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.