America/Denver

TZ identifier and aliases

America/Denver is the canonical identifier for US Mountain Time (the DST-observing variant — see America/Phoenix for the state that opts out). US/Mountain and the POSIX-style MST7MDT are older names still pointing at the same rules, along with America/Boise and the Navajo Nation's America/Shiprock/Navajo link, both of which currently share identical rules with Denver. All are deprecated in favor of America/Denver.

UTC offset and DST rule

Mountain Time is -07:00 (MST) on standard time and -06:00 (MDT) during daylight saving, following the same nationwide US rule as the other DST-observing zones: the second Sunday in March to the first Sunday in November (March 8 to November 1 in 2026). Outside that window it sits seven hours behind UTC — one hour ahead of Los Angeles and one hour behind Chicago year-round, a fixed relationship even though all three shift together each DST transition.

Using it in cron, CI and Postgres

Set TZ=America/Denver for a process that should schedule by Mountain local time. Postgres accepts it directly — SET TIME ZONE 'America/Denver'; or the timezone configuration parameter — and applies the DST rule correctly for any date rather than a fixed offset. Schedulers that support a CRON_TZ=America/Denver line (Vixie-cron and similar) schedule by local time directly; GitHub Actions' schedule: trigger always runs in UTC and has no per-workflow timezone option, so a job meant to fire at a fixed Mountain hour needs its UTC equivalent recalculated across each DST boundary. Avoid the 2:00–3:00am local window for anything that must run exactly once: it's skipped entirely on the March transition and repeats on the November one.

History and quirks

Mountain Time was one of the four original US railroad time zones from 1883, given federal standing by the Standard Time Act of 1918. Its most notable internal quirk sits entirely within Arizona: the Navajo Nation, whose territory extends into Arizona, observes DST on the same schedule as America/Denver even though the rest of Arizona (America/Phoenix) does not — meaning the two zones' offsets from UTC match for eight months of the year and diverge by an hour for the other four, so a Navajo Nation community and a non-reservation Arizona town a few miles away can show different local times depending on the date. The Hopi Reservation, entirely surrounded by the Navajo Nation, follows Phoenix's no-DST rule instead — producing a small but real patchwork of three different rules within one US state.

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.