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 part of the year. The time zone boundaries and DST observance are under the authority of the Department of Transportation. Official and highly precise timekeeping services (clocks) are provided by two federal time agencies: a Department of Commerce agency, the National Institute of Standards and Technology (NIST), 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 international time keeping organizations.
It is the combination of the time zone and daylight saving rules along with the time keeping services which determines the legal civil time for any U.S. location at any moment.
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Standard time zones in the United States are currently defined at the federal level by law 15 U.S.C. §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.[1] 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.
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) and Chamorro Standard Time (UTC+10). View the standard time zone boundaries.
http://www.timetemperature.com/tzus/time_zone.shtml
From east to west, the times zones of the contiguous United States are:
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. 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).
(Described from north to south along each boundary.)
As an energy conservation measure, 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:
The Eastern Time Zone is commonly used by the national media and other organizations as somewhat of a de facto official 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; even though both clubs are based on the Pacific Coast, 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.
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.
The 27 zones for the United States as given by zone.tab of the tz database. Columns marked * are from the zone.tab.
| No. | c.c.* | coordinates* | TZ* | comments* | UTC offset | UTC offset DST | Note |
|---|---|---|---|---|---|---|---|
| 1 | US | +404251-0740023 | America/New_York | Eastern Time | -05:00 | UTC−04 | |
| 2 | US | +421953-0830245 | America/Detroit | Eastern Time - Michigan - most locations | -05:00 | UTC−04 | |
| 3 | US | +381515-0854534 | America/Kentucky/Louisville | Eastern Time - Kentucky - Louisville area | -05:00 | UTC−04 | |
| 4 | US | +364947-0845057 | America/Kentucky/Monticello | Eastern Time - Kentucky - Wayne County | -05:00 | UTC−04 | |
| 5 | US | +394606-0860929 | America/Indiana/Indianapolis | Eastern Time - Indiana - most locations | -05:00 | UTC−04 | |
| 6 | US | +384038-0873143 | America/Indiana/Vincennes | Eastern Time - Indiana - Daviess, Dubois, Knox & Martin Counties | -05:00 | UTC−04 | |
| 7 | US | +410305-0863611 | America/Indiana/Winamac | Eastern Time - Indiana - Pulaski County | -05:00 | UTC−04 | |
| 8 | US | +382232-0862041 | America/Indiana/Marengo | Eastern Time - Indiana - Crawford County | -05:00 | UTC−04 | |
| 9 | US | +382931-0871643 | America/Indiana/Petersburg | Eastern Time - Indiana - Pike County | -05:00 | UTC−04 | |
| 10 | US | +384452-0850402 | America/Indiana/Vevay | Eastern Time - Indiana - Switzerland County | -05:00 | UTC−04 | |
| 11 | US | +415100-0873900 | America/Chicago | Central Time | -06:00 | UTC−05 | |
| 12 | US | +375711-0864541 | America/Indiana/Tell_City | Central Time - Indiana - Perry County | -06:00 | UTC−05 | |
| 13 | US | +411745-0863730 | America/Indiana/Knox | Central Time - Indiana - Starke County | -06:00 | UTC−05 | |
| 14 | US | +450628-0873651 | America/Menominee | Central Time - Michigan - Dickinson, Gogebic, Iron & Menominee Counties | -06:00 | UTC−05 | |
| 15 | US | +470659-1011757 | America/North_Dakota/Center | Central Time - North Dakota - Oliver County | -06:00 | UTC−05 | |
| 16 | US | +465042-1012439 | America/North_Dakota/New_Salem | Central Time - North Dakota - Morton County (except Mandan area) | -06:00 | UTC−05 | |
| 17 | US | +394421-1045903 | America/Denver | Mountain Time | -07:00 | UTC−06 | |
| 18 | US | +433649-1161209 | America/Boise | Mountain Time - south Idaho & east Oregon | -07:00 | UTC−06 | |
| 19 | US | +364708-1084111 | America/Shiprock | Mountain Time - Navajo | -07:00 | UTC−06 | |
| 20 | US | +332654-1120424 | America/Phoenix | Mountain Standard Time - Arizona | -07:00 | - | |
| 21 | US | +340308-1181434 | America/Los_Angeles | Pacific Time | -08:00 | UTC−07 | |
| 22 | US | +611305-1495401 | America/Anchorage | Alaska Time | -09:00 | UTC−08 | |
| 23 | US | +581807-1342511 | America/Juneau | Alaska Time - Alaska panhandle | -09:00 | UTC−08 | |
| 24 | US | +593249-1394338 | America/Yakutat | Alaska Time - Alaska panhandle neck | -09:00 | UTC−08 | |
| 25 | US | +643004-1652423 | America/Nome | Alaska Time - west Alaska | -09:00 | UTC−08 | |
| 26 | US | +515248-1763929 | America/Adak | Aleutian Islands | -10:00 | UTC−09 | |
| 27 | US | +211825-1575130 | Pacific/Honolulu | Hawaii | -10:00 | - |
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