There is no reason given but there are a few that dont observe it. Interestingly the Indian reservations in Arizona DO observe it.
Daylight saving time in the United States
An American Indian reservation is an area of land managed by a Native American tribe under the United States Department of the Interior's Bureau of Indian Affairs. There are about 310 Indian reservations in the United States, meaning not all of the country's 550-plus recognized tribes have a reservation—some tribes have more than one reservation, some share reservations, while others have none. In addition, because of past land allotments, leading to some sales to non-Indians, some reservations are severely fragmented, with each piece of tribal, individual, and privately held land being a separate enclave. This jumble of private and public real estate creates significant administrative, political, and legal difficulties.
The collective geographical area of all reservations is 55,700,000 acres (22,500,000 ha), representing 2.3% of the area of the United States 2,379,400,204 acres (962,909,100 ha). Twelve Indian reservations are larger than the state of Rhode Island which covers 776,960 acres (314,425 ha) and nine reservations larger than Delaware's 1,316,480 acres (532,760 ha). The territory of the Navajo Nation compares in size to West Virginia. Reservations are unevenly distributed throughout the country; the majority are west of the Mississippi River and occupy lands that were first reserved by treaty or 'granted' from the public domain.
Daylight saving time
Daylight saving time in the United States is the practice of setting the clock forward by one hour during the warmer part of the year, so that evenings have more daylight and mornings have less. Most areas of the United States currently observe daylight saving time (DST), the exceptions being Arizona (except for the Navajo Nation, which does observe daylight saving time), Hawaii, and the overseas territories of Puerto Rico, American Samoa, Guam, Northern Mariana Islands, and the United States Virgin Islands.
Since 2007, daylight saving time starts on the second Sunday of March and ends on the first Sunday of November, with all time changes taking place at 2:00 A.M. local time.
Daylight saving time (DST)—also summer time in British English— is the practice of advancing clocks during the lighter months so that evenings have more apparent daylight and mornings have less. Typically clocks are adjusted forward one hour near the start of spring and are adjusted backward in autumn.
The modern idea of daylight saving was first proposed in 1895 by George Vernon Hudson and it was first implemented by Germany and Austria-Hungary starting on 30 April 1916. Many countries have used it at various times since then. Much of the United States used DST in the 1950s and 1960s, and DST use expanded following the 1970s energy crisis. It has been widely used in North America and Europe since then.
Geography of the United States
A time zone is a region on Earth that has a uniform standard time for legal, commercial, and social purposes. It is convenient for areas in close commercial or other communication to keep the same time, so time zones tend to follow the boundaries of countries and their subdivisions.
Most of the time zones on land are offset from Coordinated Universal Time (UTC) by a whole number of hours (UTC−12 to UTC+14), but a few are offset by 30 or 45 minutes (for example: Nepal Standard Time, NPT). Some higher latitude countries use daylight saving time for part of the year, typically by changing clocks by an hour. Many land time zones are skewed toward the west of the corresponding nautical time zones. This also creates a permanent daylight saving time effect.
The United States is a country in the Northern Hemisphere, Western Hemisphere, and the Eastern Hemisphere. It consists of forty-eight contiguous states in North America, Alaska, a peninsula which forms the northwestern most part of North America, and Hawaii, an archipelago in the Pacific Ocean. There are several United States territories in the Pacific and Caribbean. The term "United States", when used in the geographical sense, means the continental United States, Alaska, Hawaii, Puerto Rico, Guam, and the Virgin Islands of the United States. The country shares land borders with Canada and Mexico and maritime (water) borders with Russia, Cuba, and the Bahamas in addition to Canada and Mexico.
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The Hopi Reservation, or simply Hopi, is a Native American reservation for the Hopi and Arizona Tewa people, surrounded entirely by the Navajo Nation, in Navajo and Coconino counties of Arizona, USA. The site in north-eastern Arizona has a land area of 2,531.773 sq mi (6,557.262 km²) and as of the 2000 census had a population of 6,946. The Hopi Reservation, like most of Arizona but unlike the surrounding Navajo Nation, does not observe Daylight Saving Time. Until recently, the two nations shared the Navajo-Hopi Joint Use Area. The partition of this area, commonly known as Big Mountain, by Acts of Congress in 1974 and 1996, has resulted in continuing controversy.
The system of villages is based around three mesas in the pueblo style, which has been traditionally used by the Hopi. Walpi is the oldest village on First Mesa, having been established in 1690 after the villages at the foot of mesa Koechaptevela were abandoned for fear of Spanish reprisal post 1680 Pueblo Revolt. The Tewa people live on First Mesa. Hopi also occupy the Second Mesa and Third Mesa. The community of Winslow West is off-reservation trust land of the Hopi tribe.