Convert GMT to AST

AST is 4 hours behind GMT. GMT is UTC+0 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 4 hours: 1:00 PM–5:00 PM GMT (9:00 AM–1:00 PM AST). Schedule meetings inside that window.

GMT to AST conversion table

About GMT

Greenwich Mean Time (GMT) is UTC+0. It is used in the UK and Ireland in winter, Portugal, Iceland and much of West Africa.

The trap with GMT is that the United Kingdom is not on it for most of the year. From late March to late October the UK runs BST, UTC+1, so a meeting agreed "10am GMT" with a London colleague in July is an hour out unless both parties meant the abbreviation literally. Broadcasters and airlines avoid the ambiguity by saying UTC. GMT itself is fixed at UTC+0 and does not shift; it is the UK's clocks that move away from it and back.

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 GMT switches to BST (UTC+1) from late March to late October. So the 4-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 GMT conversions

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

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