Convert ACST to AST
AST is 13 hours 30 minutes behind ACST. ACST is UTC+9:30 and AST is UTC−4.
Right now: ACST · AST
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:
| If AST means | Then |
|---|---|
| Atlantic Standard Time — assumed on this page | AST is 13.5 hours behind ACST |
| Arabia Standard Time | AST is 6.5 hours behind ACST |
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 do not overlap at all between ACST and AST. Someone has to take an early or late call — or use asynchronous communication instead.
| ACST | AST | Suitable? |
|---|---|---|
| 9:00 AM | 7:30 PM (previous day) | outside 9–5 |
| 10:00 AM | 8:30 PM (previous day) | outside 9–5 |
| 11:00 AM | 9:30 PM (previous day) | outside 9–5 |
| 12:00 PM | 10:30 PM (previous day) | outside 9–5 |
| 1:00 PM | 11:30 PM (previous day) | outside 9–5 |
| 2:00 PM | 12:30 AM | outside 9–5 |
| 3:00 PM | 1:30 AM | outside 9–5 |
| 4:00 PM | 2:30 AM | outside 9–5 |
| 5:00 PM | 3:30 AM | outside 9–5 |
ACST to AST conversion table
| ACST | AST |
|---|---|
| 12:00 AM | 10:30 AM (previous day) |
| 1:00 AM | 11:30 AM (previous day) |
| 2:00 AM | 12:30 PM (previous day) |
| 3:00 AM | 1:30 PM (previous day) |
| 4:00 AM | 2:30 PM (previous day) |
| 5:00 AM | 3:30 PM (previous day) |
| 6:00 AM | 4:30 PM (previous day) |
| 7:00 AM | 5:30 PM (previous day) |
| 8:00 AM | 6:30 PM (previous day) |
| 9:00 AM | 7:30 PM (previous day) |
| 10:00 AM | 8:30 PM (previous day) |
| 11:00 AM | 9:30 PM (previous day) |
| 12:00 PM | 10:30 PM (previous day) |
| 1:00 PM | 11:30 PM (previous day) |
| 2:00 PM | 12:30 AM |
| 3:00 PM | 1:30 AM |
| 4:00 PM | 2:30 AM |
| 5:00 PM | 3:30 AM |
| 6:00 PM | 4:30 AM |
| 7:00 PM | 5:30 AM |
| 8:00 PM | 6:30 AM |
| 9:00 PM | 7:30 AM |
| 10:00 PM | 8:30 AM |
| 11:00 PM | 9:30 AM |
About ACST
Australian Central Standard Time (ACST) is UTC+9:30. It is used in Adelaide and Darwin in winter.
Australian Central Standard Time is UTC+9:30, half an hour behind the eastern states rather than a whole hour, which surprises people every time. South Australia moves to daylight saving in October and the Northern Territory does not, so Adelaide and Darwin share this offset for half the year and are an hour apart for the other half.
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?
Neither ACST nor AST shifts with the seasons, so this 13.5-hour difference holds all year. That is unusual enough to be worth relying on: most cross-continental gaps move twice a year.
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
What time is 9:00 AM ACST in AST?
9:00 AM ACST is 7:30 PM AST (previous day).
Is AST ahead of ACST?
AST is 13 hours 30 minutes behind ACST.
Which time zone does this use?
ACST is UTC+9:30 and AST is UTC−4. Both abbreviations denote a single fixed offset, so this conversion never shifts with daylight saving — what changes is which abbreviation a place is using at the time.