Which IANA zone keeps Denver, El Paso and Albuquerque on the same clock?
America/Denver is the canonical identifier. It covers Denver (population 729,019), El Paso (678,815), Albuquerque (564,559), Colorado Springs (456,568) and Aurora (359,407). The zone observes daylight saving: clocks spring forward on the second Sunday in March and fall back on the first Sunday in November.
America/Denver is the representative zone in the Common Locale Data Repository for the America_Mountain metazone. Any system that needs one zone to stand for all of Mountain Time defaults to it.
Which other IANA zones belong to the Mountain Time metazone?
Mountain Time is not uniform. The IANA database lists nine zones that share the metazone but differ in DST observance or historical rules. Use the one that matches your location.
| IANA zone | Key cities | DST? | Notes |
|---|---|---|---|
| America/Denver | Denver, El Paso, Albuquerque, Colorado Springs, Aurora | Yes | The default for US Mountain Time |
| America/Boise | Boise, Idaho | Yes | Covers most of Idaho's mountain region |
| America/Cambridge_Bay | Cambridge Bay, Nunavut | Yes | Canadian Arctic |
| America/Ciudad_Juarez | Ciudad Juárez, Mexico | Yes | Border city matching El Paso; Mexico abolished DST nationally in 2023 but Juárez kept it to stay aligned |
| America/Dawson_Creek | Dawson Creek, British Columbia | No | Stays on MST (UTC-7) year-round |
| America/Edmonton | Calgary, Edmonton | Yes | Covers Alberta and parts of Saskatchewan |
| America/Fort_Nelson | Fort Nelson, British Columbia | No | Stays on MST (UTC-7) year-round |
| America/Inuvik | Inuvik, Northwest Territories | Yes | Canadian Arctic |
| America/Phoenix | Phoenix, Tucson, Mesa, Chandler, Gilbert, Glendale, Scottsdale | No | Arizona (except Navajo Nation) stays on MST year-round |
The most common mistake is using America/Denver for Phoenix. Phoenix is on America/Phoenix, which never switches to MDT. In July, Denver is on MDT (UTC-6) while Phoenix is on MST (UTC-7). They are one hour apart.
Does the Mountain Time IANA zone cover Phoenix correctly?
No. Phoenix needs its own zone: America/Phoenix. Arizona exempted itself from DST in 1968. The reasoning: extra evening daylight in summer increases air conditioning use, which costs more than the energy saved. So Arizona stays on MST (UTC-7) year-round.
But the Navajo Nation, which extends into Arizona, does observe DST. The Hopi Reservation, surrounded by Navajo land, does not. This creates a DST "doughnut" inside Arizona that no single IANA zone can fully describe. Handling the Hopi-Navajo boundary programmatically requires custom logic.
What about Canada and Mexico?
Canada's Mountain Time zones follow US DST rules, with two exceptions: America/Dawson_Creek and America/Fort_Nelson stay on MST year-round. Mexico's Ciudad Juárez (population 1,512,450) kept DST after the national abolition to stay synchronized with El Paso.
Do not hardcode America/Denver for all of North America. Use a lookup table that maps cities or coordinates to the correct IANA zone.
When does the Mountain Time IANA offset switch between MST and MDT?
Mountain Standard Time (MST) is UTC-7. Mountain Daylight Time (MDT) is UTC-6. "Mountain Time" in a schedule means MST in winter and MDT in summer unless specified otherwise.
On 15 January 2026, Denver is on MST (UTC-7). On 15 July 2026, Denver is on MDT (UTC-6). The offset changes, but the IANA zone stays America/Denver.
Quick reference: IANA zones for Mountain Time
| Scenario | Use this zone |
|---|---|
| Most of US Mountain Time (CO, NM, UT, WY, MT) | America/Denver |
| Arizona (except Navajo Nation) | America/Phoenix |
| Alberta, Canada | America/Edmonton |
| Ciudad Juárez, Mexico | America/Ciudad_Juarez |
| Dawson Creek or Fort Nelson, BC (no DST) | America/Dawson_Creek or America/Fort_Nelson |
| Boise, Idaho | America/Boise |
| Cambridge Bay or Inuvik, Arctic Canada | America/Cambridge_Bay or America/Inuvik |