Time in the United States
In the United States, time is divided into nine standard time zones covering the states, territories and other US possessions, with most of the country 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, but no single official map of those existed until the agency announced intentions to make one in September 2022. 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 19th century. 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 had to recalculate their own clocks 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, Illinois, when the telegraph lines transmitted time signals to all major cities.
In October 1884, the International Meridian Conference at Washington, D.C., decided that the prime meridian for longitude and timekeeping should be one that passes through the center 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 statute. The federal law also establishes the transition dates and times at which daylight saving time occurs, with federal law making its observation dependent on state or tribal law. 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.
| Time zone | Standard | DST | Region |
| Atlantic | UTC−04:00 | ' | Puerto Rico, U.S. Virgin Islands |
| Eastern | UTC−05:00 | UTC−04:00 | Connecticut, Delaware, Georgia, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, North Carolina, Ohio, Pennsylvania, Rhode Island, South Carolina, Vermont, Virginia, Washington, D.C., West Virginia; Partially: Florida, Indiana, Kentucky, Michigan, Tennessee |
| Eastern | UTC−05:00 | ' | No DST observed, not defined by 15 U.S.C. §260: Navassa Island, Bajo Nuevo Bank, Serranilla Bank |
| Central | UTC−06:00 | UTC−05:00 | 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−07:00 | UTC−06:00 | Colorado, Montana, New Mexico, Utah, Wyoming; Partially: Idaho, Kansas, Nebraska, Nevada, North Dakota, Oregon, South Dakota, Texas |
| Mountain | UTC−07:00 | ' | Arizona ' |
| Pacific | UTC−08:00 | UTC−07:00 | California, Washington; Partially: Idaho, Nevada, Oregon |
| Alaska | UTC−09:00 | UTC−08:00 | Partially: Alaska |
| Hawaii–Aleutian | UTC−10:00 | UTC−09:00 | Partially: Alaska |
| Hawaii–Aleutian | UTC−10:00 | ' | Hawaii, Not defined by 15 U.S.C. §260: Johnston Atoll |
| American Samoa | UTC−11:00 | ' | American Samoa; Not defined by 15 U.S.C. §260: Jarvis Island, Midway Atoll, Palmyra Atoll, Kingman Reef |
| — | UTC−12:00 | ' | Not defined by 15 U.S.C. §260: Baker Island, Howland Island |
| — | UTC+12:00 | ' | Not defined by 15 U.S.C. §260: Wake Island |
| Chamorro | 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, which comprises roughly the states on the Atlantic coast and the eastern two thirds of the Ohio Valley.
- Central Time Zone, which comprises roughly the Gulf Coast, Mississippi Valley, and most of the Great Plains.
- Mountain Time Zone, 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, which comprises roughly the states on the Pacific coast, plus Nevada and the Idaho Panhandle.
Zones used in states beyond the contiguous U.S.
- Alaska Time Zone, which comprises most of the state of Alaska.
- Hawaii–Aleutian Time Zone, which includes Hawaii and most of the length of the Aleutian Islands chain.
Zones used in U.S. territories
- Samoa Time Zone, which comprises American Samoa.
- Chamorro Time Zone, which comprises Guam and the Northern Mariana Islands.
- Atlantic Time Zone, which comprises Puerto Rico and the U.S. Virgin Islands.
Minor Outlying Islands
Antarctic research stations
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
Eastern–Central boundary
- roughly follows the border between Wisconsin and the Upper Peninsula of Michigan ; the Upper Peninsula counties that border Wisconsin observe central time, all other counties in the U.P. observe eastern time
- follows Lake Michigan
- divides a small portion of Northwestern Indiana near Chicago from the rest of the state
- follows the border between Central Illinois and Central Indiana, with parts of the central time zone extending into northern and southern Indiana.
- 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 and Georgia, although Phenix City, Alabama and some nearby towns unofficially observe eastern time.
- divides the Florida Panhandle along the Apalachicola River and Intracoastal Waterway just west of Tallahassee, Florida.