Arizona daylight saving time

Arizona is in the mountain zone under 49 CFR Part 71. The state exempts itself from the federal daylight-saving rule, so Phoenix and most of the state stay on Mountain Standard Time all year. In summer that lines up with Pacific Daylight Time on the clock, which is why a noon meeting in Los Angeles and Phoenix can share an hour from March to November and not in January.

The Navajo Nation observes daylight saving time. It covers parts of Apache, Navajo, and Coconino counties, so those counties are the ones a point-in-polygon sample splits between America/Phoenix and America/Denver. The Hopi reservation is surrounded by the Navajo Nation and does not observe daylight saving time, so America/Phoenix reappears inside America/Denver.

This is not a second legal standard-time zone. It is a daylight-saving exception inside one mountain zone. County pages for those three counties list both identifiers and the share of the sample that fell in each.