Reference chart
Time Zone Abbreviations and UTC Offsets
By &Pixels, the studio that builds MyCalculator.to. Offsets are standard time. The note column says what happens in summer, where anything does.
Sixteen common zones with their offsets, and the five abbreviations that mean more than one thing, which is the reason meetings get missed.
What is a UTC offset?
A UTC offset is how far a local clock runs ahead of or behind Coordinated Universal Time, written as UTC-6 or UTC+5:30. It is unambiguous, which the three-letter abbreviations are not: those are conventions with no standards body behind them, and several of them collide outright.
CST is the worst. It is Central Standard Time at UTC-6, China Standard Time at UTC+8, and Cuba Standard Time at UTC-5. An invitation that says 3pm CST is fourteen hours ambiguous before anyone has considered daylight saving.
Common zones
| Abbr | Name | Offset | Note |
|---|---|---|---|
| UTC | Coordinated Universal Time | UTC+0 | the reference, never shifts |
| GMT | Greenwich Mean Time | UTC+0 | UK winter; the UK uses BST in summer |
| BST | British Summer Time | UTC+1 | late March to late October |
| CET | Central European Time | UTC+1 | CEST, UTC+2, in summer |
| EET | Eastern European Time | UTC+2 | EEST, UTC+3, in summer |
| IST | India Standard Time | UTC+5:30 | also Irish and Israel Standard Time |
| CST | China Standard Time | UTC+8 | one zone for the whole country |
| JST | Japan Standard Time | UTC+9 | no daylight saving |
| AEST | Australian Eastern Standard | UTC+10 | AEDT, UTC+11, in southern summer |
| NZST | New Zealand Standard Time | UTC+12 | NZDT, UTC+13, in summer |
| EST | Eastern Standard Time | UTC-5 | EDT, UTC-4, in summer |
| CST | Central Standard Time | UTC-6 | the other CST, and the collision |
| MST | Mountain Standard Time | UTC-7 | Arizona stays on it year round |
| PST | Pacific Standard Time | UTC-8 | PDT, UTC-7, in summer |
| AKST | Alaska Standard Time | UTC-9 | AKDT in summer |
| HST | Hawaii Standard Time | UTC-10 | no daylight saving |
The calculators behind this
Abbreviations that mean more than one thing
| Abbr | Could mean | Offsets |
|---|---|---|
| CST | Central, China, Cuba Standard Time | UTC-6, UTC+8, UTC-5 |
| IST | India, Irish, Israel Standard Time | UTC+5:30, UTC+1, UTC+2 |
| AMT | Amazon, Armenia Time | UTC-4, UTC+4 |
| BST | British Summer, Bangladesh Standard | UTC+1, UTC+6 |
| ECT | Ecuador, European Central Time | UTC-5, UTC+1 |
Writing a time that cannot be misread
- Write the offset, not the abbreviation. UTC-6 is unambiguous. CST could be three different zones fourteen hours apart.
- Use a city or an IANA zone name. America/Chicago carries its own daylight saving rules. A three-letter code carries none.
- Check whether either end observes daylight saving. Arizona, Japan and most of Asia do not, so the gap to them changes twice a year.
- Watch the fortnight in March and the one in October. Europe and North America switch on different dates, so the gap is briefly unusual.
- Store timestamps in UTC and convert on display. Every scheduling bug of this kind comes from storing a local time and losing the context.
For a specific pair of cities on a specific date, the time zone converter handles the daylight saving rules for both ends, which is the part a table cannot do. The abbreviation lookup resolves a code when somebody has sent you one and not said where they are.
Formula & Methodology
Did you know? Nepal runs at UTC+5:45, one of a handful of zones offset by three quarters of an hour. It was set relative to the longitude of Kathmandu rather than rounded to a neat boundary, which is why scheduling software that assumes whole or half hours occasionally gets Nepal wrong.
Sources
- IANA Time Zone Database, the reference implementation of zone rules and historical changes
- International Telecommunication Union recommendation on Coordinated Universal Time
- National Institute of Standards and Technology, time and frequency division