united states time zones

United States time zones are nine standard time zones covering the states and possessions of the United States, used in conjunction with state and national legislation regarding the use of Daylight Saving Time.

State option regarding Daylight Savings Time

In response to the Uniform Time Act of 1966, each state of the US 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 savings time from 2 AM on the first Sunday in April until 2 AM on the last Sunday in October.
The remainder (Indiana, Arizona and Hawaii) use standard time throughout the year. However:

The portions of Indiana that are on Central Time observe Daylight Savings Time. Also, some Indiana counties near Cincinnati and Louisville are on Eastern Time, but do (unofficially) observe DST.
The Navajo Nation observes DST throughout its entire territory, including the portion that lies in Arizona. But the Hopi Nation, which is entirely surrounded by the Navajo Nation, does not observe DST.

Enumeration of time zones

All time zone abbreviations below are unofficial — only the full names are official.

Zones used in the contiguous U.S.

From east to west, these zones are:

Eastern Standard Time (EST; UTC−5; Zone R), which comprises roughly the states on the Atlantic coast plus some other areas away from both the Gulf of Mexico and the Mississippi River.
Central Standard Time (CST; UTC−6; Zone S), which comprises roughly the states on the Mississippi River plus nearly a full state further west.
Mountain Standard Time (MST; UTC−7; Zone T), which comprises roughly the states that include the Continental Divide and Rocky Mountains plus much of the Great Basin and the watershed of the Colorado River.
Pacific Standard Time (PST; UTC−8; Zone U), which comprises roughly the states on the Pacific coast, plus Nevada.

Zones used in states beyond the contiguous U.S.

Alaska Standard Time (AKST; UTC−9; Zone V), which comprises most of the state of Alaska.
Hawaii-Aleutian Standard Time (or unofficially Hawaii Standard Time) (HST; UTC−10; zone W), which includes Hawaii and most of the length of the Aleutian Islands chain (west of 16930'W).

Zones outside the states

Atlantic Standard Time (AST, UTC−4, Zone Q), which comprises Puerto Rico and the U.S. Virgin Islands.
Samoa Standard Time (UTC−11, Zone X), which comprises American Samoa.
Chamorro Standard Time (UTC+10, Zone K), which comprises Guam and the Northern Mariana Islands.

Boundaries between the zones

(Described from north to south along each boundary.)

EST/CST boundary

roughly follows the border between Wisconsin (to west) and the Upper Peninsula of Michigan (to east); the Upper Peninsula counties that border on Wisconsin observe CST, all other counties in the UP observe EST
follows Lake Michigan
divides a small portion of northwestern Indiana from the rest of the state
follows the border between Illinois (west) and Indiana (east)
divides a small portion of southwestern Indiana from the rest of the state
divides Kentucky roughly in half
divides a smaller portion of eastern Tennesee from a larger one
follows the border between Alabama (west) and Georgia (east)
divides most of the western panhandle of Florida from the rest of the state.

CST/MST boundary

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 from the rest of the state
follows the border between New Mexico (west) and Oklahoma (east)
follows the border between New Mexico (west) and Texas (east)
divides El Paso County and Hudspeth County from the rest of Texas