Time in the United States

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°.[6] 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 in the contiguous United States, illustration 1903

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.

• 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).

• 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.

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.

Marker showing the border of Wayne County, Kentucky, and the Eastern Time Zone, coming from Clinton County, same state, Central Time Zone

Time in Indiana: red counties belong to the Central Time Zone.

• 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.

• 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

• 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 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
• 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.