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The following abbreviations each refer to more than one timezone. This list is the primary reason why timezone abbreviations should never be used as the sole identifier in a time reference that crosses regional boundaries.
For each abbreviation, the conflicting meanings are listed with their UTC offsets. Where DST applies, the offset shown is the standard (non-DST) value unless otherwise noted.
IST — three meanings
India Standard Time: UTC+5:30. Used across all of India. India does not observe DST. The half-hour offset is a frequent source of confusion in UTC arithmetic.
Irish Standard Time: UTC+1. Used in the Republic of Ireland during summer (last Sunday in March to last Sunday in October). Outside summer, Ireland observes GMT (UTC+0), which is not abbreviated IST. The switch means that "IST" in an Irish context is a seasonal abbreviation.
Israel Standard Time: UTC+2. Used in Israel during standard time. Israel observes DST (IDT, UTC+3) from late March to late October, though the exact dates are set annually by government decree and vary year to year.
Maximum spread between IST interpretations: 3 hours 30 minutes.
CST — two primary meanings
Central Standard Time: UTC−6. Used across the central United States and Canada during standard time. Transitions to CDT (UTC−5) during daylight saving.
China Standard Time: UTC+8. Used across all of China year-round. China does not observe DST.
Maximum spread between CST interpretations: 14 hours.
AST — two meanings
Atlantic Standard Time: UTC−4. Used in Atlantic Canada (Nova Scotia, New Brunswick, Prince Edward Island) and parts of the Caribbean during standard time.
Arabia Standard Time: UTC+3. Used in Saudi Arabia, Qatar, Kuwait, Bahrain, and Yemen. These countries do not observe DST.
Maximum spread between AST interpretations: 7 hours.
BST — two meanings
British Summer Time: UTC+1. Used in the United Kingdom during DST (last Sunday in March to last Sunday in October). Outside this period, the UK uses GMT (UTC+0).
Bangladesh Standard Time: UTC+6. Used in Bangladesh year-round. Bangladesh does not observe DST.
Maximum spread between BST interpretations: 5 hours.
EST — two meanings
Eastern Standard Time: UTC−5. Used in the eastern United States and Canada during standard time. Transitions to EDT (UTC−4) during daylight saving.
Eastern Standard Time (Australia): UTC+10. Used in Queensland year-round, and in New South Wales, Victoria, Tasmania, and the ACT during standard time (when those states are not observing AEDT).
Maximum spread between EST interpretations: 15 hours.
CDT — two meanings
Central Daylight Time: UTC−5. Used in the central United States and Canada during daylight saving.
Cuba Daylight Time: UTC−4. Used in Cuba during daylight saving.
Maximum spread: 1 hour.
SST — two meanings
Samoa Standard Time: UTC−11. Used in American Samoa and Jarvis Island.
Singapore Standard Time: UTC+8. Used in Singapore year-round.
Maximum spread: 19 hours. The largest spread of any shared abbreviation.
GST — two meanings
Gulf Standard Time: UTC+4. Used in the UAE and Oman. No DST.
South Georgia Time: UTC−2. Used in South Georgia and the South Sandwich Islands.
Maximum spread: 6 hours.
WST — two meanings
Western Standard Time (Australia): UTC+8. Used in Western Australia year-round.
West Samoa Time: UTC+13. Used in Samoa (note: distinct from American Samoa).
Maximum spread: 5 hours.
A note on usage
This list is not exhaustive. New abbreviation collisions emerge when countries adopt or change their timezone designations. The IANA timezone database (the authoritative source used by most operating systems and programming languages) uses region/city identifiers — such as America/Chicago or Asia/Kolkata — precisely because abbreviations are not unique. When precision is required, use an IANA identifier or a UTC offset rather than an abbreviation.