Convert KST to AST
AST is 13 hours behind KST. KST is UTC+9 and AST is UTC−4.
Right now: KST · 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 hours behind KST |
| Arabia Standard Time | AST is 6 hours behind KST |
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 KST and AST. Someone has to take an early or late call — or use asynchronous communication instead.
| KST | AST | Suitable? |
|---|---|---|
| 9:00 AM | 8:00 PM (previous day) | outside 9–5 |
| 10:00 AM | 9:00 PM (previous day) | outside 9–5 |
| 11:00 AM | 10:00 PM (previous day) | outside 9–5 |
| 12:00 PM | 11:00 PM (previous day) | outside 9–5 |
| 1:00 PM | 12:00 AM | outside 9–5 |
| 2:00 PM | 1:00 AM | outside 9–5 |
| 3:00 PM | 2:00 AM | outside 9–5 |
| 4:00 PM | 3:00 AM | outside 9–5 |
| 5:00 PM | 4:00 AM | outside 9–5 |
KST to AST conversion table
| KST | AST |
|---|---|
| 12:00 AM | 11:00 AM (previous day) |
| 1:00 AM | 12:00 PM (previous day) |
| 2:00 AM | 1:00 PM (previous day) |
| 3:00 AM | 2:00 PM (previous day) |
| 4:00 AM | 3:00 PM (previous day) |
| 5:00 AM | 4:00 PM (previous day) |
| 6:00 AM | 5:00 PM (previous day) |
| 7:00 AM | 6:00 PM (previous day) |
| 8:00 AM | 7:00 PM (previous day) |
| 9:00 AM | 8:00 PM (previous day) |
| 10:00 AM | 9:00 PM (previous day) |
| 11:00 AM | 10:00 PM (previous day) |
| 12:00 PM | 11:00 PM (previous day) |
| 1:00 PM | 12:00 AM |
| 2:00 PM | 1:00 AM |
| 3:00 PM | 2:00 AM |
| 4:00 PM | 3:00 AM |
| 5:00 PM | 4:00 AM |
| 6:00 PM | 5:00 AM |
| 7:00 PM | 6:00 AM |
| 8:00 PM | 7:00 AM |
| 9:00 PM | 8:00 AM |
| 10:00 PM | 9:00 AM |
| 11:00 PM | 10:00 AM |
About KST
Korea Standard Time (KST) is UTC+9. It is used in South Korea, with no daylight saving.
Korea Standard Time is UTC+9, the same as Japan, and South Korea has had no daylight saving since 1988 when it was used for the Seoul Olympics. The two Koreas briefly differed: the North moved to UTC+8:30 in 2015 and returned to UTC+9 in 2018, which is the only time in modern history the peninsula ran on two clocks.
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 KST nor AST shifts with the seasons, so this 13-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 KST in AST?
9:00 AM KST is 8:00 PM AST (previous day).
Is AST ahead of KST?
AST is 13 hours behind KST.
Which time zone does this use?
KST is UTC+9 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.