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 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. As defined by US law 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. Hawaiiâ€"Aleutian Time Zone
8. Samoa Time Zone (UTCâˆ'11)
9. Chamorro Time Zone (UTC+10)

View the standard time zone boundaries here.

Standard time and daylight saving time

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; UTCâˆ'09; Zone V), which comprises most of the state of Alaska.
• Hawaii-Aleutian standard time zone: (or unofficially Hawaii Standard Time: HST) (HAST; UTCâˆ'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; UTCâˆ'04; Zone Q), which comprises Puerto Rico and the U.S. Virgin Islands.
• Samoa standard time zone (SST; UTCâˆ'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 UTCâˆ'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 (UTCâˆ'11); Johnston Atoll (UTCâˆ'10); and Navassa Island, Bajo Nuevo Bank, and Serranilla Bank (UTCâˆ'05).

Antarctic research stations

In Antarctica, the US research facility Palmer Station is in UTCâˆ'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

(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 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 Coloradoâ€"Cheyenne, Morton, and Stanton countiesâ€"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 (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.
• turns north and follows the Salmon River to the Snake River at the Oregon border (at 45°51.3′N 116°47.5′W). 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 (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

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:

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:
  • 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.

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 Dodgersâ€"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. Thus a first pitch time of 7:10 pm Pacific time would read as 10:10 pm for national purposes.

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.

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 may be 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.

Time representation

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 1959-1964 version 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.