Time zone abbreviations that cause mix-ups
IST, CST, BST and AST each mean more than one thing. Here is what they mean, and what to write instead.
Three-letter time zone abbreviations look precise, but many of them are ambiguous, several are used incorrectly most of the year, and none of them are an international standard. If an invite says "10:00 IST", the reader has to guess which of three countries the sender meant.
Abbreviations that mean more than one thing
These are the collisions most likely to appear in real international scheduling. Offsets are taken from the IANA time zone database (2026c) for January and July 2027.
| Abbreviation | Can mean | Offset | IANA zone |
|---|---|---|---|
| IST | India Standard Time | UTC+5:30 (Jan) · UTC+5:30 (Jul) | Asia/Kolkata |
| IST | Israel Standard Time | UTC+2 (Jan) · UTC+3 (Jul) | Asia/Jerusalem |
| IST | Irish Standard Time (summer) | UTC+0 (Jan) · UTC+1 (Jul) | Europe/Dublin |
| CST | Central Standard Time (North America) | UTC−6 (Jan) · UTC−5 (Jul) | America/Chicago |
| CST | China Standard Time | UTC+8 (Jan) · UTC+8 (Jul) | Asia/Shanghai |
| CST | Cuba Standard Time | UTC−5 (Jan) · UTC−4 (Jul) | America/Havana |
| BST | British Summer Time | UTC+0 (Jan) · UTC+1 (Jul) | Europe/London |
| BST | Bangladesh Standard Time | UTC+6 (Jan) · UTC+6 (Jul) | Asia/Dhaka |
| AST | Atlantic Standard Time | UTC−4 (Jan) · UTC−3 (Jul) | America/Halifax |
| AST | Arabia Standard Time | UTC+3 (Jan) · UTC+3 (Jul) | Asia/Riyadh |
"10:00 CST" sent from Chicago and read in Shanghai is off by 14 hours in winter. "IST" in an email between a Dublin office and a Bangalore office can mean either side's local time. Context usually helps, but context is exactly what is missing when an invite is forwarded.
Standard vs daylight: the EST problem
In North America, Europe and Australia, each region has two abbreviations: one for standard (winter) time and one for daylight (summer) time. The one people get wrong most is EST.
| Region | Standard (winter) | Daylight (summer) | IANA zone |
|---|---|---|---|
| Eastern | EST = UTC−5 | EDT = UTC−4 | America/New_York |
| Central | CST = UTC−6 | CDT = UTC−5 | America/Chicago |
| Mountain | MST = UTC−7 | MDT = UTC−6 | America/Denver |
| Pacific | PST = UTC−8 | PDT = UTC−7 | America/Los_Angeles |
| UK | GMT = UTC+0 | BST = UTC+1 | Europe/London |
| Central Europe | CET = UTC+1 | CEST = UTC+2 | Europe/Paris |
| Eastern Australia | AEST = UTC+10 | AEDT = UTC+11 | Australia/Sydney |
New York is on EST only from early November to mid-March. The rest of the year it is on EDT. An invite for "2 pm EST" in June is technically an hour off. Most people mean "New York time", and calendar software usually reads it that way, but a careful reader in another country may convert it literally. The same applies to PST in summer, and to anyone writing "GMT" for London between March and October (see UTC vs GMT).
A simple way to avoid the problem: write ET, CT, PT ("Eastern Time"), or name the city. Those labels are always correct whatever the season.
Why software uses names like "America/New_York"
Computers do not use abbreviations to identify time zones. They use IANA zone names, built from a region and
a representative city: Europe/London, Asia/Kolkata, America/Sao_Paulo. Each
name carries the full history of that place's offsets and daylight saving rules, so software can work out the
correct local time for any date, past or future.
The city in the name does not have to be the capital. It is usually the most populous city in the area that has
always shared the same rules. That is why India is Asia/Kolkata, not New Delhi, and why most of the
eastern US is America/New_York. You can type any IANA name into the
TimeSlotSync converter as well as a city name.
How to write a time that nobody can misread
- Name a city: "10:00 London time". It is unambiguous and correct all year.
- Give two zones for cross-border meetings: "10:00 New York / 15:00 London".
- Use UTC with an offset for technical audiences: "14:00 UTC".
- Use a calendar invite rather than a time in text wherever possible. The calendar converts it for each attendee automatically.
- Avoid bare abbreviations, especially IST, CST, BST and AST, in anything that will be forwarded.