US Time Zones Explained: A Complete Guide
The US uses nine time zones from Eastern to Chamorro — here's how each one works and which states use it.
Nine zones, not four
Most people can name the four zones of the contiguous US — Eastern, Central, Mountain, and Pacific — but the country actually spans nine when Alaska, Hawaii, and the Pacific and Caribbean territories are included.
Eastern Time (UTC-5) covers the most populous corridor, from Maine to Florida. Central Time (UTC-6) stretches from the Great Lakes to the Gulf Coast. Mountain Time (UTC-7) covers the Rockies. Pacific Time (UTC-8) covers the West Coast.
Beyond those four: Alaska Time (UTC-9) covers nearly all of Alaska; Hawaii-Aleutian Time (UTC-10) covers Hawaii and the far-western Aleutians; Atlantic Time (UTC-4) covers Puerto Rico and the US Virgin Islands; Samoa Time (UTC-11) covers American Samoa; and Chamorro Time (UTC+10) covers Guam and the Northern Mariana Islands.
How the boundaries were drawn
The zone system dates to 1883, when American railroads adopted standard time zones to solve the chaos of thousands of local solar times. Congress made the system federal law in 1918.
The boundaries were never perfectly straight lines of longitude — they follow state and county borders, which is why so many states end up split between two zones today.
Daylight saving adds a second layer
Seven of the nine zones observe daylight saving time, shifting an hour ahead each spring. Hawaii-Aleutian (in Hawaii specifically), Atlantic, Samoa, and Chamorro Time do not — those regions run on a single fixed offset all year.
Split states complicate the map further
Fourteen states have territory in more than one zone, from Florida's Panhandle (Central Time, unlike the rest of the state's Eastern Time) to Alaska's far-western Aleutian Islands (Hawaii-Aleutian Time, unlike the rest of the state's Alaska Time). These splits almost always trace back to which nearby city or railroad hub a border community historically did business with.
Arizona adds a different kind of complication: it's not split across zones, but its refusal to observe daylight saving means it effectively shares Pacific Time with California every summer and Mountain Time with Colorado every winter, despite never leaving the Mountain zone itself.
Understanding these nine zones and their splits is the foundation for almost every other US time-zone question, from figuring out a meeting time to understanding why a state's clocks don't match its neighbor's for part of the year.
Frequently asked questions
How many time zones does the US have?
Nine, including Alaska, Hawaii-Aleutian, Atlantic (Puerto Rico/USVI), Samoa, and Chamorro (Guam/Northern Mariana Islands) alongside the four contiguous zones.
Which US zone is furthest ahead of UTC?
Chamorro Time, at UTC+10, covering Guam and the Northern Mariana Islands.
Which US zone is furthest behind UTC?
Samoa Time, at UTC-11, covering American Samoa.
Which zones don't observe daylight saving?
Hawaii-Aleutian (in Hawaii specifically), Atlantic, Samoa, and Chamorro Time stay on a single fixed offset all year.