An IANA time zone is a named entry in the IANA Time Zone Database — the authoritative global catalog of time zone rules used by virtually every operating system, programming language, and application on the internet. You've seen them even if you didn't know the name: 'America/New_York', 'Europe/London', 'Asia/Tokyo'.
The Format: Area/City
IANA time zones follow an Area/City format. The area is a continent or ocean region (America, Europe, Asia, Africa, Pacific, Atlantic, Indian, Arctic, Antarctica). The city is the largest or most recognizable city in that time zone that has historical continuity.
Cities are used rather than zone names or abbreviations for a crucial reason: abbreviations are ambiguous. 'CST' alone could mean Central Standard Time (UTC−6, North America), China Standard Time (UTC+8), or Cuba Standard Time (UTC−5). 'America/Chicago' is unambiguous.
Why IANA Zones Matter
Time zone rules change frequently — countries and regions modify their DST rules, shift their offset, or abolish DST entirely. The IANA database tracks every historical change and is updated several times a year. When you store an IANA zone name rather than a fixed UTC offset, your software automatically applies the correct rules for any date, past or future.
How to Find the Right Zone
The IANA database includes over 600 named zones, but most are links to canonical zones that share the same rules. When choosing a zone for a city, pick the canonical zone whose rules match. For New York, London, and Tokyo, the canonical names match the city names exactly. For smaller cities, you use the canonical zone for your region — someone in Cleveland would use 'America/New_York', not 'America/Cleveland'.
