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The Uniform Time Act (Pub.L. 89-387, April 13, 1966, 80 Stat. 107, 15 U.S.C. § 260a) is a 1966 United States federal law whose effect was to simplify the official pattern of where and when Daylight saving time (DST) is applied within the U.S. Previous to this law, each state worked out its own scheme for the dates of beginning and ending DST, and in some cases, which parts of the state should use it.

The law as originally written required states that observe DST to begin it at 02:00 local time on the last Sunday in April and to end it at 02:00 local time on the last Sunday in October. The law was later amended in 1986 to move the uniform start date for DST to the first Sunday in April (effective 1987). The latest amendment, part of the Energy Policy Act of 2005, will extend DST by four weeks by moving the uniform start date for DST to the second Sunday in March and the end date to the first Sunday in November (effective 2007). However, this extension may not be permanent because the Department of Energy is required to study the impact of the DST extension no later than nine months after it takes effect.

The law does not require that all states observe DST. Individual states may exempt themselves from DST and observe standard time year-round by passing a state law, provided:

• if the state lies entirely within a time zone, that the exemption apply statewide, or
• if the state is divided by a time zone boundary, that the exemption apply statewide or to the entire part of the state on one side of the boundary.

While there are a few areas where de facto differences within states exist, the overall pattern is simple enough, and close enough to actual practice, that it is effective and efficient

• for repeat travelers, and repeat users of long-distance telephone service, to learn the basic pattern, and
• for those in areas where de facto exceptions apply to mention the fact to those likely to be affected by it.

The most notable de facto exception is that Arizona has chosen not to use DST statewide. However, the various Indian nations within the state have the right to use DST, or not use it, as they see fit. The Navajo Nation has chosen to use DST throughout its territory, which includes parts of Arizona, New Mexico, and Utah. On the other hand, the Hopi Nation, whose territory is surrounded by that of the Navajo Nation, has chosen not to use DST.

• 15 U.S.C. § 260a – Text of the Uniform Time Act