How many hours is MST behind AST?
As abbreviations, AST is always 3 hours ahead of MST: MST is UTC−7 and AST is UTC−4. To convert, add 3 hours.
Check which label you actually need: MST (Mountain Time is on MDT today). A time written with an abbreviation that is not in use is usually meant as the local time, so convert between the places instead: right now AST is 2 hours ahead of MST.
When the MST to AST time difference changes
For most of the year AST is 2 hours ahead of MST. From 1 November to 31 December 2026, AST is 3 hours ahead of MST instead, because the two places change their clocks on different dates. Meetings booked by the usual gap land an hour off in those weeks.
| Dates | AST vs MST | Length |
|---|---|---|
| 8 March 2026 to 31 October 2026 | 2 hours ahead | 238 days |
| 1 November 2026 to 31 December 2026 | 3 hours ahead | 61 days |
| 1 January 2027 to 13 March 2027 | 3 hours ahead | 72 days |
| 14 March 2027 to 6 November 2027 | 2 hours ahead | 238 days |
| 7 November 2027 to 31 December 2027 | 3 hours ahead | 55 days |
Calling hours for MST and AST
- When AST is 2 hours ahead of MST: 6 shared office hours, 09:00 to 15:00 in MST (11:00 to 17:00 in AST).
- When AST is 3 hours ahead of MST: 5 shared office hours, 09:00 to 14:00 in MST (12:00 to 17:00 in AST).