Time zones and clocks
Why CST, IST and BST mean different things in different places
Three-letter time zone codes were never standardised, so the same letters were adopted independently on different continents. Here are the clashes you will actually meet, and the habit that avoids them.
Last reviewed October 5, 2026About 2 min to readSite version v2026.10.1
There is no official list
No international body assigns time zone abbreviations. Each country, and sometimes each newspaper, picked codes that made sense locally, and English-language codes built from "Standard", "Summer" and a region name were bound to collide. The IANA time zone database records the abbreviations in local use but is explicit that they are not unique identifiers; it identifies zones by city names such as Asia/Kolkata precisely because the abbreviations cannot be trusted.
Software that parses a code like "CST" has to guess which one you meant, and different programs guess differently. That is why a calendar invitation written with abbreviations can land an hour, or thirteen hours, away from the time intended.
The clashes you will actually meet
| Code | Meanings | Offsets |
|---|---|---|
| CST | Central Standard Time (North America); China Standard Time; Cuba Standard Time | UTC-6; UTC+8; UTC-5 |
| IST | India Standard Time; Irish Standard Time (summer); Israel Standard Time | UTC+5:30; UTC+1; UTC+2 |
| BST | British Summer Time; Bangladesh Standard Time | UTC+1; UTC+6 |
| PST | Pacific Standard Time; Philippine Standard Time | UTC-8; UTC+8 |
| AST | Atlantic Standard Time; Arabia Standard Time | UTC-4; UTC+3 |
| EST | Eastern Standard Time (North America); also written for Australian Eastern Standard Time, which is properly AEST | UTC-5; UTC+10 |
| GST | Gulf Standard Time; historically Guam Standard Time | UTC+4; UTC+10 |
| SAST | South Africa Standard Time; occasionally misread as South Australia | UTC+2; UTC+9:30 |
The Pacific and Philippine clash is the most expensive one: the two PSTs are sixteen hours apart, so a mistaken reading does not shift a meeting, it moves it to a different day.
Standard versus summer codes
Even an unambiguous region has two codes. Eastern Time in North America is EST (UTC-5) in winter and EDT (UTC-4) in summer. People routinely write "EST" all year, so "3 PM EST" in July usually means 3 PM Eastern Daylight Time, which is a different instant from what the letters say. The safe reading of a seasonal code is "the Eastern zone, whatever it is doing on that date", and that is how World Time Lab's quick-pick chips treat it: picking EST gives you the zone, and the converter applies the correct seasonal offset for the chosen date.
Britain has the mirror problem. GMT is the winter zone and BST the summer one, but "GMT" is often used to mean London time in July, when London is an hour ahead of GMT. The UTC vs GMT guide covers that trap in detail.
What to write instead
- Name a city. "3 PM Chicago time" cannot be misread. A city implies a zone and its seasonal rules, and anyone can look it up.
- Or give the UTC offset for that date. "3 PM UTC-5" is exact. Write the offset that applies on the day, not the zone's winter offset.
- For software and formal documents, use the IANA name. America/Chicago, Asia/Kolkata and Europe/London are unambiguous and are what calendars, databases and this site use internally.
- If you must use a code, add the region. "CST (China)" or "IST (India)" costs five characters and prevents a half-day error.
When you type an ambiguous code into the converter, it does not guess. It asks which region you mean, then keeps the label you chose beside that clock. The abbreviation pages list every meaning of each code with a live clock for each one.
See it on the site
- Abbreviations explainedEvery common code with each of its meanings and a live clock for each.
- CST: Central, China or CubaThe three-way clash, resolved with live clocks and offsets.
- PST to EST chartA conversion table that states which PST and which EST it uses.