AST to MST
-3hAST
ASTSOURCEMST
MSTGreen rows indicate typical business hours (9 AM - 5 PM)
AST and MST on the World Map
The map below highlights the AST (Atlantic Standard Time) and MST (Mountain Standard Time) timezone regions across the globe.
AST – Atlantic Standard Time
UTC-04:00Atlantic Standard Time (AST) is the time zone for the Canadian Atlantic provinces and the Caribbean, 4 hours behind UTC.
- •Puerto Rico uses AST year-round and does not observe DST.
- •AST is 1 hour ahead of EST.
- •Halifax, the capital of Nova Scotia, is the largest city in AST Canada.
MST – Mountain Standard Time
UTC-07:00Mountain Standard Time (MST) is the time zone for the Mountain region of North America, 7 hours behind UTC.
- •Arizona does not observe daylight saving time, staying on MST year-round.
- •The Navajo Nation in Arizona does observe DST, unlike the rest of the state.
- •Denver, the 'Mile High City', is the largest city fully in MST.
- •MST covers 6 US states entirely and parts of several others.
Understanding the -3h Gap: AST vs MST
AST runs on Atlantic Standard Time (AST) at UTC-04:00. MST runs on Mountain Standard Time (MST) at UTC-07:00. That puts AST 3 hours behind MST — a difference of -3h at the current offsets.
In practice, when it is 9:00 AM in AST it is 6:00 AM in MST. Midnight in AST is 9:00 PM in MST. Neither zone observes daylight saving time, so the offset between them stays constant all year.
AST to MST time conversion table
| AST (AST) | MST (MST) | Day |
|---|---|---|
| 12:00 AM | 9:00 PM | previous day |
| 6:00 AM | 3:00 AM | same day |
| 7:00 AM | 4:00 AM | same day |
| 8:00 AM | 5:00 AM | same day |
| 9:00 AM | 6:00 AM | same day |
| 12:00 PM | 9:00 AM | same day |
| 3:00 PM | 12:00 PM | same day |
| 5:00 PM | 2:00 PM | same day |
| 6:00 PM | 3:00 PM | same day |
| 9:00 PM | 6:00 PM | same day |
Why people convert AST to MST time
- Scheduling a call from AST to MST: a 10:00 AM start in AST lands at 7:00 AM in MST, so you can confirm the slot without doing the arithmetic yourself.
- Finding shared working hours: both AST and MST are inside the 9-to-5 working day between 12:00 PM and 5:00 PM AST time (9:00 AM–2:00 PM MST). That is roughly 5 hours of usable overlap.
- Keeping family and friends in sync: knowing that 8:00 PM in AST is 5:00 PM in MST makes it easy to avoid calling at 3 AM.
- Always state the zone abbreviation, not just the hour. "3:00 PM AST" is unambiguous; "3 PM" across a -3h gap is not.
- Block 12:00 PM–5:00 PM AST as the recurring meeting window if you work primarily from AST and need MST colleagues present.
About AST (Atlantic Standard Time)
Atlantic Standard Time (AST) covers Canada's Atlantic provinces (Nova Scotia, New Brunswick, Prince Edward Island, and most of Labrador), as well as several Caribbean nations. Puerto Rico and the US Virgin Islands use AST year-round without daylight saving time. The Canadian provinces switch to Atlantic Daylight Time (ADT) in summer.
About MST (Mountain Standard Time)
Mountain Standard Time (MST) covers the Rocky Mountain region of the United States and Canada. Arizona is notable for not observing daylight saving time, staying on MST year-round (except the Navajo Nation). The time zone includes major cities like Denver, Phoenix, Calgary, and Salt Lake City. During summer, most areas switch to Mountain Daylight Time (MDT, UTC-06:00).
Best time to call between AST and MST
Both AST and MST sit inside the 9 AM – 5 PM working day between 12:00 PM–5:00 PM AST, which is 9:00 AM–2:00 PM MST. That gives roughly 5 hours of shared time — the safest window for a live call or meeting.
Business hours overlap
AST and MST share 5 hours of working-hours overlap (12:00 PM–5:00 PM AST), so scheduling a live meeting is straightforward.
AST is 3 hours behind MST (AST UTC-04:00 → MST UTC-07:00). The shared working window is 12:00 PM–5:00 PM AST. Remember to re-check around daylight-saving transitions.
🕐 Daylight Saving Time Info
AST
☀ Currently in DST
Standard: UTC-04:00 · DST: UTC-03:00
Clocks fall back on Sun, Nov 1, 2026
MST
☀ Currently in DST
Standard: UTC-07:00 · DST: UTC-06:00
Clocks fall back on Sun, Nov 1, 2026
Learn More About Time
Dive deeper into the fascinating world of timekeeping, time zones, and how they shape our global society:
Frequently Asked Questions
What is the time difference between AST and MST?
AST (AST, UTC-04:00) is 3 hours behind MST (MST, UTC-07:00). The gap is currently -3h.
How do I convert AST time to MST time?
Add -3h to AST local time. For example, 9:00 AM in AST is 6:00 AM in MST.
What time is it in MST right now?
MST uses MST at UTC-07:00. Use the live converter at the top of this page — it updates in real time and already accounts for the -3h offset from AST.
What is the best time to call MST from AST?
Call between 12:00 PM and 5:00 PM AST — that maps to 9:00 AM–2:00 PM MST and keeps both parties inside the working day.
Which is behind, AST or MST?
AST is 3 hours behind MST. When it is 12:00 PM in AST it is 9:00 AM in MST.
Does daylight saving time affect the AST–MST difference?
Neither zone observes daylight saving time, so the -3h offset between AST and MST is fixed all year.
Which other cities share AST's time zone?
AST also covers Halifax, San Juan, Santo Domingo, Caracas, La Paz, Fredericton, Charlottetown.
Which countries use AST, AST's zone?
AST is used in Canada (Atlantic provinces), Puerto Rico, US Virgin Islands, Dominican Republic, Venezuela, Bolivia, Barbados, Trinidad and Tobago.
Is there a day difference between AST and MST?
Not normally. The -3h gap only pushes the date over the previous day.
What is the time difference between AST and MST?
MST is -3h from AST.
What timezone is AST in?
AST operates on its local timezone.
What timezone is MST in?
MST operates on its local timezone.
Which countries use AST (Atlantic Standard Time)?
AST is used in Canada (Atlantic provinces), Puerto Rico, US Virgin Islands, Dominican Republic, Venezuela, Bolivia, and 2 more countries.
Does AST observe daylight saving time?
Yes, AST switches to ADT during summer months.
What are some interesting facts about AST?
Puerto Rico uses AST year-round and does not observe DST. AST is 1 hour ahead of EST.
Which cities are in the MST timezone?
Major cities in the MST timezone include Denver, Phoenix, Salt Lake City, Calgary, Edmonton, Albuquerque, El Paso, Tucson.
What is the UTC offset for MST?
Mountain Standard Time (MST) is at UTC-07:00. During daylight saving time, it shifts to MDT.
What should I know about the MST timezone?
Arizona does not observe daylight saving time, staying on MST year-round. The Navajo Nation in Arizona does observe DST, unlike the rest of the state.
When is it midnight in AST, what time is it in MST?
When it is 12:00 AM (midnight) in AST (AST), it is -3h later in MST (MST).
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See every conversion from AST →·See every conversion from MST →