Time Zones in the United States: The Nine Zones and How They Began

The United States has nine standard time zones defined in federal law. Six of them cover the 50 states: Eastern, Central, Mountain, Pacific, Alaska and Hawaii-Aleutian. The other three, Atlantic, Samoa and Chamorro, keep time for US territories in the Caribbean and the Pacific.

Knowing which zone a place used matters more than it seems when you work with dates. A birth, a broadcast or a full moon can land on different calendar days depending on the clock that recorded it. The sections below explain the zones, how they started and how to convert between them without slipping a day.

The nine US time zones at a glance

Each zone is defined by its offset from Coordinated Universal Time (UTC), the world reference clock. The names come from 15 U.S.C. 263; the offsets come from the section just before it.

Zone	Standard offset	Daylight saving time?	Main places
Atlantic	UTC−4	No	Puerto Rico, US Virgin Islands
Eastern	UTC−5	Yes (UTC−4)	East Coast, most of Indiana, Michigan and Florida
Central	UTC−6	Yes (UTC−5)	Midwest, most of Texas, Gulf Coast
Mountain	UTC−7	Yes, except most of Arizona	Rocky Mountain states
Pacific	UTC−8	Yes (UTC−7)	California, Washington, most of Oregon and Nevada
Alaska	UTC−9	Yes (UTC−8)	Most of Alaska
Hawaii-Aleutian	UTC−10	Hawaii no, Aleutians yes	Hawaii and the far western Aleutian Islands
Samoa	UTC−11	No	American Samoa
Chamorro	UTC+10	No	Guam, Northern Mariana Islands

The Aleutian line is precise. Federal regulations put the part of the Aleutian chain west of 169 degrees 30 minutes west longitude in the Hawaii-Aleutian zone, so a small slice of Alaska shares a standard offset with Honolulu.

Chamorro time is the odd one out. Guam and the Northern Mariana Islands lie west of the International Date Line, so their clocks run ahead of UTC rather than behind it. Guam is 15 hours ahead of New York in winter, which means its calendar has already turned to the next day for most of the hours when the East Coast is still on the current one.

From railroad time to the Standard Time Act of 1918

Before 1883, most American towns set clocks by local solar noon, and each railroad ran on its own reference time. A traveler changing lines could face several “correct” times in one station. The railroads fixed the problem themselves. On Sunday, November 18, 1883, US and Canadian railroads switched to four standard zones: Eastern, Central, Mountain and Pacific.

Railroad time was a private agreement, not law. Many cities adopted it because trains and telegraph schedules depended on it, but nothing in federal law required it for another 35 years.

That changed with the Standard Time Act, signed on March 19, 1918. It created official time zones for the continental United States and Alaska, gave the Interstate Commerce Commission power to set the boundary lines, and started the first national daylight saving time on March 31, 1918, as a wartime measure. Congress repealed the daylight saving part in August 1919 by overriding President Woodrow Wilson’s veto, though the zones stayed.

Daylight saving time came back nationally during World War II as year-round “War Time,” which ran from 1942 until September 1945. After the war, states and cities chose for themselves, and the result was a patchwork of local clock rules.

The Uniform Time Act of 1966 and who sets the lines

The Uniform Time Act, enacted on April 13, 1966, and effective April 1, 1967, ended most of that patchwork. It required standard time within each zone, set one national schedule for daylight saving time and let a state opt out only if the whole state did so. A 1972 amendment let a state that straddles two zones exempt just the part lying in one zone.

Oversight of time zones moved from the Interstate Commerce Commission to the new Department of Transportation. The Department still handles every boundary change. A county or state files a request, and DOT weighs it against the “convenience of commerce,” a broad test that looks at where residents work, attend school, shop and get medical care, and where airports and rail and bus stations sit. DOT then publishes a proposed rule, takes public comment and holds a hearing. Approved changes usually take effect at a daylight saving changeover so that clocks move only once.

The daylight saving schedule itself has changed several times since 1966:

1. 1967 to 1986: last Sunday in April to last Sunday in October, apart from an emergency trial of year-round daylight saving time that began in January 1974 during the energy crisis.
2. 1987 to 2006: first Sunday in April to last Sunday in October.
3. Since 2007: second Sunday in March to first Sunday in November, a change made by the Energy Policy Act of 2005.

Clocks change at 2:00 a.m. local time. For example, in 2026 daylight saving time began on Sunday, March 8, and ends on Sunday, November 1. The daylight saving time dates page lists the changeover days for every year, and the 2007 calendar shows the first year of the current March start.

Federal law lets a state stay on standard time all year, but it has no option for permanent daylight saving time. Bills called the Sunshine Protection Act would change that. A version passed the Senate in March 2022 but not the House, and the House passed H.R. 139 on July 14, 2026. Check the bill’s current status before treating any change as settled.

