chambers County, and Lee County unofficially observe Eastern Time
Alaska7
Arizona2
Florida2Gulf County is in two zones
Idaho2Idaho County is in two zones
Indiana11
Kansas2Note: Kearny County has no zone of its own, despite a change in the late 1980s.
Kentucky4
Michigan2
Nebraska2Cherry County is in two zones
Nevada2Elko County is in two zones
North Dakota5
Oregon2Malheur County is in two zones
South Dakota2
Tennessee2
Texas2parts of Culberson County are on MT

Time representation[edit]

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

See also[edit]

References[edit]

External links[edit]