Arizona and Hawaii are the only U.S. states that do not observe Daylight Saving Time.

The Uniform Time Act of 1966 established a national framework for Daylight Saving Time across the United States. While most states adopted the practice, the law permitted states to opt out by passing local ordinances. Hawaii officially opted out in 1967 because its proximity to the equator means daylight hours vary little between summer and winter, making the time shift impractical.Arizona followed suit in 1968 after a brief and unpopular experiment with the time change. The state's decision was driven by climate necessity. If Arizona observed Daylight Saving Time, the sun would set as late as 9:00 PM during summer, forcing residents to endure extreme heat exceeding 38°C (100°F) for an additional hour. This extended period of high temperatures significantly increases air conditioning costs and strains energy resources.By maintaining Mountain Standard Time year-round, Arizona ensures earlier sunsets, allowing the ground to cool sooner and reducing energy consumption. It's worth noting that the Navajo Nation, which spans parts of Arizona, New Mexico, and Utah, does observe Daylight Saving Time to maintain consistency across its tribal lands. The Hopi Reservation, entirely surrounded by the Navajo Nation, does not observe it and instead follows Arizona's standard time, creating a unique geographic anomaly known as a "time zone donut" in northeastern Arizona.For most of the United States, Daylight Saving Time remains debated, but for Hawaii and Arizona, the decision stems from geographical and climatological necessity. These exemptions are codified under Title 15 of the United States Code, guaranteeing that these states remain on permanent standard time.