Time in the United States

Time in the United States, by law, is divided into nine standard time zones covering the states, territories and other US possessions, with most of the United States observing daylight saving time (DST) for approximately the spring, summer, and fall 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 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.

Time in the United States
Map of U.S. time zones since November 2007.
Current Time
Atlantic1:22 pm, August 18, 2020 [refresh]
Eastern1:22 pm, August 18, 2020 [refresh]
Central12:22 pm, August 18, 2020 [refresh]
Mountain11:22 am, August 18, 2020 [refresh][lower-alpha 1]
Pacific10:22 am, August 18, 2020 [refresh]
Alaska9:22 am, August 18, 2020 [refresh]
Hawaii–Aleutian8:22 am, August 18, 2020 [refresh][lower-alpha 2]
Samoa6:22 am, August 18, 2020 [refresh]
Chamorro3:22 am, August 19, 2020 [refresh]

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.

History
1913 time zone map of the United States, showing boundaries different from today
See also: Standard time in the United States

Before the adoption of four standard time zones for the continental United States, many towns and cities set their clocks to noon when the sun passed their local meridian, pre-corrected for the equation of time on the date of observation, to form local mean solar time. Noon occurred at different times but time differences between distant locations were barely noticeable prior to the 19th century because of long travel times and the lack of long-distance instant communications prior to the development of the telegraph.

The use of local solar time became increasingly awkward as railways and telecommunications improved.[1] American railroads maintained many different time zones during the late 1800s. Each train station set its own clock making it difficult to coordinate train schedules and confusing passengers. Time calculation became a serious problem for people traveling by train (sometimes hundreds of miles in a day), according to the Library of Congress. Train drivers must recalculate their own clock in order to know departure time. Every city in the United States used a different time standard so there were more than 300 local sun times to choose from. Time zones were therefore a compromise, relaxing the complex geographic dependence while still allowing local time to be approximate with mean solar time. Railroad managers tried to address the problem by establishing 100 railroad time zones, but this was only a partial solution to the problem.[1]

Weather service chief Cleveland Abbe introduced four standard time zones for his weather stations, an idea which he offered to the railroads.[2] Operators of the new railroad lines needed a new time plan that would offer a uniform train schedule for departures and arrivals. Four standard time zones for the continental United States were introduced at noon on November 18, 1883, in Chicago, IL, when the telegraph lines transmitted time signals to all major cities.[3][4]

In October 1884, the International Meridian Conference at Washington, D.C. adopted a proposal which stated that the prime meridian for longitude and timekeeping should be one that passes through the centre of the transit instrument at the Greenwich Observatory in the United Kingdom (UK). The conference therefore established the Greenwich Meridian as the prime meridian and Greenwich Mean Time (GMT) as the world's time standard. The U.S. time-zone system grew from this, in which all zones referred back to GMT on the prime meridian.[1]

From GMT to UTC

In 1960, the International Radio Consultative Committee formalized the concept of Coordinated Universal Time (UTC), which became the new international civil time standard. UTC is, within about 1 second, mean solar time at 0°.[5] UTC does not observe daylight saving time.

For most purposes, UTC is considered interchangeable with GMT, but GMT is no longer precisely defined by the scientific community. UTC is one of several closely related successors to GMT.

United States and regional time zones

Standard time zones in the United States and other regions are currently defined at the federal level by law 15 USC §260.[6] 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.[7] As of August 9, 2007, the standard time zones are defined in terms of hourly offsets from UTC.[8] 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 and its surrounding areas use nine standard time zones. As defined by U.S. law[9] they are:

Time ZoneDSTStandard
Atlantic (not observed) UTC−04:00 Puerto Rico, U.S. Virgin Islands
Eastern UTC−04:00 UTC−05:00 Connecticut, Delaware, District of Columbia, Georgia, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, North Carolina, Ohio, Pennsylvania, Rhode Island, South Carolina, Vermont, Virginia, West Virginia; Partially: Florida, Indiana, Kentucky, Michigan, Tennessee; No DST observed, not defined by 15 U.S.C. §260: Navassa Island, Bajo Nuevo Bank, Serranilla Bank
Central UTC−05:00 UTC−06:00 Alabama, Arkansas, Illinois, Iowa, Louisiana, Minnesota, Mississippi, Missouri, Oklahoma, Wisconsin; Partially: Florida, Indiana, Kansas, Kentucky, Michigan, Nebraska, North Dakota, South Dakota, Tennessee, Texas
Mountain UTC−06:00 UTC−07:00 Arizona (no DST outside of Navajo Nation), Colorado, Montana, New Mexico, Utah, Wyoming; Partially: Idaho, Kansas, Nebraska, North Dakota, Oregon, South Dakota, Texas
Pacific UTC−07:00 UTC−08:00 California, Nevada, Washington (state); Partially: Idaho, Oregon
Alaska UTC−08:00 UTC−09:00 Partially: Alaska
Hawaii‑Aleutian UTC−09:00 UTC−10:00 Hawaii (no DST observed in Hawaii); Partially: Alaska; No DST observed, not defined by 15 U.S.C. §260: Johnston Atoll
American Samoa (not observed) UTC−11:00 American Samoa; Not defined by 15 U.S.C. §260: Jarvis Island, Midway Atoll, Palmyra Atoll, Kingman Reef
  (not observed) UTC−12:00 Not defined by 15 U.S.C. §260: Baker Island, Howland Island
  (not observed) UTC+12:00 Not defined by 15 U.S.C. §260: Wake Island
Chamorro (not observed) UTC+10:00 Guam, Northern Mariana Islands

