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 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 are provided by two federal agencies: the National Institute of Standards and Technology ; and the United States Naval Observatory. 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.

History

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. 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, 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.
Weather service chief Cleveland Abbe introduced four standard time zones for his weather stations, an idea which he offered to the railroads. 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.
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. The conference therefore established the Greenwich Meridian as the prime meridian and Greenwich Mean Time 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.

From GMT to UTC

In 1960, the International Radio Consultative Committee formalized the concept of Coordinated Universal Time, which became the new international civil time standard. UTC is, within about 1 second, mean solar time at 0°. 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. 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.
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 they are:
Time ZoneDSTStandard
Atlantic'Puerto Rico, U.S. Virgin Islands
EasternConnecticut, 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
CentralAlabama, Arkansas, Illinois, Iowa, Louisiana, Minnesota, Mississippi, Missouri, Oklahoma, Wisconsin; Partially: Florida, Indiana, Kansas, Kentucky, Michigan, Nebraska, North Dakota, South Dakota, Tennessee, Texas
MountainArizona ', Colorado, Montana, New Mexico, Utah, Wyoming; Partially: Idaho, Kansas, Nebraska, North Dakota, Oregon, South Dakota, Texas
PacificCalifornia, Nevada, Washington ; Partially: Idaho, Oregon
AlaskaPartially: Alaska
Hawaii‑AleutianHawaii ; Partially: Alaska; No DST observed, not defined by 15 U.S.C. §260: Johnston Atoll
American Samoa'American Samoa; Not defined by 15 U.S.C. §260: Jarvis Island, Midway Atoll, Palmyra Atoll, Kingman Reef
''Not defined by 15 U.S.C. §260: Baker Island, Howland Island
''Not defined by 15 U.S.C. §260: Wake Island
Chamorro'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:
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 when on these islands. Baker Island and Howland Island are in, while Wake Island is in. 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 ; Johnston Atoll ; and Navassa Island, Bajo Nuevo Bank, and Serranilla Bank.

Antarctic research stations

In Antarctica, the U.S. research facility Palmer Station is in, while McMurdo Station and Amundsen–Scott South Pole Station use in order to coordinate with their main supply base in New Zealand.

Boundaries between the zones

Eastern–Central boundary

Daylight saving time 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:
The Energy Policy Act of 2005 extended daylight saving time 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
2019Mar 10Nov 3