This guide covers IANA Time Zone Database Explained | 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a count — it is computed from the IANA Time Zone Database for the date stated, not copied from a table that may be out of date.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 is a set of files that describes every time zone in the world: its name, its offset from UTC, its daylight saving rules, and its entire history of changes. It is maintained by IANA and a volunteer editor, and it is the data source behind the clock on almost every computer, phone and server on Earth.

A zone in the database is identified by a name in the form Region/City — America/New_York, Europe/London, Asia/Kolkata, Australia/Sydney. The city is the place the zone's rules are anchored to, usually the largest city in the zone. Two cities that share the same rules share a zone: Boston and New York are both America/New_York, and San Francisco and Los Angeles are both America/Los_Angeles.

Each zone file contains a list of rules and transitions: the dates on which the offset changed, and what it changed to. For America/New_York, that includes the adoption of standard time in 1883, the introduction of daylight saving in 1918, the wartime changes, and the extension of daylight saving in 2007. The database keeps the full history because a computer that needs to know the time in New York on a date in 1965 needs the 1965 rules, not today's.

The database is updated whenever a government changes its rules — which happens more often than people think. Morocco changes its daylight saving dates around Ramadan, Türkiye abolished the change in 2016, and Mexico abolished it in 2022. Each change produces a new release (named by year and letter, like 2026a), and operating systems ship the update to your device. That is why your phone always knows the right time, even in countries that just changed their clocks.

In practice

The database is a set of text files, and its most famous file is the list of zone rules — the dates on which each zone changes its clocks, going back decades. When a government announces a change, a volunteer writes the new rule, the change is reviewed, and a new version of the database is released. Operating systems and programming languages then ship the update, and your phone applies it the next time it updates.

The most dramatic recent example is Morocco, which abolished daylight saving in June 2026 by decree, with the last clock change in September 2026. A change like that needs a database update, and every device in the world needs to receive it — otherwise phones in Casablanca would keep moving their clocks. The system works because the database is maintained centrally and distributed everywhere, but it depends on governments giving notice, and they do not always do so.

For most people, the database is invisible — and that is how it should be. You only notice it when it is wrong: a calendar entry an hour out after a rule change, a flight time that does not match the airport board, a meeting scheduled for the wrong hour. When that happens, the cause is almost always a rule change that has not yet reached your device. Updating your operating system is the fix.

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

How does a time zone change reach my phone?

A volunteer writes the new rule, it is reviewed, and a new version of the database is released. Operating systems ship the update, and your phone applies it when it updates.

What happens if my device has old time zone data?

A calendar entry can be an hour out after a rule change, or a flight time can disagree with the airport board. Updating your operating system is the fix.

What is the difference between a zone and an offset?

A zone is a named set of rules (America/New_York); an offset is a number (UTC−05:00). The same zone can have two offsets a year, and two zones can share an offset.

Why are zone names place names?

So the zone keeps its identity when its rules change. New York has been America/New_York through every change of its daylight saving dates.

Is the database free?

Yes. It is public data, maintained by volunteers and distributed with every major operating system and programming language.

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.