In the United States, time is divided into nine standardtime zones covering the states, territories and other US possessions, with most of the country observingdaylight saving time (DST) for approximately the spring, summer, and fall months. The time zone boundaries and DST observance are regulated by theDepartment of Transportation, but no single official map of those existed until the agency announced intentions to make one in September 2022.[1] Official and highly precise timekeeping services (clocks) are provided by two federal agencies: theNational Institute of Standards and Technology (NIST) (an agency of theDepartment of Commerce); and theUnited 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 legalcivil time for any U.S. location at any moment.
Before the adoption of fourstandard time zones for the continental United States, many towns and cities set their clocks to noon when the sun passed their localmeridian, pre-corrected for theequation of time on the date of observation, to formlocal 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 oflocal solar time became increasingly awkward as railways and telecommunications improved.[2] 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 (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.[2]
Weather service chiefCleveland Abbe introduced four standard time zones for his weather stations, an idea which he offered to the railroads.[3] 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, inChicago, Illinois, when the telegraph lines transmitted time signals to all major cities.[4][5]
In October 1884, theInternational 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 theGreenwich Observatory in the United Kingdom. The conference therefore established theGreenwich Meridian as the prime meridian andGreenwich 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.[2]
In 1960, the International Radio Consultative Committee formalized the concept ofCoordinated 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 zones in the United States and other regions are currently defined at the federal level by statute15 U.S.C.§ 260. The federal law also establishes the transition dates and times at whichdaylight saving time occurs, with federal law making its observation dependent on state or tribal law. It is ultimately the authority of thesecretary 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.[7] As of August 9, 2007, the standard time zones are defined in terms of hourly offsets fromUTC.[8] Prior to this they were based upon themean solar time at several meridians 15° apart west ofGreenwich (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 areas[9]
Mountain Time Zone (Zone T), which comprises roughly the states and portions of states that include theRocky Mountains and the western quarter of the Great Plains.
roughly follows the border betweenWisconsin (to the south and west) and theUpper Peninsula (U.P.) ofMichigan (to the north and east); the Upper Peninsula counties that border Wisconsin (namelyGogebic,Iron,Dickinson, andMenominee counties) observe central time, all other counties in the U.P. observe eastern time
follows the border betweenCentral Illinois (west) andCentral Indiana (east), with parts of the central time zone extending into northern and southern Indiana.
follows the border betweenNew Mexico (west) andOklahoma (east) (although the zone legally begins at the Oklahoma–New Mexico state line,Kenton in extreme northwesternCimarron County, Oklahoma unofficially observes mountain time)
follows the border between New Mexico (west) andTexas (east)
follows the Salmon River west to the town ofRiggins, 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 theSnake River at the Oregon border (at45°51.3′N116°47.5′W / 45.8550°N 116.7917°W /45.8550; -116.7917). This loop to the north creates a curious situation where one can enter a more westerly time zone by travelingeast over one of the six bridges across this portion of the Salmon River.
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 betweenOregon (west) and Idaho (east)
turns east and follows the border between Idaho (north) and Nevada (south) along the42nd parallel north to longitude 114.041726 W.
turns south and follows the border betweenNevada (west) andUtah (east), except for following the west city limit line ofWest Wendover dividing it from the rest ofNevada, 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 betweenNevada (west) andArizona (east)
follows the border betweenCalifornia (west) and Arizona (east), mostly defined by the Colorado River, to the border between the U.S. and Mexico.
Daylight saving time (DST) begins on the second Sunday of March and ends on the first Sunday of November.
Clocks are 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 the corresponding end dates:
Dates for daylight saving time
Year
Start
End
2025
Mar 9
Nov 2
2026
Mar 8
Nov 1
2027
Mar 14
Nov 7
2028
Mar 12
Nov 5
2029
Mar 11
Nov 4
2030
Mar 10
Nov 3
2031
Mar 9
Nov 2
2032
Mar 14
Nov 7
In response to theUniform Time Act of 1966, each state has officially chosen to apply one of two rules over its entire territory:
Most use the standard time for their zone (or zones, where a state is divided between two zones), except for using daylight saving time during the summer months. Originally this ran from the last Sunday in April until the last Sunday in October. Two subsequent amendments, in 1986 and 2005, have shifted these days so that daylight saving time now runs from the second Sunday in March until the first Sunday in November.Arizona time: Year-round Mountain Standard Time (red) and DST (yellow)
Arizona andHawaii use standard time throughout the year. However:
TheNavajo Nation observes DST throughout its entire territory, including the portion that lies in Arizona. But theHopi Nation, which is entirely surrounded by the Navajo Nation and is entirely in Arizona, doesnot observe DST. (See map inset right.)
In 2005, Indiana passed legislation that took effect on April 2, 2006, that placed the entire state on daylight saving time (seeTime in Indiana). Before then,Indiana officially used standard time year-round, with the following exceptions:
The portions of Indiana that were on Central Time observed daylight saving time.
Some Indiana counties nearCincinnati andLouisville were on Eastern Time but did (unofficially) observe DST.
TheEnergy Policy Act of 2005 extended daylight saving time for an additional month beginning in 2007.
