AST to MST Converter & Meeting Planner
Quick answer: 9:00 AM AST = 6:00 AM MST (fixed -04:00 standard offset — not DST-adjusted). Use the interactive converter below for DST-aware local-time scheduling.
Planning a call or meeting between Atlantic Standard Time (AST) and Mountain Standard Time (MST)? You need to account for the standard time difference. Mountain Standard Time is normally 3 hours behind Atlantic Standard Time. Whether you are coordinating between teams in Halifax and San Juan and Denver and Phoenix, our live tool has you covered. Use our interactive overlap studio below to find the perfect waking hours for both regions, taking into account any seasonal Daylight Saving Time (DST) changes.
24-Hour Conversion Table
Quick reference: 12:00 AM AST = 9:00 PM MST. The full 24-hour AST to MST table below uses fixed standard offsets for exact abbreviation searches.
Example: 3:00 PM AST = 12:00 PM MST.
| AST Time | MST Time |
|---|---|
| 12:00 AM (00:00) | 9:00 PM (21:00)-1 day |
| 1:00 AM (01:00) | 10:00 PM (22:00)-1 day |
| 2:00 AM (02:00) | 11:00 PM (23:00)-1 day |
| 3:00 AM (03:00) | 12:00 AM (00:00) |
| 4:00 AM (04:00) | 1:00 AM (01:00) |
| 5:00 AM (05:00) | 2:00 AM (02:00) |
| 6:00 AM (06:00) | 3:00 AM (03:00) |
| 7:00 AM (07:00) | 4:00 AM (04:00) |
| 8:00 AM (08:00) | 5:00 AM (05:00) |
| 9:00 AM (09:00) | 6:00 AM (06:00) |
| 10:00 AM (10:00) | 7:00 AM (07:00) |
| 11:00 AM (11:00) | 8:00 AM (08:00) |
| 12:00 PM (12:00) | 9:00 AM (09:00) |
| 1:00 PM (13:00) | 10:00 AM (10:00) |
| 2:00 PM (14:00) | 11:00 AM (11:00) |
| 3:00 PM (15:00) | 12:00 PM (12:00) |
| 4:00 PM (16:00) | 1:00 PM (13:00) |
| 5:00 PM (17:00) | 2:00 PM (14:00) |
| 6:00 PM (18:00) | 3:00 PM (15:00) |
| 7:00 PM (19:00) | 4:00 PM (16:00) |
| 8:00 PM (20:00) | 5:00 PM (17:00) |
| 9:00 PM (21:00) | 6:00 PM (18:00) |
| 10:00 PM (22:00) | 7:00 PM (19:00) |
| 11:00 PM (23:00) | 8:00 PM (20:00) |
Best time to call Denver from Halifax
For most work calls, compare the normal business day in Halifax (AST) with the business day in Denver (MST). AST commonly covers North America, Caribbean, while MST is mainly associated with North America.
⚠️ Note: 2026-07-05 is not a normal workday in one or both regions based on their configured calendar. The overlap below should be treated as a reference, not a guaranteed office-hours window.
✅ The most practical overlap on 2026-07-05 is usually 12:00 PM - 5:00 PM AST, which corresponds to 9:00 AM - 2:00 PM MST. This window sits inside the typical AST workday (9:00 AM - 5:00 PM) and the MST workday (9:00 AM - 5:00 PM).
- DST Safety: AST or MST may shift during Daylight Saving Time. Always confirm the converted time for your specific date.
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Frequently Asked Questions
What is the time difference between AST and MST?
During standard time, Mountain Standard Time is 3 hours behind Atlantic Standard Time.
Does the Caribbean observe Daylight Saving Time?
The vast majority of Caribbean nations and territories — including Puerto Rico, the Dominican Republic, the US Virgin Islands, Trinidad and Tobago, and Barbados — do not observe Daylight Saving Time and remain on AST (UTC-4) year-round. This means these territories share the same clock as Halifax only during the Atlantic Standard Time window in winter, diverging by one hour once Canada and Bermuda advance to ADT each spring.
What is the difference between AST and ADT?
AST (Atlantic Standard Time) is the standard winter offset at UTC-4:00, observed from the first Sunday in November through the second Sunday in March across all AST regions. ADT (Atlantic Daylight Time) is the summer offset at UTC-3:00, applied by Eastern Canadian provinces and Bermuda when they advance their clocks by one hour during Daylight Saving Time.
Does Arizona observe Daylight Saving Time?
Most of Arizona — including Phoenix, Tucson, and the state government — does not observe Daylight Saving Time, remaining on MST (UTC-7) year-round under a state law in effect since 1968. The significant exception is the Navajo Nation, which spans parts of northeastern Arizona, Utah, and New Mexico and does observe DST, creating one of the most complex intrastate time zone situations in the United States.
What is the difference between MST and MDT?
MST (Mountain Standard Time) is the winter offset at UTC-7:00, observed from the first Sunday in November through the second Sunday in March. MDT (Mountain Daylight Time) is the summer offset at UTC-6:00, applied when most Mountain Time regions advance their clocks by one hour during Daylight Saving Time.