How U.S. state time zones are decided
Time-zone boundaries inside the United States are defined by the U.S. Department of Transportation under the Uniform Time Act. Boundaries generally follow state lines, but where a state has historic, economic, or geographic ties on both sides of a meridian, the boundary cuts through the state along county lines. A state can petition the DOT to change the boundary; this happens occasionally as commerce shifts.
Whether a state observes daylight saving time is a separate decision: a state can stay on standard time year-round (Hawaii and most of Arizona), but it cannot pick its own DST start and end dates. For the full DST mechanics, see the daylight saving time guide.
Single-zone states
The states (and the District of Columbia) that sit entirely in one zone:
Eastern Time
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.
Central Time
Alabama, Arkansas, Illinois, Iowa, Louisiana, Minnesota, Mississippi, Missouri, Oklahoma, Wisconsin.
Mountain Time
Colorado, Montana, New Mexico, Utah, Wyoming. Arizona is also in the Mountain zone but does not observe DST, so its clock matches Pacific Time for the half of the year that DST is in effect — except on the Navajo Nation, which does observe DST.
Pacific Time
California, Nevada, Washington.
Alaska Time
Most of Alaska. The westernmost Aleutian Islands are in the Hawaii–Aleutian zone — see below.
Hawaii–Aleutian Time
Hawaii uses this zone and does not observe DST. The portion of Alaska's Aleutian Islands west of about 169° 30′ W also sits here and does observe DST.
States that span two zones
Roughly a dozen states straddle a zone boundary. The list below names the two zones and identifies, in plain terms, which side of the state holds which.
| State | Zones | How the boundary runs |
|---|---|---|
| Florida | Eastern / Central | The Panhandle west of the Apalachicola River is on Central; the rest is on Eastern. |
| Indiana | Eastern / Central | Most of Indiana is on Eastern Time. A handful of northwestern and southwestern counties (those with strong commuting ties to Chicago and Evansville) are on Central. |
| Kentucky | Eastern / Central | The eastern half is on Eastern Time; the western half (including Louisville-area counties out to the Ohio border) is on Central. |
| Michigan | Eastern / Central | Most of Michigan is on Eastern Time. Four counties in the western Upper Peninsula bordering Wisconsin are on Central. |
| Tennessee | Eastern / Central | East Tennessee (Knoxville, Chattanooga) is on Eastern; Middle and West Tennessee (Nashville, Memphis) are on Central. |
| Kansas | Central / Mountain | Almost all of Kansas is on Central. Four counties in the far west are on Mountain. |
| Nebraska | Central / Mountain | Most of Nebraska is on Central; the western Panhandle is on Mountain. |
| North Dakota | Central / Mountain | Most of North Dakota is on Central; the southwestern corner is on Mountain. |
| South Dakota | Central / Mountain | The eastern half is on Central; the western half (including the Black Hills and Rapid City) is on Mountain. |
| Texas | Central / Mountain | Almost all of Texas is on Central. The far western corner around El Paso (and a couple of neighboring counties) is on Mountain. |
| Idaho | Mountain / Pacific | The northern Panhandle is on Pacific Time; the rest of the state is on Mountain. |
| Oregon | Pacific / Mountain | Almost all of Oregon is on Pacific Time. Most of Malheur County in the far east is on Mountain. |
| Alaska | Alaska / Hawaii–Aleutian | Most of Alaska is on Alaska Time. The westernmost Aleutian Islands are on Hawaii–Aleutian. |
The Arizona / Navajo Nation special case
Arizona is the most-asked-about case. The state opted out of daylight saving time in 1968. The Navajo Nation, which extends across parts of Arizona, New Mexico, and Utah, observes DST throughout. That creates a small geographic puzzle: from March to November, driving from Tuba City (Navajo Nation) to Flagstaff (Arizona, no DST) crosses an effective one-hour boundary, even though both are within the same state and even though Arizona's "standard" zone is technically Mountain. The Hopi Reservation, surrounded by the Navajo Nation, follows Arizona's no-DST rule.
U.S. territories
- Puerto Rico and the U.S. Virgin Islands — Atlantic Standard Time (
UTC-4), no DST. Year-round, this matches Eastern Daylight Time in summer and is one hour ahead of Eastern Standard Time in winter. - Guam and the Northern Mariana Islands — Chamorro Standard Time (
UTC+10), no DST. - American Samoa — Samoa Standard Time (
UTC-11), no DST. - Wake Island — Wake Island Time (
UTC+12), no DST. - Baker Island, Howland Island — uninhabited, technically
UTC-12.
Quick lookup tips
- If a state is split, the boundary almost always follows county lines. Local courthouses, school districts, and TV stations make excellent tie-breakers when you're not sure which side a small town is on.
- The IANA zone identifier is the safest way to be precise in writing or software.
America/Detroithandles Michigan correctly even for the small set of counties on Central Time, because those counties have their own identifier (America/Menominee). The same is true for Indiana — there are several IANA zones inside the state. - Don't assume "the state capital sets the zone." It usually does, but in Florida, Tennessee, and a couple of other states the capital and a major commercial city are on different sides of the line.
Related
To see the live time in every U.S. zone right now, head to the homepage. To convert a specific moment between any two zones, use the converter. For the practical scheduling implications of all this, see scheduling across U.S. time zones. For the abbreviations used in writing (EST, EDT, CST, CDT, etc.), the abbreviations reference covers them in one place.