Time in the United States

Last updated

Time in the United States of America
Map of U.S. time zones since November 2007
Current time
Chamorro 6:02 am, January 28, 2024 [refresh]
Atlantic 4:02 pm, January 27, 2024 [refresh]
Eastern 3:02 pm, January 27, 2024 [refresh]
Central 2:02 pm, January 27, 2024 [refresh]
Mountain 1:02 pm, January 27, 2024 [refresh] [lower-alpha 1]
Pacific 12:02 pm, January 27, 2024 [refresh]
Alaska 11:02 am, January 27, 2024 [refresh]
Hawaii–Aleutian 10:02 am, January 27, 2024 [refresh] [lower-alpha 2]
Samoa 9:02 am, January 27, 2024 [refresh]

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 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: 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.


Rand McNally Standard Map of the United States, 1921, showing boundaries different from today Rand McNally Standard Map of the United States 1921 UTA.jpg
Rand McNally Standard Map of the United States, 1921, showing boundaries different from today

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. [2] 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. Train drivers must 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 chief Cleveland 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, in Chicago, IL, when the telegraph lines transmitted time signals to all major cities. [4] [5]

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. [2]

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°. [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.

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 15 U.S.C.   § 260. 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. [7] As of August 9, 2007, the standard time zones are defined in terms of hourly offsets from UTC. [8] 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 areas [9]
Time zoneStandardDSTRegion
Atlantic UTC−04:00 (not observed) Puerto Rico, U.S. Virgin Islands
Eastern UTC−05:00 UTC−04:00 Connecticut, Delaware, District of Columbia, 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−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 Arizona (no DST outside of Navajo Nation), Colorado, Montana, New Mexico, Utah, Wyoming; Partially: Idaho, Kansas, Nebraska, Nevada, North Dakota, Oregon, South Dakota, Texas
Pacific UTC−08:00 UTC−07:00 California, Washington (state); Partially: Idaho, Nevada, Oregon
Alaska UTC−09:00 UTC−08:00 Partially: Alaska
Hawaii–Aleutian UTC−10:00 UTC−09:00 Hawaii (no DST observed in Hawaii); Partially: Alaska; No DST observed, not defined by 15 U.S.C. §260: Johnston Atoll
American Samoa UTC−11:00 (not observed) American Samoa; Not defined by 15 U.S.C. §260: Jarvis Island, Midway Atoll, Palmyra Atoll, Kingman Reef
UTC−12:00 (not observed)Not defined by 15 U.S.C. §260: Baker Island, Howland Island
UTC+12:00 (not observed)Not defined by 15 U.S.C. §260: Wake Island
Chamorro UTC+10:00 (not observed) Guam, Northern Mariana Islands

Zones used in the contiguous U.S.

Standard time in the contiguous United States, illustration 1903 Standard time in the United States illustration 1906.jpg
Standard time in the contiguous United States, illustration 1903

From east to west, the four time zones of the contiguous United States are:

Zones used in states beyond the contiguous U.S.

Zones used in U.S. territories

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.)

EasternCentral boundary

Marker showing the border of Wayne County, Kentucky, and the Eastern Time Zone, coming from Clinton County, same state, Central Time Zone Wayne Co KY.JPG
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. Current time zones in Indiana.svg
Time in Indiana: red counties belong to the Central Time Zone.

CentralMountain boundary

MountainPacific boundary

Daylight saving time (DST)

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
2023Mar 12Nov 5
2024Mar 10Nov 3
2025Mar 9Nov 2
2026Mar 8Nov 1
2027Mar 14Nov 7
2028Mar 12Nov 5
2029Mar 11Nov 4

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.

The Sunshine Protection Act of 2021 passed the United States Senate in March of 2022. The bill would make Daylight Time the time year-round in the entire United States. The bill is yet to be voted on by the House of Representatives. [10] [ needs update ]

Previous DST change dates
2006Apr 2Oct 29
2007Mar 11Nov 4
2008Mar 9Nov 2
2009Mar 8Nov 1
2010Mar 14Nov 7
2011Mar 13Nov 6
2012Mar 11Nov 4
2013Mar 10Nov 3
2014Mar 9Nov 2
2015Mar 8Nov 1
2016Mar 13Nov 6
2017Mar 12Nov 5
2018Mar 11Nov 4
2019Mar 10Nov 3
2020Mar 8Nov 1
2021Mar 14Nov 7
2022Mar 13Nov 6

See also

Explanatory notes

  1. Time in Arizona (besides the Navajo Nation): 1:02 pm, January 27, 2024 [refresh]
  2. Time in Hawaii: 10:02 am, January 27, 2024 [refresh]

Related Research Articles

<span class="mw-page-title-main">Time zone</span> Area that observes a uniform standard time

A time zone is an area which observes a uniform standard time for legal, commercial and social purposes. Time zones tend to follow the boundaries between countries and their subdivisions instead of strictly following longitude, because it is convenient for areas in frequent communication to keep the same time.

<span class="mw-page-title-main">Japan Standard Time</span> Standard time zone in Japan

Japan Standard Time, or Japan Central Standard Time, is the standard time zone in Japan, 9 hours ahead of UTC (UTC+09:00). Japan does not observe daylight saving time, though its introduction has been debated on several occasions. During World War II, the time zone was often referred to as Tokyo Standard Time.

