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US Time Zones Explained

The continental US alone spans four time zones, and the full country β€” including Alaska, Hawaii, and its territories β€” spans considerably more.

The 50 states use six official time zones

From east to west: Eastern, Central, Mountain, Pacific, Alaska, and Hawaii-Aleutian. The contiguous 48 states use only the first four of these.

Each mainland zone is one hour behind the one to its east

Crossing from Eastern into Central time moves clocks back an hour, and so on moving west through Mountain and Pacific β€” which is why a flight across the country can seem to arrive close to when it left.

Most of Arizona does not observe daylight saving time

Since 1968, most of Arizona has stayed on Mountain Standard Time year-round rather than shifting to daylight saving time, a policy adopted partly to avoid extending hot summer evenings.

Hawaii also stays on standard time year-round

Hawaii-Aleutian Standard Time does not shift for daylight saving, so Hawaii's time difference from the mainland actually changes between winter and summer, depending on whether the mainland zone is currently observing daylight saving time.

US territories add three more time zones to the total

Puerto Rico and the US Virgin Islands use Atlantic time, Guam and the Northern Mariana Islands use Chamorro time, and American Samoa uses Samoa time β€” bringing the full US total to nine time zones when territories are included.

Zone boundaries generally follow state or county lines, not straight geography

Rather than following strict longitude lines, US time zone borders are often drawn along state or county boundaries so that a single state or metro area is not needlessly split, though some states do straddle two zones.

Why Arizona and Hawaii opted out of daylight saving

States and territories are allowed under federal law to opt out of observing daylight saving time (though not to adopt permanent daylight saving on their own). Arizona and Hawaii cite reduced benefit from an extra hour of evening daylight in a hot climate, since it can increase air conditioning use in the evening rather than saving energy.

Time zones are policy decisions, not just geography

Time zone boundaries and daylight saving observance are set by federal and state law and can change β€” Congress has periodically proposed making daylight saving time permanent nationwide, though no such change had been enacted as of this writing.

Frequently Asked Questions

How many time zones does the US observe in total?

Six across the 50 states (Eastern, Central, Mountain, Pacific, Alaska, and Hawaii-Aleutian), plus three more used only in US territories, for nine total.

Why not just use one time zone for the whole country?

A single national time zone would put sunrise and sunset at wildly different clock times on the east and west coasts, since the country spans multiple hours of actual solar time from coast to coast.

Does all of the Navajo Nation follow a different rule than the rest of Arizona?

Yes β€” the Navajo Nation observes daylight saving time even though it is surrounded by Arizona, which generally does not, while the neighboring Hopi Reservation (surrounded by the Navajo Nation) does not observe it either.

Is daylight saving time going to become permanent nationwide?

It has been proposed in Congress more than once, but as of this writing no such change has been enacted into law, so the twice-yearly clock change continues in states that observe daylight saving time.