On 1 November 2026, Denver clocks fall back an hour. On 8 March 2026, they jump forward. The two settings have different names, different offsets, and different consequences for anyone scheduling a call with Phoenix. Getting them mixed up costs an hour.

MST is Mountain Standard Time, fixed at UTC-7. It runs from the first Sunday of November to the second Sunday of March.

MDT is Mountain Daylight Time, fixed at UTC-6. It runs from the second Sunday of March to the first Sunday of November.

Together they form Mountain Time (MT). That is the umbrella name. "Let's meet at 2 PM Mountain Time" is correct. "2 PM MST" in July is wrong, and if your calendar app trusts that abbreviation, your meeting is off by an hour.

The UTC offset difference: -7 vs -6

One hour. That is the whole difference.

Abbreviation Offset from UTC When it is used
MST UTC-7 Standard time (winter)
MDT UTC-6 Daylight time (summer)

Denver in January sits 2 hours behind New York. Denver in July also sits 2 hours behind New York. Both zones shift together, so the gap holds.

The trouble starts when one place shifts and the other does not.

Compare Denver and Phoenix. Phoenix stays on MST (UTC-7) year-round. In January, Denver and Phoenix show the same clock: both are on MST. In July, Denver shows MDT (UTC-6). Phoenix still shows MST (UTC-7). Phoenix is now one hour behind Denver, even though both cities sit inside the Mountain zone on paper. This is the single most common scheduling error in the zone.

When does MST switch to MDT?

The United States and Canada change clocks on the same dates. Mexico's border municipalities that use Mountain Time follow these dates too.

Book these dates. The switch happens at 2:00 AM local time.

  • Sunday, 8 March 2026: clocks spring forward to 3:00 AM. MDT begins.
  • Sunday, 1 November 2026: clocks fall back to 1:00 AM. MST begins.
  • Sunday, 14 March 2027: clocks spring forward to 3:00 AM. MDT begins.
  • Sunday, 7 November 2027: clocks fall back to 1:00 AM. MST begins.

Every city in the Mountain zone that observes DST runs on MDT (UTC-6) from March to November. From November to March, they run on MST (UTC-7).

Why mixing up MST and MDT breaks your schedule

Book a meeting for "2 PM MST" in July and you have actually booked it for 3 PM MDT. An hour gone. If your meeting tool interprets "MST" as UTC-7, it shows the wrong time to every participant.

Three situations where this bites hardest:

Remote teams split between Arizona and Colorado. Phoenix (always MST) and Denver (MST in winter, MDT in summer) share a clock for half the year and sit an hour apart for the other half. Before you send the invite, check whether you are in the November to March window or the March to November window.

Software that labels everything "MST." Many systems stamp "MST" on timestamps year-round, even during daylight months. That is technically wrong but common. If your code reads a timestamp marked "MST" in August, do not trust the abbreviation. Use the IANA zone identifier America/Denver and ISO 8601 offsets like -06:00 instead.

Flight bookings. A Denver departure at 10:00 AM in January is 10:00 AM MST. The same departure in July is 10:00 AM MDT. Same local time, different UTC offset. If your connection relies on a UTC-based global schedule, the distinction matters.

Stop saying MST when you mean MT

This is the most widespread error in the zone. People use "Mountain Standard Time" as a catch-all. They do not mean standard time. They just do not know the correct term.

Mountain Time (MT) is the correct generic label. It covers both MST and MDT.

Say "I live in MST" and you are claiming you live on UTC-7 year-round. True for Phoenix. True for Tucson, Mesa, Chandler, Gilbert, Glendale, and Scottsdale. False for Denver, Salt Lake City, Calgary, Edmonton, or Ciudad Juárez. Those places switch to MDT for eight months of the year.

Rule of thumb: Use "Mountain Time" for general conversation. Use "MST" or "MDT" only when you specifically mean standard time or daylight time. If you are unsure which one is active, say "Mountain Time" and let the other person check.

Is Mountain Time on MST or MDT right now?

Check the current UTC offset for Denver. If it is UTC-6, you are in MDT. If it is UTC-7, you are in MST.

Or check the date:

  • Between the second Sunday of March and the first Sunday of November: MDT
  • Between the first Sunday of November and the second Sunday of March: MST

For Arizona, the answer never changes. Phoenix is always MST. That makes Arizona time identical to Pacific Daylight Time in summer (both are UTC-7), but the name is still MST, not PDT.

MST vs MDT at a glance

MST MDT
Full name Mountain Standard Time Mountain Daylight Time
UTC offset UTC-7 UTC-6
In effect November to March March to November
When to use the abbreviation Only when you mean standard time Only when you mean daylight time
Umbrella term Mountain Time (MT) Mountain Time (MT)
Arizona Uses MST year-round Never uses MDT