84% of Arizona is always on Mountain Standard Time (MST/UTC-7). The Navajo Nation, which is in parts of four states including 16% of Arizona, observes daylight saving time (DST) from the second Sunday of March until the first Sunday of November. During that part of the year, when it's 9 AM MST in Phoenix, it's 10 AM MDT (UTC-6) in the Navajo Nation. During the rest of the year, it's MST across the whole state.