Convert MST to AST

AST is 3 hours ahead of MST. MST is UTC−7 and AST is UTC−4.

Which AST is this?

AST names more than one zone. This page reads it as Atlantic Standard Time at UTC−4, which is the reading most converters take. There is one other: Arabia Standard Time at UTC+3, used in Saudi Arabia, Iraq, Kuwait, Bahrain, Qatar and Yemen.

The letters on a meeting invitation do not say which one is meant. If yours is not Atlantic Standard Time, the answer above is wrong, and not by a little:

The large converter sites disagree about this as well: search for a AST conversion and some of the results answer for one reading and some for another, none of them saying which they chose. Where it matters, ask which country the sender is in rather than which letters they typed.

Best time to schedule a meeting

Standard 9–5 working hours overlap for 5 hours: 9:00 AM–2:00 PM MST (12:00 PM–5:00 PM AST). Schedule meetings inside that window.

MST to AST conversion table

About MST

Mountain Standard Time (MST) is UTC−7. It is used in Denver and Salt Lake City in winter, and Phoenix all year.

Mountain Standard Time is UTC−7. Denver and Salt Lake City use it in winter and switch to MDT in summer; Arizona does not switch at all and stays on MST year-round, which means Phoenix agrees with Denver in winter and with Los Angeles in summer. The Navajo Nation, inside Arizona, does observe daylight saving, so the state contains both. Any scheduling system that maps "Arizona" to "Mountain Time" without the no-DST flag will be an hour out for eight months.

About AST

Atlantic Standard Time (AST) is UTC−4. It is used in Halifax, Puerto Rico and the eastern Caribbean in winter.

Atlantic Standard Time is UTC−4, one hour ahead of Eastern. It covers Canada's Atlantic provinces, Puerto Rico and much of the eastern Caribbean, and it is the reason a call from Halifax to Toronto is an hour out from what either party expects. The abbreviation is also used for Arabia Standard Time at UTC+3, seven hours away in the other direction, which is a genuinely dangerous collision in scheduling.

Does this gap change with the seasons?

This gap is seasonal. The abbreviations themselves are fixed offsets, but the region on MST switches to MDT (UTC−6) from March to November. So the 3-hour figure on this page is correct for the part of the year when both places are actually on these abbreviations, and changes by an hour outside it.

AST does not shift, so the whole change comes from one side. A recurring meeting fixed in AST will appear to move by an hour for everyone on the other side, twice a year, without anyone changing anything.

This is the reason scheduling systems store an IANA zone such as Region/City rather than an offset or an abbreviation: the zone knows its own rules and the offset does not.

Frequently asked questions

Other MST conversions

• MST to UTC
• MST to GMT
• MST to EST
• MST to EDT
• MST to CST
• MST to CDT
• MST to MDT
• MST to PST
• MST to AKST
• MST to HST
• MST to BRT
• MST to BST
• MST to CET
• MST to CEST
• MST to EET
• MST to EEST

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