Why Arizona and Hawaii skip daylight saving time

Hawaii exempted itself in 1967. At about 21 degrees north latitude, Honolulu’s day length swings far less between June and December than it does on the mainland, so there is little extra evening light to move around.

Arizona tried daylight saving time from April to October 1967 and exempted itself by state law in 1968. The reason was heat. An extra hour of evening sun in a desert summer meant air conditioners ran longer and children played outside in the hottest part of the day. Arizona State University’s account of the decision notes that energy use went up during the trial year rather than down.

The result is a state whose relationship with its neighbors flips twice a year. From March to November, Phoenix shows the same time as Los Angeles. From November to March, it matches Denver.

Arizona has its own exceptions. The Navajo Nation, which spreads across parts of Arizona, Utah and New Mexico, observes daylight saving time so its lands keep one clock. The Hopi Reservation, which is surrounded by the Navajo Nation, does not. In summer, a drive across northeastern Arizona can cross an hour change and back again.

The territories also skip the switch. NIST lists Puerto Rico, the US Virgin Islands, Guam and American Samoa among the places with no daylight saving time, and the Congressional Research Service adds the Northern Mariana Islands.

States split between two time zones

Time zone lines follow county borders and commuting patterns, not straight meridians, so more than a dozen states are split. A few common examples:

• Eastern and Central: Florida’s western panhandle, a group of counties in northwestern and southwestern Indiana, western Kentucky, Middle and West Tennessee, and a few Michigan counties bordering Wisconsin.
• Central and Mountain: El Paso and far western Texas, western Kansas and Nebraska, and the western parts of North and South Dakota.
• Mountain and Pacific: northern Idaho, most of Malheur County in Oregon, and West Wendover in Nevada.

Indiana is the classic research trap. The state did not observe daylight saving time statewide until April 2, 2006, so older records from Indiana can be an hour off from what a modern converter assumes. When a historical time matters, check the rule in force in that county in that year.

Converting times across US time zones: two worked examples

The safest method is to go through UTC. Convert the source time to UTC, then apply each destination’s offset for that date, remembering which places observe daylight saving time.

Example 1: 8:00 p.m. Eastern on July 4, 2026. Eastern Daylight Time is UTC−4, so the moment is 00:00 UTC on July 5.

Example 2: 8:00 p.m. Eastern on January 15, 2027. Eastern Standard Time is UTC−5, so the moment is 01:00 UTC on January 16.

Place	July 4, 2026	January 15, 2027
San Juan, Puerto Rico	8:00 p.m.	9:00 p.m.
New York	8:00 p.m.	8:00 p.m.
Chicago	7:00 p.m.	7:00 p.m.
Denver	6:00 p.m.	6:00 p.m.
Phoenix	5:00 p.m.	6:00 p.m.
Los Angeles	5:00 p.m.	5:00 p.m.
Anchorage	4:00 p.m.	4:00 p.m.
Honolulu	2:00 p.m.	3:00 p.m.
Pago Pago, American Samoa	1:00 p.m.	2:00 p.m.
Hagåtña, Guam	10:00 a.m., July 5	11:00 a.m., January 16

Every row that changes between the two columns is a place without daylight saving time: Puerto Rico, Arizona, Hawaii, American Samoa and Guam. Their clocks stand still while the mainland moves, so the gap to New York shrinks or grows by an hour.

How a time zone can change the date on a calendar

Old Calendars reports moon phases, equinoxes and solstices in US Eastern Time. That choice is convenient for most readers, but it means the date can differ by one day farther west.

Take the full moon of October 2026. NASA’s phase table puts it at 04:12 UTC on October 26. In New York that is 12:12 a.m. on Monday, October 26. In Los Angeles it is 9:12 p.m. on Sunday, October 25, and in Honolulu it is 6:12 p.m. on October 25. All three clocks describe the same instant, but only the East Coast calls it October 26. The moon phases guide and the equinoxes and solstices guide explain how those event times are calculated.

The daylight saving changeovers create two more traps for anyone reading old records:

• The missing hour. On the second Sunday in March, clocks jump from 2:00 a.m. to 3:00 a.m., so a time such as 2:30 a.m. does not exist that night in places that observe daylight saving time.
• The repeated hour. On the first Sunday in November, clocks fall back from 2:00 a.m. to 1:00 a.m., so 1:30 a.m. happens twice. A hospital record or police log from that night needs a note saying which 1:30 it means.

When you research a specific day, such as a birthday in the 2006 calendar or anything in our guide to what happened on the day you were born, note the place, the local time and whether daylight saving time was in effect there. Those three details settle almost every date mismatch.