This guide covers How Time Zones Work | 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a count — it is computed from the IANA Time Zone Database for the date stated, not copied from a table that may be out of date.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.

Time zones work because the Earth rotates. The planet turns 360 degrees in 24 hours, so every 15 degrees of longitude the local solar time shifts by one hour. Time zones are the system that turns that fact into clocks people can use: each zone is a region where everyone agrees to use the same standard time, roughly matched to the Sun's position.

The reference point is the Prime Meridian at Greenwich, London — 0 degrees longitude, defined as UTC. Places east of Greenwich are ahead of UTC (Tokyo is UTC+09:00, Sydney UTC+10:00 in winter), and places west are behind it (New York UTC−05:00 in winter, Los Angeles UTC−08:00). The offset is the whole system: to convert between any two places, you add or subtract the difference in their offsets.

In practice, time zone boundaries follow politics, not geometry. China spans five geographical time zones but uses one (UTC+08:00). Spain sits on Greenwich's longitude but uses Central European Time (UTC+01:00) because of a decision made in 1940. India uses UTC+05:30, Nepal UTC+05:45. The zones are where governments put them, and governments can and do move them.

Daylight saving adds a second layer: in summer, many zones shift one hour forward, so their offset changes for part of the year. That means the difference between two cities can change — New York and London are 5 hours apart in winter and 4 hours apart for three weeks in March, when the US has changed its clocks and Europe has not yet. Any tool that converts times must know the date, not just the cities, because the answer depends on it.

In practice

The mechanism is simple, but the details are political. The Earth is divided into 24 hourly slices in theory, but the boundaries follow borders, not lines of longitude. Spain sits on the Greenwich meridian and uses Central European Time; China spans five geographical zones and uses one; the border between India and Nepal is a 15-minute jump. The map of time zones is a map of politics, not of astronomy.

The second mechanism is the offset, and it is the one that matters day to day. Every zone is defined by its offset from UTC: New York is UTC−05:00 in winter, London is UTC+0, Tokyo is UTC+09:00. To convert, you add or subtract the offset. The offset is what your phone shows, what your calendar uses, and what changes when daylight saving starts — and it is the only number you need.

The third mechanism is the date line, and it is the one that confuses people most. The International Date Line runs through the Pacific, roughly along 180 degrees longitude, and crossing it changes the date by a day. Fly from Los Angeles to Tokyo and you arrive two days after you left; fly back and you arrive before you left. The date line is not straight — it bends around Kiribati, Samoa and Tonga so that whole countries stay on the same date — and it is, like everything else about time zones, a political line, not a natural one.

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 do time zones work?

The Earth is divided into zones so local time roughly matches the Sun's position. Each zone is defined by its offset from UTC, and the boundaries follow country and state lines, not lines of longitude.

Why is there an International Date Line?

Because the Earth is round and the day has to start somewhere. The line runs through the Pacific at roughly 180 degrees longitude, and crossing it changes the date by a day.

How do I convert between two time zones?

Convert the first city's time to UTC by subtracting its offset, then convert UTC to the second city by adding its offset. The time zone converter does it for any two cities.

Why do some time zones have 30-minute offsets?

Because the countries sit between the neat 15-degree slices and chose an offset that puts noon closest to the middle of the day — India (UTC+05:30), Iran (UTC+03:30), Nepal (UTC+05:45).

What happens at the date line?

Crossing westward you gain a day; crossing eastward you lose one. Fly from Los Angeles to Tokyo and you arrive two calendar days after you left.

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.