How many time zones does the United States have?

The fifty states are covered by six standard zones: Eastern, Central, Mountain, Pacific, Alaska, and Hawaii-Aleutian. Counting the territories adds three more, since Puerto Rico and the US Virgin Islands use Atlantic Standard Time, Guam and the Northern Mariana Islands use Chamorro Standard Time, and American Samoa is further west again.

What is the time difference between the East and West Coast?

Three hours. When it is 12:00 noon in New York it is 9:00 a.m. in Los Angeles, and that gap holds all year because both coasts change their clocks on the same dates. Alaska is one hour behind the West Coast and Hawaii is two hours behind it in winter.

Who sets United States time zone boundaries?

The Department of Transportation. Since the Uniform Time Act of 1966 it has held the authority to move a county or region from one zone to another, deciding largely on where the area's business, schools and transport links point. Requests come from local governments and are published for comment before any change.

Why does Arizona not match its neighbours in summer?

Arizona stays on Mountain Standard Time all year rather than moving its clocks forward in March. From March to November it therefore reads the same as California on Pacific Daylight Time, and only matches Colorado and New Mexico during the winter months. The Navajo Nation within Arizona does observe the change.

US Time Zones Explained

Ask what time it is in the United States and you get several correct answers at once. The country reaches from the Atlantic seaboard to the middle of the Pacific, so the US time zones divide the day into six standard bands across the fifty states, with further zones for the territories. Get the zone wrong and a nine o'clock meeting arrives at six in the morning for half the people invited.

This guide sets out each of the US time zones, the offsets from Coordinated Universal Time that define them, who draws the lines between them, and how daylight saving reshuffles the arrangement twice a year. To see every zone side by side at this moment, open the live US time board.

What Are the US Time Zones?

The US time zones are six standard zones covering the fifty states: Eastern, Central, Mountain, Pacific, Alaska, and Hawaii–Aleutian. Each is one hour behind the zone to its east, so noon in New York is nine in the morning in California.

Three further zones cover territory beyond the states. Puerto Rico and the US Virgin Islands keep Atlantic Standard Time, Guam and the Northern Mariana Islands keep Chamorro Standard Time, and American Samoa sits further west still. Those are covered in detail in the guide to time in US territories.

The Six Standard Zones and Their Offsets

Every zone is defined by its distance from Coordinated Universal Time, written as UTC minus a number of hours. Each has a standard abbreviation for winter and a daylight abbreviation for summer.

Eastern Time: UTC−5 as EST, UTC−4 as EDT. Covers the seaboard from Maine to Florida and inland to Ohio and Michigan.
Central Time: UTC−6 as CST, UTC−5 as CDT. The broadest zone by population share of the interior, holding Chicago, Houston and Dallas.
Mountain Time: UTC−7 as MST, UTC−6 as MDT. Covers Colorado, Utah, Wyoming, New Mexico, Montana and Arizona.
Pacific Time: UTC−8 as PST, UTC−7 as PDT. Covers California, Washington and most of Nevada and Oregon.
Alaska Time: UTC−9 as AKST, UTC−8 as AKDT. Covers almost all of Alaska, including Anchorage and Juneau.
Hawaii–Aleutian Time: UTC−10 as HST. Hawaii stays on it all year; the far western Aleutian Islands shift to HDT in summer.

Because the mainland spans four of the US time zones, the practical working day is squeezed: the East Coast is three hours ahead of the West Coast, a gap examined in the guide to calling across US time zones.

Who Decides Where the US Time Zones Begin and End?

The Department of Transportation sets and amends the boundaries of the US time zones. A county or region that wants to move zones petitions the department, which weighs the request mainly on where the area's commerce, schools and media naturally point.

The framework is older than the department itself. The Standard Time Act of 1918 gave the United States its first legal standard zones, replacing a patchwork of railway and local times, and briefly introduced daylight saving. The Uniform Time Act of 1966 then imposed a single national pattern on the seasonal change while letting whole states opt out of it. Authority over the boundary lines passed from the Interstate Commerce Commission to the Department of Transportation under that same law, which is why time zone petitions are handled today by a transport agency rather than a scientific body.

The test the department applies is the convenience of commerce, a phrase that does a great deal of work. In practice it means a county can be moved if its residents commute, trade and send their children to school across the line, and it means longitude on its own is never a sufficient argument. Boundaries therefore bend well away from the meridians they nominally follow, which is why several of the US time zones have edges that zig-zag through the middle of states rather than running straight from Canada to the Gulf.

How Daylight Saving Reshapes the Map

For most of the year the US time zones sit exactly one hour apart, but the seasonal change complicates that. Under the Energy Policy Act of 2005, clocks move forward on the second Sunday in March and back on the first Sunday in November, in both cases at 2:00 a.m. local time.

Two places sit the change out. Arizona keeps Mountain Standard Time all year, so it matches California in summer and Colorado in winter, though the Navajo Nation within its borders does observe the change. Hawaii also stays on standard time throughout. The list of exceptions, and the reasoning behind each, is set out in the article on states that don't observe daylight saving. Proposals under the banner of the Sunshine Protection Act have repeatedly sought to make daylight time permanent nationwide, but none has become law.

Which States Are Split Between US Time Zones?

Around a dozen states are divided between two zones, with the line usually following county boundaries rather than a straight meridian. Florida's panhandle, western Kentucky and western Tennessee sit in Central Time while the rest of each state keeps Eastern.

Other splits run the other way. Nebraska, Kansas, North Dakota and South Dakota all have western counties on Mountain Time, and Texas places El Paso on Mountain while the rest of the state keeps Central. Oregon's Malheur County looks east to Idaho and keeps Mountain Time, and Idaho itself is divided, with the northern panhandle on Pacific. Indiana's arrangement was contested for decades and is the most tangled of them all.

Reading the Zones Without Getting Caught Out

A few habits prevent most mistakes when working across the US time zones.

Write ET, CT, MT or PT when you mean the zone generally, and reserve EST or EDT for the specific half of the year.
Name the city as well as the zone; "3 p.m. Denver time" is harder to misread than an abbreviation.
Remember that Arizona and Hawaii drift relative to their neighbours for eight months of the year.
Check the offset rather than the label when scheduling with another country, since the abbreviations are not unique worldwide.

For a single zone at a glance, the dedicated Eastern Time clock page shows the current time and whether standard or daylight time is in force.

Conclusion

The US time zones are six standard zones for the states, plus Atlantic, Chamorro and Samoa time across the territories, each defined by a fixed offset from Coordinated Universal Time and each redrawn at the edges by the Department of Transportation when local commerce demands it. Daylight saving lifts most of them by an hour from March to November, while Arizona and Hawaii hold still. To check every zone at once, open the live US time zones board, or start from the ustime.now home page.

US Time Zones Explained: All Six Zones, Offsets and Boundaries