The IANA Time Zone Database: How Computers Know UK Time
Almost every device that shows the time uses a shared database of time zone rules, maintained by volunteers and known as the IANA time zone database or tz database.
What is in it
The database records the history and future of every zone, including daylight saving start and end dates, past changes of rules and unusual cases. Each entry has an identifier such as Europe/London, America/New_York or Asia/Kolkata.
Why cities, not countries
Zones are named after a representative city because countries merge, split and change rules, while cities tend to stay put. The UK has a single entry, Europe/London, and Belfast, Jersey, Guernsey and the Isle of Man are links to it.
That is also why you should choose Europe/London in a calendar app instead of a fixed GMT offset: the first changes with the seasons, and the second does not.
Frequently asked questions
Who maintains the time zone database?
A community of volunteers maintains it, and it is published through the Internet Assigned Numbers Authority, IANA.
Clock change guides
- What Is Daylight Saving Time (DST)? A Plain-English GuideWhat daylight saving time is, how it works in the UK, Europe and the US, why it exists, its history, who uses it and the argument over scrapping it.
- Why Do the Clocks Go Back in October? The Real ReasonThe clocks go back in autumn to return the UK to GMT, so winter mornings are lighter. The history, the rule, what time it happens and what changes for you.
- When Do the Clocks Go Forward Again After Going Back?When the UK clocks go forward again after the autumn change, how long the UK stays on GMT, when evenings get lighter and how to remember to reset your clocks.