US Time Zones Explained: How America Keeps Time

The Six Mainland Time Zones and the Five Territory Zones

Count the zones aloud and most people stop at four. That tally misses Alaska, Hawaii, and five territories. The correct number is eleven. The six that cover the fifty states handle daily life.

The six state zones are Eastern, Central, Mountain, Pacific, Alaska, and Hawaii-Aleutian. Each sits one hour behind the zone to its east when both are on winter time. Noon in Washington, D.C. is 11:00 a.m. in Chicago, 10:00 a.m. in Denver, 9:00 a.m. in San Francisco, 8:00 a.m. in Anchorage, and 7:00 a.m. in Honolulu.

Five additional zones cover the territories: Atlantic Standard Time (Puerto Rico and US Virgin Islands), Chamorro Standard Time (Guam and Northern Mariana Islands), Samoa Standard Time (American Samoa), plus two zones with no permanent population. Wake Island uses one. Uninhabited Baker Island and Howland Island use the other.

UTC Offsets and How Boundaries Actually Work

Every time zone is defined by its offset from Coordinated Universal Time, written as UTC plus or minus a number of hours. The offset tells you how far ahead or behind the zone runs relative to the global clock.

Eastern Standard Time is UTC−5. When it is noon in London (UTC+0 in winter), it is 7:00 a.m. in Washington. Central Standard Time is UTC−6, one hour behind Eastern. Mountain Standard Time is UTC−7. Pacific Standard Time is UTC−8. Alaska Standard Time is UTC−9. Hawaii-Aleutian Standard Time is UTC−10.

The range across all US zones is wide. American Samoa sits at UTC−11, the farthest behind UTC of any permanently inhabited US territory. Guam and the Northern Mariana Islands sit at UTC+10, ahead of UTC. When it is Monday morning in Guam, it is still Sunday afternoon in American Samoa.

Why Zone Names and Abbreviations Differ

The names of the zones are permanent. Eastern Time is always Eastern Time, whether the clocks are on winter time or summer time. The abbreviations change.

Eastern Standard Time (EST) becomes Eastern Daylight Time (EDT) when clocks spring forward. Central Standard Time (CST) becomes Central Daylight Time (CDT). Mountain Standard Time (MST) becomes Mountain Daylight Time (MDT). Pacific Standard Time (PST) becomes Pacific Daylight Time (PDT). Alaska Standard Time (AKST) becomes Alaska Daylight Time (AKDT). Hawaii-Aleutian is HST all year; the Aleutian Islands portion shifts to HDT in summer.

This matters because "ET" is safe year-round. "EST" in July is incorrect and signals carelessness. Schedule a meeting for 3:00 p.m. EST in July and your West Coast colleagues will show up at noon Pacific Daylight Time. They read the signifier correctly. You did not. Use "ET", "CT", "MT", "PT" to cover the whole year. The guide on ET vs EST vs EDT explains the distinction in detail.

Why Time Zone Boundaries Zigzag

Look at a map of US time zones and the boundaries bend. They do not follow the meridians of longitude that theoretically define them. Commerce decides the shape.

Before 1883, every city kept its own solar time. Railroads ran on roughly 50 different local times. Scheduling chaos and collisions were routine. On 18 November 1883, the railroads voluntarily adopted four time zones. It was not federal law until the Standard Time Act of 1918, which codified the railroad zones and introduced daylight saving time for World War I fuel conservation.

The Uniform Time Act of 1966 transferred authority over time zone boundaries to the Department of Transportation. The department applies a simple test: the convenience of commerce. A county can petition to switch zones if its residents commute, trade, and send their children to school across the line. Longitude alone is never a sufficient argument.

That is why several states are split between two zones. Florida's panhandle runs on Central Time while the rest of the state uses Eastern. Western Kansas uses Mountain Time while the rest uses Central. Indiana was a mess until 2006, when the entire state moved to Eastern Time with daylight saving, except for 12 counties near Chicago and Evansville that stayed Central. About 15 states have counties in different time zones. The page on states split between two time zones lists every one.

US Territories and Their Time Zones

The territories follow different rules from the states. None of them observe daylight saving time.

Puerto Rico and the US Virgin Islands use Atlantic Standard Time, UTC−4, all year. That puts them one hour ahead of Eastern Standard Time in winter but matches Eastern Daylight Time in summer. When Washington is on EDT, Puerto Rico shares the same clock time.

Guam and the Northern Mariana Islands use Chamorro Standard Time, UTC+10. American Samoa uses Samoa Standard Time, UTC−11. The difference between Guam and American Samoa is 21 hours. That gap exceeds the difference between any two states.

The full breakdown of each territory's time zone, including the uninhabited ones, is on the US territories time explained page.

How to Read a US Time Zone Map

A US time zone map shows the six main zones as color-coded bands across the country. The boundaries are not straight. They follow county lines, rivers, and economic regions.

The Eastern zone runs from the Atlantic coast west to the Appalachians, then bulges through Ohio, Indiana, and Michigan. The Central zone covers the Great Lakes to the Gulf of Mexico, including Chicago, Dallas, and Houston. The Mountain zone covers the Rocky Mountain states from Montana to New Mexico, plus Arizona. The Pacific zone covers California, Oregon, Washington, and most of Nevada.

Alaska is a single zone except for the western Aleutian Islands, which use Hawaii-Aleutian Time. Hawaii is on Hawaii-Aleutian Time with no daylight saving. The map also shows the splits: Florida's panhandle, the western edge of Kansas and Nebraska, the Oregon-Idaho border.

Which States Fall Into Each Zone?

Arizona does not observe daylight saving time, except for the Navajo Nation which does. Hawaii does not observe it either. The states that don't observe daylight saving page explains why.