Every time zone,
by UTC offset.

Frequently asked questions

Common questions about how time zones work, why they look the way they do, and how to use them correctly in software. Can't find what you need? Email [email protected].

How many time zones are there in the world?

About 38 distinct UTC offsets are in use, but the IANA database lists roughly 350 named zones to capture every variation in history and DST rules. This page lists the most commonly referenced ones, grouped by offset.

What's the difference between UTC and GMT?

UTC (Coordinated Universal Time) is the modern atomic-clock reference standard. GMT (Greenwich Mean Time) is the historical, astronomy-based time at the Greenwich meridian. In practice they are almost always within a second of each other, and most software treats them as equivalent — but UTC is the value to use.

Why do some zones have half-hour or quarter-hour offsets?

Local geography. India sits roughly midway between hour-aligned offsets and chose to compromise at UTC+05:30. Nepal went a step further at UTC+05:45 to stay distinct from India. The Chatham Islands use UTC+12:45 for the same reason, and Australia has both UTC+09:30 (Adelaide) and UTC+08:45 (Eucla).

Why are some abbreviations like 'IST' or 'CST' used for multiple zones?

The abbreviations are conventions, not unique identifiers. IST refers to Indian, Israel, and Irish Standard Time; CST means Central (US), China, or Cuba Standard Time depending on context. This is why software should always store the full IANA zone name.

Which time zones don't observe daylight saving time?

Equatorial regions, most of Asia, all of Africa, and most of South America. In Europe, Iceland and Russia don't change clocks. In North America, Arizona (except the Navajo Nation), Hawaii, and Saskatchewan stay on standard time year-round. Among DST observers, the start and end dates vary by country.

What is the IANA time zone database?

A collaborative public catalog of every named time zone on Earth, with the full history of offset and DST changes for each. It's maintained by IANA (Internet Assigned Numbers Authority) and updated several times a year as governments change their clock rules. Every modern operating system, language, and database derives its time-zone behaviour from it.

What is 'Anywhere on Earth' time?

AoE (UTC−12:00) is a calendar convenience: as long as it's still a given date somewhere on Earth, AoE considers the date current. It's used in academic deadlines so that nobody anywhere in the world has missed the cutoff.

Are time zones a fixed feature of the Earth?

No. Time zones are political constructs, and governments change them. North Korea reverted its standard time by 30 minutes in 2015, then reversed the change in 2018. Russia consolidated and re-split its zones in 2011 and 2014. Samoa skipped December 30, 2011 entirely to jump from the Americas-side to the Asia-side of the Date Line.