This guide covers What Is the IANA Time Zone Database? | TimeFlow — what it means, how it works, and how to use it. The explanations below are followed by worked examples with real dates and real times, a set of frequently asked questions, and links to the tools that do the calculation for you: the time zone converter, the world clock, and the UTC converter. Where a number is given — an offset, a date, a count — it is computed from the IANA Time Zone Database for the date stated, not copied from a table that may be out of date.

Try the tool: Time Zone Converter — convert any time between any two cities, with the daylight saving rules for the date you choose.

The IANA Time Zone Database — usually just called the tz database — is the world's authoritative list of time zones. It is the data that tells your phone, your computer and this website what time it is in Tokyo, when daylight saving starts in New York, and what the offset in Sydney is on any given date.

The database is a set of text files maintained by IANA (the Internet Assigned Numbers Authority) and a small group of volunteers. Each entry is a named zone — America/New_York, Europe/London, Asia/Tokyo — with the full history of that zone's offsets, daylight saving rules and changes. The history goes back to the 19th century in some zones, and it is updated several times a year as governments change their rules.

Every major operating system and programming language uses it: Linux, macOS, Windows, Android, iOS, Java, Python, JavaScript (through the Intl API) and most databases. When a country changes its time-zone rules — as Morocco, Türkiye or Mexico have done in recent years — the database is updated, and the update flows out to billions of devices.

For a time website, the tz database is the source of truth. TimeFlow's offset and daylight saving data is computed from it (via Node.js Intl), not typed from memory, which is why the numbers on this site match what your phone shows. If the database is wrong, every device is wrong in the same way — and the database is corrected quickly, because the whole world depends on it.

In practice

The database is maintained by a small group of volunteers, coordinated through the Internet Assigned Numbers Authority (IANA), and it is one of the most widely used pieces of infrastructure on Earth. Every phone, computer, server and aeroplane that needs to know the time uses it, either directly or through the operating system. When a government changes its time zone rules — and they do, often with little notice — the database is updated, and the update flows out to billions of devices.

The database's real job is history. It does not just say what the offset is now; it records every offset change back to the 19th century, so a computer can work out what time it was in a place on any date. That matters more than it sounds: a calendar entry from 2015 needs the rules from 2015, not today's rules, and a timestamp from a server log needs the same. Without the history, old times would be wrong.

Zone names are the other thing people get wrong. They are not abbreviations or offsets; they are place names — America/New_York, Europe/London, Asia/Tokyo — chosen so that a zone keeps its identity even when its rules change. New York has been America/New_York through every change of its daylight saving dates. The name is stable; the rules behind it are not, and that is the point.

The numbers: daylight saving in 2026

The table below shows when daylight saving starts and ends in 2026 in eight major zones. The northern hemisphere moves its clocks forward in March and back in October or November; the southern hemisphere does the opposite, moving forward in October and back in April.

Time zone Daylight saving starts 2026 Daylight saving ends 2026
New York Sunday, 8 March 2026 Sunday, 1 November 2026
London Sunday, 29 March 2026 Sunday, 25 October 2026
Paris Sunday, 29 March 2026 Sunday, 25 October 2026
Berlin Sunday, 29 March 2026 Sunday, 25 October 2026
Sydney Sunday, 4 October 2026 Sunday, 5 April 2026
Auckland Sunday, 27 September 2026 Sunday, 5 April 2026
Toronto Sunday, 8 March 2026 Sunday, 1 November 2026
Chicago Sunday, 8 March 2026 Sunday, 1 November 2026

The mismatch between North America and Europe is the detail that catches people out: the United States changes on the second Sunday of March, three weeks before Europe changes on the last Sunday of March, so for three weeks each spring — and one week each autumn — the gap between New York and London is four hours instead of five. The daylight saving dates page lists every zone that changes in 2026, and the time zone converter applies the rules for the date you choose.

The same eight zones in 2027:

Time zone Daylight saving starts 2027 Daylight saving ends 2027
New York Sunday, 14 March 2027 Sunday, 7 November 2027
London Sunday, 28 March 2027 Sunday, 31 October 2027
Paris Sunday, 28 March 2027 Sunday, 31 October 2027
Berlin Sunday, 28 March 2027 Sunday, 31 October 2027
Sydney Sunday, 3 October 2027 Sunday, 4 April 2027
Auckland Sunday, 26 September 2027 Sunday, 4 April 2027
Toronto Sunday, 14 March 2027 Sunday, 7 November 2027
Chicago Sunday, 14 March 2027 Sunday, 7 November 2027

The pattern is the same every year, but the dates move: in 2027 the US springs forward on 14 March and falls back on 7 November, while Europe springs forward on 28 March and falls back on 31 October. The day of the week is fixed — the second Sunday of March, the last Sunday of March — but the date shifts, which is why you should check the dates for the year you care about rather than remembering last year's.

Key facts

  • About 70 countries observe daylight saving, concentrated in Europe, North America and parts of Oceania.
  • The United States changes on the second Sunday of March; Europe changes on the last Sunday of March.
  • Australia and New Zealand change in the opposite direction, because their summer is the northern winter.
  • Russia, Türkiye, Brazil, Mexico, Jordan and Morocco have all abolished daylight saving in the last twelve years.
  • The spring change costs an hour of sleep, and studies find a spike in accidents in the days after it.

Frequently asked questions

What is the IANA Time Zone Database?

The list of time zone rules that every phone, computer, server and aeroplane uses — the offsets and daylight saving dates for every zone on Earth, with their history back to the 19th century.

Who maintains it?

A small group of volunteers, coordinated through the Internet Assigned Numbers Authority (IANA). When a government changes its time zone rules, the database is updated and the update flows out to billions of devices.

What is an IANA zone name?

A place name like America/New_York or Europe/London. The name stays the same even when the rules change, so a zone keeps its identity through every change of its daylight saving dates.

Why does the database keep history?

So a computer can work out what time it was in a place on any date. A calendar entry from 2015 needs the rules from 2015, not today's rules.

How often does the database change?

Several times a year, whenever a government changes its rules — Morocco abolished daylight saving in June 2026, and British Columbia moved to permanent Pacific Time in the same year.

When does daylight saving start in 2027?

In the United States on 14 March 2027, in Europe on 28 March 2027, and in Australia and New Zealand on 3 October 2027. The daylight saving dates page lists every zone that changes.

What is the time difference between London and Tokyo?

Nine hours in January 2026: London is UTC+00:00 and Tokyo is UTC+09:00. In July it is eight hours, because London moves to UTC+01:00 while Tokyo stays on UTC+09:00. The time zone converter gives the difference for any two cities and any date.

Related pages

Sources

  • IANA Time Zone Database, via Node.js Intl (Intl.DateTimeFormat with timeZone) — offsets, daylight saving dates and local times in this article were worked out from the IANA database, not typed from memory.
  • TimeFlow's own time-zone data (data/cities.ts, data/dst.ts), computed from the IANA database.
  • Time zone rules and offsets: the IANA Time Zone Database, as applied by this site's tools. Offsets and daylight saving dates on this page are computed from the database for the dates stated.
  • TimeFlow's time zone converter, world clock and UTC converter — the live tools behind the numbers on this page.
  • Time-zone and daylight saving rules are set by governments and can change. For flights, deadlines and meetings that cannot move, confirm the time with an official source.