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 (DST) 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 (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.

History

Before the adoption of four standard time zones for the contiguous 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 to 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 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 contiguous 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 (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.

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°. 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.Public Law 110–69—America COMPETES Act (August 9, 2007). Sec. 3013) 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.

+Standard time zones of the United States and its surrounding areasStandard Time Zone Boundaries 49CFR71 Time zone Standard DST Region Atlantic UTC−04:00 (not observed) 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 (not observed) 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 (not observed) Arizona (no DST outside of Navajo Nation) Pacific UTC−08:00 UTC−07:00 California, Washington; Partially: Idaho, Nevada, Oregon Alaska UTC−09:00 UTC−08:00Partially: Alaska Hawaii–Aleutian UTC−10:00 UTC−09:00(not observed) Hawaii, Not defined by 15 U.S.C. §260: Johnston Atoll (no DST observed in Hawaii and 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 (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. Alaska Time Zone (UTC−09:00; Zone V), which comprises most of the state of Alaska. Hawaii–Aleutian Time Zone (no daylight saving in Hawaii, UTC−10:00; zone W), which includes Hawaii and most of the length of the Aleutian Islands chain (west of 169°30′W).

Zones used in U.S. territories Samoa Time Zone (UTC−11:00; Zone X), which comprises American Samoa. Chamorro Time Zone (UTC+10:00; Zone K), which comprises Guam and the Northern Mariana Islands. Atlantic Time Zone (UTC−04:00; Zone Q), which comprises Puerto Rico and the U.S. Virgin Islands.

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

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

roughly follows the border between Wisconsin (to the south and west) and the Upper Peninsula (U.P.) of Michigan (to the north and east); the Upper Peninsula counties that border Wisconsin (namely Gogebic, Iron, Dickinson, and Menominee counties) 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 (west) and Central Indiana (east), 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 (west) and Georgia (east), 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.

Central–Mountain 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 counties—observe central time) follows the border between New Mexico (west) and Oklahoma (east) (although the zone legally begins at the Oklahoma–New Mexico state line, Kenton in extreme northwestern Cimarron County, Oklahoma unofficially observes mountain time) follows the border between New Mexico (west) and Texas (east) divides El Paso County, Hudspeth County, and a portion of northwestern Culberson County that includes eastern Guadalupe Mountains National Park from the rest of Texas

Mountain–Pacific boundary follows the border between northern Idaho (to the west) and northwestern Montana (to the east) turns west at (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 ). 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 six 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