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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.
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.
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 (sometimes hundreds of miles in a day), according to the Library of Congress. 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 DC 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.
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°.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.
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 U.S. lawthey are:
|Atlantic||(not observed)||UTC−04:00||Puerto Rico, U.S. Virgin Islands|
|Eastern||UTC−04:00||UTC−05:00||Connecticut, Delaware, 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; Partially: Idaho, Oregon|
|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|
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 (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).
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.
(Described from north to south along each boundary.)
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 AM on the following start dates and set back one hour at 2:00 AM on these ending dates:
|2019||Mar 10||Nov 3|
|2020||Mar 8||Nov 1|
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 (DST) for an additional month beginning in 2007.
Previous DST change dates include:
|2006||Apr 2||Oct 29|
|2007||Mar 11||Nov 4|
|2008||Mar 9||Nov 2|
|2009||Mar 8||Nov 1|
|2010||Mar 14||Nov 7|
|2011||Mar 13||Nov 6|
|2012||Mar 11||Nov 4|
|2013||Mar 10||Nov 3|
|2014||Mar 9||Nov 2|
|2015||Mar 8||Nov 1|
|2016||Mar 13||Nov 6|
|2017||Mar 12||Nov 5|
|2018||Mar 11||Nov 4|
|2019||Mar 10||Nov 3|
A time zone is a region of the globe that observes a uniform standard time for legal, commercial, and social purposes. Time zones tend to follow the boundaries of countries and their subdivisions instead of longitude, because it is convenient for areas in close commercial or other communication to keep the same time.
Japan Standard Time is the standard time zone in Japan, 9 hours ahead of UTC. There is no daylight saving time, though its introduction has been debated several times. During World War II, it was often called Tokyo Standard Time.
The North American Central Time Zone (CT) is a time zone in parts of Canada, the United States, Mexico, Central America, some Caribbean Islands, and part of the Eastern Pacific Ocean.
The Eastern Time Zone (ET) is a time zone encompassing part or all of 22 states in the eastern part of the contiguous United States, parts of eastern Canada, the state of Quintana Roo in Mexico, Panama in Central America, and certain Caribbean and Atlantic islands, along with certain countries and parts of countries in South America. Places that use Eastern Standard Time (EST) when observing standard time (autumn/winter) are five hours behind Coordinated Universal Time (UTC−05:00).
The Mountain Time Zone of North America keeps time by subtracting seven hours from Coordinated Universal Time (UTC) when standard time is in effect, and by subtracting six hours during daylight saving time (UTC−06:00). The clock time in this zone is based on the mean solar time at the 105th meridian west of the Greenwich Observatory. In the United States, the exact specification for the location of time zones and the dividing lines between zones is set forth in the Code of Federal Regulations at 49 CFR 71.
The Pacific Time Zone (PT) is a time zone encompassing parts of western Canada, the western United States, and western Mexico. Places in this zone observe standard time by subtracting eight hours from Coordinated Universal Time (UTC−08:00). During daylight saving time, a time offset of UTC−07:00 is used.
The Hawaii–Aleutian Time Zone observes Hawaii–Aleutian Standard Time (HST), by subtracting ten hours from Coordinated Universal Time (UTC−10:00). The clock time in this zone is based on the mean solar time of the 150th meridian west of the Greenwich Observatory.
Central European Time (CET), used in most parts of Europe and a few North African countries, is a standard time which is 1 hour ahead of Coordinated Universal Time (UTC). The time offset from UTC can be written as UTC+01:00. The same standard time, UTC+01:00, is also known as Middle European Time and under other names like Berlin Time, Warsaw Time and Romance Standard Time (RST), Paris Time or Rome Time.
Time in New Zealand is divided by law into two standard time zones. The main islands use New Zealand Standard Time (NZST), 12 hours in advance of Coordinated Universal Time (UTC) / military M (Mike), while the outlying Chatham Islands use Chatham Standard Time (CHAST), 12 hours 45 minutes in advance of UTC / military M^ (Mike-Three).
The Atlantic Time Zone is a geographical region that keeps standard time—called Atlantic Standard Time (AST)—by subtracting four hours from Coordinated Universal Time (UTC), resulting in UTC−04:00. During part of the year, some portions of the zone observe daylight saving time, referred to as Atlantic Daylight Time (ADT), by moving their clocks forward one hour to result in UTC−03:00. The clock time in this zone is based on the mean solar time of the 60th meridian west of the Greenwich Observatory.
Australia uses three main time zones: Australian Western Standard Time, Australian Central Standard Time, and Australian Eastern Standard Time. Time is regulated by the individual state governments, some of which observe daylight saving time (DST). Australia's external territories observe different time zones.
Canada is divided into six time zones, based on proposals by Scottish Canadian railway engineer Sir Sandford Fleming, who pioneered the use of the 24-hour clock, the world's time zone system, and a standard prime meridian. Most of Canada operates on standard time from the first Sunday in November to the second Sunday in March and daylight saving time the rest of the year.
The Indonesian archipelago geographically stretches across four time zones from UTC+06:00 in Aceh to UTC+09:00 in Western New Guinea. However, the Indonesian government only recognizes three time zones in its territory:
The U.S. state of Indiana is divided between Eastern and Central time zones. The official dividing line has generally moved progressively west from its original location on the Indiana–Ohio border, to a position dividing Indiana down the middle, and finally to its current location along much of the Indiana–Illinois border. In April 2006, several southwestern and northwestern counties reverted to Central time, although by late 2007 all but two had returned to Eastern time.
Iran Standard Time (IRST) or Iran Time (IT) is the time zone used in Iran. Iran uses a UTC offset UTC+03:30. IRST is defined by the 52.5 degrees east meridian, the same meridian which defines the Iranian calendar and is the official meridian of Iran.
Mexico uses four main time zones since February 2015. Most of the country observes Daylight Saving Time.
Daylight saving time in the United States is the practice of setting the clock forward by one hour during the warmer part of the year, so that evenings have more daylight and mornings have less. Most areas of the United States and Canada observe daylight saving time (DST), the exceptions being Arizona, Hawaii, and the overseas territories of American Samoa, Guam, the Northern Mariana Islands, Puerto Rico, and the United States Virgin Islands. The Uniform Time Act of 1966 established the system of uniform Daylight Saving Time throughout the US.
Portugal has two time zones and observes daylight saving time. Continental Portugal and Madeira use UTC+00:00, while the Azores use UTC–01:00. Daylight saving time is observed nationwide from the last Sunday in March to the last Sunday in October, so that every year, continental Portugal and Madeira temporarily use UTC+01:00, and the Azores temporarily use UTC+00:00.
The U.S. state of Idaho is covered by two time zones, as described below. All locations observe daylight saving time.
Denmark, including the dependencies Faroe Islands and Greenland, uses six different time zones.