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By Law, the United States of America and its territories use nine time zones, with most of the county observing DST (daylight saving time) for much of spring, fall, and summer months. From west to east, the nine US time zones are Chamorro Standard Time (UTC+10), Samoa Standard Time, Hawaii-Aleutian Standard Time (HST), Alaskan Standard Time (AKST), Pacific Standard Time (PST), Mountain Standard Time, Central Standard Time, Eastern Standard Time (EST), and Atlantic Standard Time (AST). The contiguous US observes four time zones, while Hawaii, Alaska, and each of the five territories has its time zone. The Department of Transportation regulates the DST and time zone boundaries in the US. 

Contiguous Time Zones

The contiguous US comprises 48 states located within continental North America. These states fall within four time zones. All or most of the 23 states in the eastern US, including those on the Atlantic coast, observe Eastern Time Zone (UTC-04:00). However, Indiana, Florida, Kentucky, Tennessee partially use this time zone. The second time zone on the contiguous US is the Central Standard Time (UTC-05:00), observed fully by ten states and partly by the other ten states occupying the Mississippi Valley, Gulf Coast, and parts of the Great Plains. 

Washington, Nevada, California, and parts of Oregon and Idaho observe Pacific Time Zone (UTC-07:00) while the states straddled by the Rocky Mountains and Great Plains’ western quarter have Mountain Time Zone (UTC-06:00). The DST is observed in all of the contiguous states except Arizona (observed only in the Navajo Nation). The time zones observed by the 48 states use generic names, without highlighting the difference between DST and standard time designations. For instance, Eastern Time refers to EST or EDT.

Time Zones Outside The Contiguous US

The state of Alaska observes Alaskan Time Zone (UTC-09:00), while Hawaii and parts of the Aleutian Islands have Hawaii-Aleutian Time Zone (UTC-09:00). However, there is no DST in Hawaii. The US territories observe Samoa Time Zone (American Samoa), Atlantic Time Zone (US Virginia Islands and Puerto Rico), and Chamorro Time Zone (the Northern Mariana Islands and Guam).

The US also possesses outlying islands which do not observe any of the nine time zones. For instance, Howland Island and Baker Island use UTC-12:00 and are sometimes assigned Anywhere on Earth (AoE).

Daylight Saving Time

The Daylight Saving Time (DST) is observed in the whole of US states and territories except in Hawaii, Puerto Rico, American Samoa, US Virgin Island, and Arizona (observed only in Navajo Nation). The DST runs from 2 pm on the 2nd Sunday in March to 2 am on the first Sunday in November.