<span class="mw-page-title-main">Central Time Zone</span> Time zone in North America

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.

<span class="mw-page-title-main">Eastern Time Zone</span> North American time zone (UTC−5 and UTC−4)

The Eastern Time Zone (ET) is a time zone encompassing part or all of 23 states in the eastern part of the United States, parts of eastern Canada, and the state of Quintana Roo in Mexico.

<span class="mw-page-title-main">Mountain Time Zone</span> Time zone of North America

The Mountain Time Zone of North America keeps time by subtracting seven hours from Coordinated Universal Time (UTC) when standard time (UTC−07:00) 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.

<span class="mw-page-title-main">Hawaii–Aleutian Time Zone</span> UTC−10:00 during standard time; UTC−09:00 during daylight saving

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.

<span class="mw-page-title-main">Central European Time</span> Standard time (UTC+01:00)

Central European Time (CET) is a standard time of Central, and parts of Western Europe, which is one hour ahead of Coordinated Universal Time (UTC). The time offset from UTC can be written as UTC+01:00. It is used in most parts of Europe and in a few North African countries. CET is also known as Middle European Time and by colloquial names such as Amsterdam Time, Berlin Time, Brussels Time, Budapest Time,Madrid Time, Paris Time, Rome Time,Prague time,Warsaw Time or Romance Standard Time (RST).

<span class="mw-page-title-main">Summer time in Europe</span> Variation of standard clock time

Summer time in Europe is the variation of standard clock time that is applied in most European countries in the period between spring and autumn, during which clocks are advanced by one hour from the time observed in the rest of the year, with a view to making the most efficient use of seasonal daylight. It corresponds to the notion and practice of daylight saving time (DST) to be found in some other parts of the world.

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).

<span class="mw-page-title-main">Atlantic Time Zone</span> Time zone (UTC−04:00)

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. AST is observed in parts of North America and some Caribbean islands. 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 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.

<span class="mw-page-title-main">Moscow Time</span> Time zone in western Russia (UTC+3)

Moscow Time is the time zone for the city of Moscow, Russia, and most of western Russia, including Saint Petersburg. It is the second-westernmost of the eleven time zones of Russia. It has been set to UTC+03:00 without DST since 26 October 2014; before that date it had been set to UTC+04:00 year-round on 27 March 2011.

<span class="mw-page-title-main">Time in Australia</span> Time zones in Australia

Australia uses three main time zones: Australian Eastern Standard Time, Australian Central Standard Time and Australian Western Standard Time.

<span class="mw-page-title-main">Time in Canada</span> Time zones of Canada

Canada is divided into six time zones. Most areas of the country's provinces and territories operate on standard time from the first Sunday in November to the second Sunday in March and daylight saving time the rest of the year.

Malaysian Standard Time or Malaysian Time (MYT) is the standard time used in Malaysia. It is 8 hours ahead of Coordinated Universal Time (UTC). Malaysia does not observe daylight saving time.

<span class="mw-page-title-main">Time in Indiana</span> Time zones used in Indiana

The U.S. state of Indiana is divided into 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.

<span class="mw-page-title-main">Iran Standard Time</span> Identifier for a time offset from UTC of +03:30

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.

<span class="mw-page-title-main">UTC+00:00</span> Identifier for the UTC +0 offset

UTC+00:00 is an identifier for a time offset from UTC of +00:00. This time zone is the basis of UTC and all other time zones are based on it. In ISO 8601, an example of the associated time would be written as 2069-01-01T12:12:34+00:00. It is also known by the following geographical or historical names:

<span class="mw-page-title-main">Daylight saving time in the United States</span> Practice of setting the clock forward by one hour in the United states

Most of the United States observes daylight saving time, the practice of setting the clock forward by one hour when there is longer daylight during the day, so that evenings have more daylight and mornings have less. Exceptions include 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.

<span class="mw-page-title-main">Daylight saving time by country</span>

Daylight saving time (DST), also known as summer time, is the practice of advancing clocks during part of the year, typically by one hour around spring and summer, so that daylight ends at a later time of the day. As of 2024, DST is observed in most of Europe, most of North America and parts of Africa and Asia around the Northern Hemisphere summer, and in parts of South America and Oceania around the Southern Hemisphere summer. It was also formerly observed in other areas.

<span class="mw-page-title-main">Time in Idaho</span>

The U.S. state of Idaho is covered by two time zones, as described below. All locations observe daylight saving time.


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  2. 1 2 3 "Why Do We Have Time Zones?".
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  4. "Greenwich Mean Time (GMT)—time, facts, history". greenwichmeantime.com. Retrieved April 18, 2019.
  5. "The Central Standard Building". Open House Chicago. Retrieved November 9, 2019.
  6. Guinot, Bernard (August 2011). "Solar time, legal time, time in use". Metrologia . 48 (4): 181–185. Bibcode:2011Metro..48S.181G. doi:10.1088/0026-1394/48/4/s08.
  7. "15 U.S. Code Subchapter IX—STANDARD TIME".
  8. Public Law 110–69—America COMPETES Act (August 9, 2007). Sec. 3013)
  9. Standard Time Zone Boundaries 49CFR71
  10. Reichert, Corinne. "Senate Unanimously Passes Bill to Make Daylight Saving Time Permanent". CNET. Retrieved March 16, 2022.