IANAタイムゾーンデータベース — 真実の情報源

What Is the IANA Time Zone Database?

The IANA Time Zone Database — also known as tzdata, the tz database, or the Olson database (after its original creator, Arthur David Olson) — is a collaborative compilation of all historical and current time zone rules around the world.

It is maintained by the Internet Assigned Numbers Authority (IANA) with contributions from a global community of volunteers. Every country's time zone offset, daylight saving time schedule, and any historical changes are recorded here.

Why It Matters

Without the IANA database, every operating system, programming language, and application would have to independently track hundreds of time zone rules. Instead, they all rely on tzdata as a single, authoritative source:

• Linux/macOS: ship with tzdata pre-installed at /usr/share/zoneinfo/
• Python: the zoneinfo module (Python 3.9+) reads from tzdata
• Java: uses the IANA database through its java.time API
• JavaScript: Intl.DateTimeFormat relies on tzdata embedded in V8/SpiderMonkey
• PostgreSQL / MySQL: use tzdata for timestamp conversions

Time Zone Identifiers

The IANA database uses human-readable identifiers in the format Area/Location. Examples:

• Asia/Seoul — Korean Standard Time (UTC+9)
• America/New_York — Eastern Time (UTC−5/−4)
• Europe/London — GMT/BST (UTC+0/+1)
• Pacific/Auckland — New Zealand Time (UTC+12/+13)

These identifiers are preferred over abbreviations like "EST" because abbreviations are ambiguous — "EST" could mean Eastern Standard Time (UTC−5) or Australian Eastern Standard Time (UTC+10).

How Often Is It Updated?

The database is updated whenever a government announces a time zone change. Recent updates include Samoa's date line jump (2011), Russia's elimination of DST (2014), North Korea's half-hour shift (2015 and 2018), and Turkey permanently adopting UTC+3 (2016).

Accessing the Data

The database is freely available at https://www.iana.org/time-zones. It is released as a compressed archive containing plain-text source files and compiled binary zone information files (zoneinfo format). On Linux, you can update it with your package manager: apt update tzdata or yum update tzdata.