IST is India Standard Time. It is also Irish Standard Time and Israel Standard Time. CST covers North America, China and Cuba. Abbreviations are convenient, unofficial and genuinely ambiguous — which is why every serious system uses IANA identifiers instead.
IST is the classic example. India runs at UTC+5:30, Ireland at UTC+1 in summer, and Israel at UTC+2. All three are written IST, and a meeting invitation that says "3pm IST" is genuinely undecidable without context.
CST is nearly as bad. North American Central Time is UTC−6, China Standard Time is UTC+8, and Cuba Standard Time is UTC−5. A fourteen-hour error is easy to make. BST means British Summer Time in London and Bangladesh Standard Time in Dhaka. AST covers both Atlantic Standard Time in Canada and Arabia Standard Time in Riyadh.
The alternative is the IANA time zone database, which names zones after a region rather than a label: Asia/Kolkata, Europe/Dublin, Asia/Jerusalem. Those identifiers are unique, they carry the full history of daylight saving rules for that place, and they are what operating systems, programming languages and databases actually use. When you write Asia/Kolkata there is no argument about what you meant.
For human communication, the practical compromise is to write the abbreviation with a city or an offset attached: "3pm IST (India, UTC+5:30)". It costs a few characters and removes the ambiguity entirely.
Key facts
IST is three zones
India (UTC+5:30), Ireland (UTC+1) and Israel (UTC+2).
CST is three zones
North America (UTC−6), China (UTC+8) and Cuba (UTC−5).
BST is two
British Summer Time (UTC+1) and Bangladesh Standard Time (UTC+6).
Abbreviations are unofficial
No standards body maintains them, unlike IANA identifiers.
IANA names are unique
Asia/Kolkata identifies exactly one set of rules.
Add a city when writing
"3pm IST (India)" removes the ambiguity in four words.
How to get it right
1
Use IANA identifiers in systems
Store Europe/London, not GMT or BST.
2
Qualify abbreviations in writing
Add the city or the offset in brackets.
3
Watch for the summer label
EST in July usually means EDT — the writer means Eastern Time.
4
Prefer calendar invitations
They carry the zone with them and remove the problem entirely.
Questions people ask
Most often India Standard Time, UTC+5:30. But Irish Standard Time and Israel Standard Time share the abbreviation, so context matters. On this site, IST pages cover India.
No. CST is Central Standard Time (UTC−6) and CDT is Central Daylight Time (UTC−5). The same region uses both, in different seasons.
A named entry in the international time zone database, such as America/New_York. It stores every daylight saving rule that region has used, which is why software relies on it.
Add a qualifier. "2pm EST (New York)" is unambiguous; "2pm EST" in July is technically wrong, because New York is on EDT then.
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