Convert IST to ACST
ACST is 4 hours ahead of IST. IST is UTC+5:30 and ACST is UTC+9:30.
Right now: IST · ACST
Which IST is this?
IST names more than one zone. This page reads it as India Standard Time at UTC+5:30, which is the reading most converters take. There are others: Israel Standard Time at UTC+2, used in Israel in winter; Irish Standard Time at UTC+1, used in Ireland in summer.
The letters on a meeting invitation do not say which one is meant. If yours is not India Standard Time, the answer above is wrong, and not by a little:
| If IST means | Then |
|---|---|
| India Standard Time — assumed on this page | ACST is 4 hours ahead of IST |
| Israel Standard Time | ACST is 7.5 hours ahead of IST |
| Irish Standard Time | ACST is 8.5 hours ahead of IST |
The large converter sites disagree about this as well: search for a IST 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: 9:00 AM–1:00 PM IST (1:00 PM–5:00 PM ACST). Schedule meetings inside that window.
| IST | ACST | Suitable? |
|---|---|---|
| 9:00 AM | 1:00 PM | working hours |
| 10:00 AM | 2:00 PM | working hours |
| 11:00 AM | 3:00 PM | working hours |
| 12:00 PM | 4:00 PM | working hours |
| 1:00 PM | 5:00 PM | outside 9–5 |
| 2:00 PM | 6:00 PM | outside 9–5 |
| 3:00 PM | 7:00 PM | outside 9–5 |
| 4:00 PM | 8:00 PM | outside 9–5 |
| 5:00 PM | 9:00 PM | outside 9–5 |
IST to ACST conversion table
| IST | ACST |
|---|---|
| 12:00 AM | 4:00 AM |
| 1:00 AM | 5:00 AM |
| 2:00 AM | 6:00 AM |
| 3:00 AM | 7:00 AM |
| 4:00 AM | 8:00 AM |
| 5:00 AM | 9:00 AM |
| 6:00 AM | 10:00 AM |
| 7:00 AM | 11:00 AM |
| 8:00 AM | 12:00 PM |
| 9:00 AM | 1:00 PM |
| 10:00 AM | 2:00 PM |
| 11:00 AM | 3:00 PM |
| 12:00 PM | 4:00 PM |
| 1:00 PM | 5:00 PM |
| 2:00 PM | 6:00 PM |
| 3:00 PM | 7:00 PM |
| 4:00 PM | 8:00 PM |
| 5:00 PM | 9:00 PM |
| 6:00 PM | 10:00 PM |
| 7:00 PM | 11:00 PM |
| 8:00 PM | 12:00 AM (next day) |
| 9:00 PM | 1:00 AM (next day) |
| 10:00 PM | 2:00 AM (next day) |
| 11:00 PM | 3:00 AM (next day) |
About IST
India Standard Time (IST) is UTC+5:30. It is used in all of India and Sri Lanka, on a half-hour offset.
India Standard Time is UTC+5:30 — one of the handful of zones offset by half an hour, which breaks any code that assumes whole-hour offsets and any mental arithmetic that does the same. India has no daylight saving and one time zone for the entire country, despite spanning nearly 30 degrees of longitude. The abbreviation is also used for Irish Standard Time and Israel Standard Time, both of which are entirely different offsets.
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.
Does this gap change with the seasons?
Neither IST nor ACST shifts with the seasons, so this 4-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 IST in ACST?
9:00 AM IST is 1:00 PM ACST.
Is ACST ahead of IST?
ACST is 4 hours ahead of IST.
Which time zone does this use?
IST is UTC+5:30 and ACST is UTC+9:30. 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.