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 part of the year. The time zone boundaries and DST observance are under the authority of the Department of Transportation. Official and highly precise time keeping 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 highly 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.

United States time zones

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 2007-08-09, 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.

Zones used in the contiguous U.S.

From east to west, these zones are:

• Eastern standard time zone: (EST; UTC-5; Zone R), which comprises roughly the states on the Atlantic coast and the eastern two thirds of the Ohio Valley.
• Central standard time zone: (CST; UTC-6; Zone S), which comprises roughly the Gulf Coast, Tennessee Valley, and Great Plains.
• Mountain standard time zone: (MST; UTC-7; Zone T), which comprises roughly the states that include the Rocky Mountains.
• Pacific standard time zone: (PST; UTC-8; 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-9; 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-4; 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.

Boundaries between the zones

(Described from north to south along each boundary.)

EST/CST boundary

• roughly follows the border between Wisconsin (to west) and the Upper Peninsula of Michigan (to east); the Upper Peninsula counties that border on Wisconsin observe CST, all other counties in the UP observe EST
• 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 roughly in half roughly along a line that is west of Louisville, KY 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 most of the western panhandle of Florida just west of Tallahassee, FL from the rest of the state.

CST/MST 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

MST/PST boundary

• follows the border between northern Idaho (west) and northwestern Montana (east)
• turns west (just south of Nez Perce Pass), following the Salmon River downstream to the Oregon border at the Snake River, dividing the Idaho Panhandle from the rest of the state
• follows the Snake River upstream between Oregon (west) and Idaho (east)
• divides the northern 80% of Malheur County (north of 42°25'N) from the rest of Oregon
• follows the border between Oregon (west) and Idaho (east)
• follows the border (42°N) between Idaho (north) and Nevada (south)
• follows the border between Nevada (west) and Utah (east), except for following city limit line of West Wendover dividing it from the rest of Nevada
• follows the border between Nevada (west) and Arizona (east)
• follows the border between California (west) and Arizona (east)

Daylight saving time

See also: Daylight saving time in the United States

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:

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.
  Image:AZTZ.gif

  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, does not observe DST. (See map inset right.)
• Before 2006, 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.
  • 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

Notes

See also

• History of time in the United States
• Time in Indiana
• List of U.S. states by time zone

External links

• U.S. Navy time zone page
• Standard Time Zone Boundaries 49CFR71
• Standard Time Law 15USC260-267
• The Official U.S. Time