MST to AST
+3hMST
MSTSOURCEAST
ASTGreen rows indicate typical business hours (9 AM - 5 PM)
MST and AST on the World Map
The map below highlights the MST (Mountain Standard Time) and AST (Atlantic Standard Time) timezone regions across the globe.
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.
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.
Time Conversion Guide: MST to AST
MST runs on Mountain Standard Time (MST) at UTC-07:00. AST runs on Atlantic Standard Time (AST) at UTC-04:00. That puts MST 3 hours ahead of AST — a difference of +3h at the current offsets.
In practice, when it is 9:00 AM in MST it is 12:00 PM in AST. Midnight in MST is 3:00 AM in AST. Neither zone observes daylight saving time, so the offset between them stays constant all year.
MST to AST time conversion table
| MST (MST) | AST (AST) | Day |
|---|---|---|
| 12:00 AM | 3:00 AM | same day |
| 6:00 AM | 9:00 AM | same day |
| 7:00 AM | 10:00 AM | same day |
| 8:00 AM | 11:00 AM | same day |
| 9:00 AM | 12:00 PM | same day |
| 12:00 PM | 3:00 PM | same day |
| 3:00 PM | 6:00 PM | same day |
| 5:00 PM | 8:00 PM | same day |
| 6:00 PM | 9:00 PM | same day |
| 9:00 PM | 12:00 AM | next day |
Why people convert MST to AST time
- Scheduling a call from MST to AST: a 10:00 AM start in MST lands at 1:00 PM in AST, so you can confirm the slot without doing the arithmetic yourself.
- Finding shared working hours: both MST and AST are inside the 9-to-5 working day between 9:00 AM and 2:00 PM MST time (12:00 PM–5:00 PM AST). That is roughly 5 hours of usable overlap.
- Coordinating a MST-based team with AST contacts: quoting times in the other side's local clock — not your own — removes the single biggest source of missed meetings.
- Always state the zone abbreviation, not just the hour. "3:00 PM MST" is unambiguous; "3 PM" across a +3h gap is not.
- Block 9:00 AM–2:00 PM MST as the recurring meeting window if you work primarily from MST and need AST colleagues present.
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).
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.
Best time to call between MST and AST
Both MST and AST sit inside the 9 AM – 5 PM working day between 9:00 AM–2:00 PM MST, which is 12:00 PM–5:00 PM AST. That gives roughly 5 hours of shared time — the safest window for a live call or meeting.
Business hours overlap
MST and AST share 5 hours of working-hours overlap (9:00 AM–2:00 PM MST), so scheduling a live meeting is straightforward.
MST is 3 hours ahead of AST (MST UTC-07:00 → AST UTC-04:00). The shared working window is 9:00 AM–2:00 PM MST. Remember to re-check around daylight-saving transitions.
🕐 Daylight Saving Time Info
MST
☀ Currently in DST
Standard: UTC-07:00 · DST: UTC-06:00
Clocks fall back on Sun, Nov 1, 2026
AST
☀ Currently in DST
Standard: UTC-04:00 · DST: UTC-03:00
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 MST and AST?
MST (MST, UTC-07:00) is 3 hours ahead of AST (AST, UTC-04:00). The gap is currently +3h.
How do I convert MST time to AST time?
Add +3h to MST local time. For example, 9:00 AM in MST is 12:00 PM in AST.
What time is it in AST right now?
AST uses AST at UTC-04:00. Use the live converter at the top of this page — it updates in real time and already accounts for the +3h offset from MST.
What is the best time to call AST from MST?
Call between 9:00 AM and 2:00 PM MST — that maps to 12:00 PM–5:00 PM AST and keeps both parties inside the working day.
Which is ahead, MST or AST?
MST is 3 hours ahead of AST. When it is 12:00 PM in MST it is 3:00 PM in AST.
Does daylight saving time affect the MST–AST difference?
Neither zone observes daylight saving time, so the +3h offset between MST and AST is fixed all year.
Which other cities share MST's time zone?
MST also covers Denver, Phoenix, Salt Lake City, Calgary, Edmonton, Albuquerque, El Paso, Tucson and 2 more.
Which countries use MST, MST's zone?
MST is used in United States (Mountain), Canada (Alberta), Mexico (Chihuahua, Sonora).
Is there a day difference between MST and AST?
Not normally. The +3h gap only pushes the date over for late-evening times.
What is the time difference between MST and AST?
AST is +3h from MST.
What timezone is MST in?
MST operates on its local timezone.
What timezone is AST in?
AST operates on its local timezone.
Which countries use MST (Mountain Standard Time)?
MST is used in United States (Mountain), Canada (Alberta), Mexico (Chihuahua, Sonora).
Does MST observe daylight saving time?
Yes, MST switches to MDT during summer months.
What are some interesting facts about MST?
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.
Which cities are in the AST timezone?
Major cities in the AST timezone include Halifax, San Juan, Santo Domingo, Caracas, La Paz, Fredericton, Charlottetown.
What is the UTC offset for AST?
Atlantic Standard Time (AST) is at UTC-04:00. During daylight saving time, it shifts to ADT.
What should I know about the AST timezone?
Puerto Rico uses AST year-round and does not observe DST. AST is 1 hour ahead of EST.
When is it midnight in MST, what time is it in AST?
When it is 12:00 AM (midnight) in MST (MST), it is +3h later in AST (AST).
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See every conversion from MST →·See every conversion from AST →