America/Phoenix

TZ identifier and aliases

America/Phoenix is the canonical identifier for Arizona's fixed Mountain Standard Time. US/Arizona and the bare POSIX name MST both still resolve to the same rules but are deprecated — use America/Phoenix in new code. America/Creston, a small town in British Columbia that also skips daylight saving, currently shares identical rules and is folded into the same entry by most tz tooling.

UTC offset and DST rule

America/Phoenix is -07:00 year-round with no daylight saving time — Arizona is one of only two US states that opt out (Hawaii is the other). That means its relationship to neighboring zones changes twice a year: it matches Pacific Daylight Time (-07:00) for the roughly eight months DST is in effect elsewhere, and matches Mountain Standard Time the rest of the year, since it's simply staying still while everyone else moves.

Using it in cron, CI and Postgres

Set TZ=America/Phoenix for a process that should schedule by fixed Arizona local time — the main advantage over a plain -07:00 offset in config is that America/Phoenix self-documents intent and stays correct if the underlying tz database is ever updated, where a hardcoded offset silently wouldn't. Postgres accepts it via SET TIME ZONE 'America/Phoenix'; or the timezone setting. Because this zone never changes offset, it's one of the few US zones where a cron job scheduled for any hour, including 2:00–3:00am, behaves identically every day of the year — no skipped or duplicated local times to worry about, which is one real operational advantage of a no-DST zone for anything scheduling-sensitive.

History and quirks

Arizona adopted daylight saving briefly during the WWII-era nationwide "War Time" (1942–1945) and again for one year in 1967 under the original Uniform Time Act, before its legislature exercised the Act's opt-out provision and the state has observed permanent standard time ever since — a decision often attributed to the desert climate, where an extra hour of evening daylight in summer means an extra hour of punishing heat rather than a welcome one. The Navajo Nation, whose territory extends into northeastern Arizona, breaks from the rest of the state and observes DST on the America/Denver schedule instead, while the Hopi Reservation — entirely enclosed within Navajo Nation land — reverts to Phoenix's no-DST rule, producing a small three-way patchwork of local time rules within one corner of the state that has confused travelers and schedulers for decades.

Offsets and DST transitions on this page are computed from the IANA time zone database bundled with your browser; the rules described above were verified against tzdata 2026b when this page was published.