When do clocks spring forward and fall back in the Mountain zone?
The second Sunday of March at 2:00 AM local time triggers the switch to daylight time. Clocks jump forward one hour to 3:00 AM. Standard time returns on the first Sunday of November at 2:00 AM local time. Clocks fall back one hour to 1:00 AM.
These dates follow the US Energy Policy Act of 2005, effective from 2007. Canada and the northern border region of Mexico keep the same schedule.
Upcoming dates:
| Event | Date | Local time change |
|---|---|---|
| DST begins (spring forward) | Sunday, 8 March 2026 | 02:00 → 03:00 |
| DST ends (fall back) | Sunday, 1 November 2026 | 02:00 → 01:00 |
| DST begins (spring forward) | Sunday, 14 March 2027 | 02:00 → 03:00 |
| DST ends (fall back) | Sunday, 7 November 2027 | 02:00 → 01:00 |
What happens at 2:00 AM on changeover day?
At 2:00 AM the clock jumps. In March, 2:00 AM becomes 3:00 AM instantly. The hour between 2:00 and 3:00 does not exist. In November, 2:00 AM becomes 1:00 AM, and the hour from 1:00 to 2:00 AM repeats.
For developers and system administrators: the IANA zone America/Denver captures these rules automatically. Use the zone identifier, not the abbreviation MST or MDT. The abbreviation changes with the season.
Why Phoenix and most of Arizona skip daylight saving
Arizona stays on standard time (UTC-7) all year. The state legislature codified this in 1968 (Arizona Revised Statutes §1-242). Summer heat makes extra evening daylight a liability: air conditioning demand rises, not falls.
The exception within the exception: the Navajo Nation, which extends into the state, does observe DST. The Hopi Reservation, surrounded entirely by the Navajo Nation, does not. A driver crossing 20 miles of reservation land can pass through three timekeeping zones in thirty minutes.
Other places that skip DST inside the zone: - Dawson Creek and Fort Nelson, British Columbia (stay on MST year-round) - Most of Mexico after the national DST abolition in 2022, except northern border municipalities like Ciudad Juárez which keep DST to align with El Paso
What UTC offset applies during MST daylight saving time?
| Period | Abbreviation | UTC offset | Example cities |
|---|---|---|---|
| Winter (standard) | MST | UTC-7 | Denver, Phoenix, Calgary |
| Summer (daylight) | MDT | UTC-6 | Denver, Calgary, Ciudad Juárez |
When it is 12:00 UTC: - During MST (winter): 5:00 AM in Denver, 5:00 AM in Phoenix - During MDT (summer): 6:00 AM in Denver, 5:00 AM in Phoenix (Arizona stays on MST)
This is the source of endless scheduling confusion. In July, a 2:00 PM call with someone in Denver (MDT, UTC-6) and someone in Phoenix (MST, UTC-7) means the Denver participant joins at 2:00 PM and the Phoenix participant at 1:00 PM, even though both are "Mountain Time."
MST daylight saving time: what to do and what to skip
| Situation | Correct action |
|---|---|
| Scheduling a meeting for July with people in Denver and Phoenix | Say "2:00 PM Denver time / 1:00 PM Phoenix time". Do not say "Mountain Time". |
| Writing software that handles timestamps | Use IANA zone America/Denver for most of the zone, America/Phoenix for Arizona. |
| Planning travel to the Navajo Nation in April | The Nation is on MDT (UTC-6) even though the surrounding Arizona land is on MST (UTC-7). |
| Checking a time from a Canadian source | Alberta (America/Edmonton) follows US DST rules. Dawson Creek does not. |
| Converting "MST" in a data feed during July | The abbreviation is almost certainly wrong. The correct summer abbreviation for Denver is MDT. |