The United States operates six standard time zones, identified in the IANA tz database as America/New_York, America/Chicago, America/Denver, America/Los_Angeles, America/Anchorage, and Pacific/Honolulu. Most states observe DST, shifting clocks forward from the second Sunday in March to the first Sunday in November.
Key facts about time in the United States
- Timezone: Eastern (ET), Central (CT), Mountain (MT), Pacific (PT), Alaska (AKT), Hawaii-Aleutian (HAT)
- UTC offset: UTC-5 to UTC-10 (standard); UTC-4 to UTC-9 (DST, where observed)
- DST: Yes (except Arizona and Hawaii), second Sunday in March to first Sunday in November
- IANA identifier:
America/New_York,America/Chicago,America/Denver,America/Los_Angeles,America/Anchorage,Pacific/Honolulu - Capital: Washington, D.C.
Before November 18, 1883, every city in the United States kept its own local time, set by when the sun reached its highest point. The IANA tz database records this directly: America/Chicago shows an LMT offset of -5:50:36 before switching to the -6:00 standard on that date, meaning Chicago’s local noon did not align with any other city’s noon. A train arriving in Pittsburgh from Philadelphia could cross through many different local times on a single journey.
Railroad companies maintained separate timetables for each city they served. Getting on a train meant reconciling what time the railroad used with what time the town kept. Schedules were unreliable, connections were missed, and the industry bore the cost in delays and accidents.
Then the industry acted.
The Day of Two Noons
William F. Allen, who worked with the General Time Convention, a voluntary association of railroad executives, proposed a solution: divide North America into four time zones, each one hour apart, each running on a single standard time.
On November 18, 1883, railway stations across the United States and Canada adjusted their clocks. The IANA tz database records each transition precisely. Chicago’s LMT was -5:50:36, and the new Central Standard Time was -6:00:00, meaning Chicago’s clocks were set back nine minutes and twenty-four seconds. In cities where noon arrived before the new standard, the clock was turned back to wait for the new noon. Newspapers reported the effect as “the Day of Two Noons.”
This was not a government program. The federal government had no authority over timekeeping at that time. The change was made by a private industry association, adopted voluntarily by the railroads, and then accepted city by city as mayors and merchants recognized that railroad time was the only clock that mattered for commerce.
The United States Congress did not formally adopt the railroad timezone system as law until the Standard Time Act of 1918, thirty-five years later.
The Six Zones
The continental United States uses four main time zones, each one hour apart:
Eastern Time (America/New_York): UTC-5 (EST) in winter, UTC-4 (EDT) in summer. Home to the largest US population concentration. When the NYSE opens at 9:30 AM ET, most of American financial life begins.
Central Time (America/Chicago): UTC-6 (CST) in winter, UTC-5 (CDT) in summer. The middle of the continent. Chicago, where railroad time was first debated, anchors this zone.
Mountain Time (America/Denver): UTC-7 (MST) in winter, UTC-6 (MDT) in summer. The Rockies zone. Some areas, particularly in Arizona, have complicated histories of non-standard or non-DST observation.
Pacific Time (America/Los_Angeles): UTC-8 (PST) in winter, UTC-7 (PDT) in summer. California and the West Coast.
Alaska Time (America/Anchorage): UTC-9 (AKST) in winter, UTC-8 (AKDT) in summer. Alaska is 665,384 square miles according to the US Census Bureau, larger than Texas, California, and Montana combined. At its western extreme, the Aleutian Islands continue almost to the International Date Line.
Hawaii-Aleutian Time (Pacific/Honolulu): UTC-10 (HST) year-round. Hawaii does not observe DST. The western Aleutian Islands use a separate IANA zone, America/Adak, which does observe DST under US rules.
The Arizona Exception
Arizona does not observe daylight saving time. The state repealed DST under Arizona Laws 1968, ch. 183, effective March 21, 1968. The IANA tz database records America/Phoenix on -7:00 MST year-round, with no DST rules applied since that date.
The Navajo Nation, which spans northeastern Arizona into New Mexico and Utah, does observe DST. The US National Institute of Standards and Technology confirms this directly: “the Navajo Indian Reservation in Arizona does observe DST.” The IANA database notes that this applies due to the Navajo Nation’s large size and location across three states.
The Hopi Reservation, which sits entirely within the Navajo Nation in Arizona, is a separate sovereign nation. It follows Arizona state practice and does not observe DST. Driving through northeastern Arizona, a traveler can move between DST (Navajo Nation) and non-DST (Hopi Reservation) and back again within a short distance.
Indiana: A Complicated History
Indiana spent decades in a state of timezone variation by county. The IANA tz database records eight separate Indiana zones, reflecting different local histories across the state. America/Indiana/Indianapolis, the primary zone, shows a sequence of changes: Central Standard Time until the 1950s, then shifts between EST and CST without DST, then permanent EST from 1969, then US Eastern Time with DST starting in 2006.
The 2006 transition is confirmed by the IANA entry: the zone moved from -5:00 - EST to -5:00 US E%sT, meaning Indianapolis began observing US DST rules that year. The debate over that change was substantial, as communities had organized economic and social schedules around their existing time practices.
DST: Legislative History
The United States adopted daylight saving time nationally for the first time with the Standard Time Act of 1918, as a wartime measure. The act was repealed in 1919. DST was reintroduced in 1942 under federal authority. Peacetime DST was left to state and local discretion from 1945 to 1966.
The Uniform Time Act of 1966 standardized DST across the United States, while preserving the right of states to opt out entirely.
The Energy Policy Act of 2005 extended DST. The IANA tz database records the change precisely: spring transition moved from “first Sunday of April” to “second Sunday of March,” and fall transition moved from “last Sunday of October” to “first Sunday of November.” Both changes took effect starting in 2007.
The Sunshine Protection Act, which would make DST permanent year-round, passed the US Senate on March 15, 2022. It did not pass the House and did not become law. The debate about permanent standard time versus permanent summer time continues, with the American Academy of Sleep Medicine and other medical organizations generally advocating for permanent standard time.
The Broadcasting Clock
American network television scheduling has been organized around the Eastern and Pacific timezone division for decades. Prime time runs 8 to 11 PM Eastern, 7 to 10 PM Central. The Eastern timezone sets the national programming clock for a country spanning more than 60 degrees of longitude. State of the Union addresses at 9 PM Eastern begin at 6 PM Pacific, still in the dinner hour.
The six-hour span from Eastern to Hawaii-Aleutian means a single national broadcast event plays across a wide range of local contexts simultaneously.