Zones used in the contiguous U.S.

From east to west, the four time 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 most of the Great Plains.
  • Mountain time zone (Zone T), which comprises roughly the states and portions of states that include the Rocky Mountains and the western quarter of the Great Plains.
  • Pacific time zone (Zone U), which comprises roughly the states on the Pacific coast, plus Nevada and the Idaho panhandle.

Zones used in states beyond the contiguous U.S.

Zones used in U.S. territories

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±00:00) when on these islands. Baker Island and Howland Island are in UTC−12:00, while Wake Island is in UTC+12:00. 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 (UTC−11:00); Johnston Atoll (UTC−10:00); and Navassa Island, Bajo Nuevo Bank, and Serranilla Bank (UTC−05:00).

Antarctic research stations
See also: Time in Antarctica

In Antarctica, the U.S. research facility Palmer Station is in UTC−03:00, while McMurdo Station and Amundsen–Scott South Pole Station use UTC+12:00 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
Marker showing the border of Wayne County, Kentucky, and the eastern time zone
Time in Indiana: red and pink areas belong to the central time zone.

Central–Mountain boundary

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. This puts almost all of northern Idaho in the Pacific time zone, except for the small loop described next.
  • turns north and follows the Salmon River to the Snake River at the Oregon border (at 45°51.3′N 116°47.5′W). 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 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), mostly defined by the Colorado River, to the border between the U.S. and Mexico.

Daylight saving time (DST)
Main article: Daylight saving time in the United States

Daylight saving time (DST) begins on the second Sunday of March and ends on the first Sunday of November.

Clocks will be set ahead one hour at 2:00 a.m. on the following start dates and set back one hour at 2:00 a.m. on these ending dates:

YearStartEnd
2020Mar 8Nov 1
2021Mar 14Nov 7
2022Mar 13Nov 6
2023Mar 12Nov 5
2024Mar 10Nov 3
2025Mar 9Nov 2
2026Mar 8Nov 1
2027Mar 14Nov 7
2028Mar 12Nov 5
2029Mar 11Nov 4

In response to the Uniform Time Act of 1966, each state 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 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 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 (ET), but did (unofficially) observe DST.

The Energy Policy Act of 2005 extended daylight saving time (DST) for an additional month beginning in 2007.

Previous DST change dates include:

YearStartEnd
2006Apr 2Oct 29
2007Mar 11Nov 4
2008Mar 9Nov 2
2009Mar 8Nov 1
2010Mar 14Nov 7
2011Mar 13Nov 6
2012Mar 11Nov 4
2013Mar 10Nov 3
2014Mar 9Nov 2
2015Mar 8Nov 1
2016Mar 13Nov 6
2017Mar 12Nov 5
2018Mar 11Nov 4
2019 Mar 10 Nov 3
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See also

Notes
  1. Time in Arizona (besides the Navajo Nation): 10:22 am, August 18, 2020 [refresh]
  2. Time in Hawaii: 7:22 am, August 18, 2020 [refresh]

References
  1. "Why Do We Have Time Zones?".
  2. Debus, Allen G. (1968). World Who's Who in Science: A Biographical Dictionary of Notable Scientists from Antiquity to the Present (1st ed.). Chicago, IL: A. N. Marquis Company. p. 2. ISBN 0-8379-1001-3.
  3. "Greenwich Mean Time (GMT)—time, facts, history". greenwichmeantime.com. Retrieved April 18, 2019.
  4. "The Central Standard Building". Open House Chicago. Retrieved November 9, 2019.
  5. Guinot, Bernard (August 2011). "Solar time, legal time, time in use". Metrologia. 48 (4): 181–185. doi:10.1088/0026-1394/48/4/s08.
  6. 15 USC §260.
  7. "15 U.S. Code Subchapter IX—STANDARD TIME".
  8. Public Law 110–69—America COMPETES Act (August 9, 2007). Sec. 3013)
  9. Standard Time Zone Boundaries 49CFR71
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