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a count — it is computed from the IANA Time Zone Database for the date stated, not copied from a table that may be out of date.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

A time zone is a region of the Earth where the same standard time is used. The idea is simple: the Sun can only be overhead in one place at a time, so noon in London cannot also be noon in Tokyo. Time zones divide the planet into slices so that local time roughly matches the position of the Sun — midday falls near the middle of the day wherever you are.

In theory the world has 24 time zones, one for each hour of the day, each 15 degrees of longitude wide. In practice there are more than 24, because governments draw the boundaries. Time zone borders follow country and state lines, not neat strips of longitude, and some zones are offset by 30 or 45 minutes instead of a whole hour. India is on UTC+05:30, Nepal on UTC+05:45, and parts of Australia on UTC+08:45. The result is that the world currently uses 38 different UTC offsets, from UTC−12:00 to UTC+14:00.

Every time zone is defined by its offset from UTC — Coordinated Universal Time. New York is UTC−05:00 in winter and UTC−04:00 in summer; London is UTC+0 in winter and UTC+01:00 in summer; Tokyo is UTC+09:00 all year. The offset is what matters, not the name: two places can share an offset but be in different zones, and one place can change its offset twice a year when daylight saving starts and ends.

Time zones exist because the railway and the telegraph made local solar time unworkable. Before the 19th century every town kept its own time, set by the Sun — Bristol was 10 minutes behind London. That was fine when travel was slow, but it made railway timetables impossible. Standard time was the fix, and it is why we can now schedule a call between New York and Sydney without doing astronomy.

In practice

The most useful way to think about a time zone is as a political decision, not a natural fact. The Sun does not care about borders, but people do: China spans five geographical time zones and uses one, Spain sits on the Greenwich meridian and uses Central European Time, and the border between India and Nepal is a 15-minute jump. When you convert a time, you are converting between two governments' choices, not between two longitudes.

Time zones also explain why the same clock time can mean very different things in different places. 9am in Sydney in January is a hot summer morning; 9am in London in January is a dark winter morning. The clock says the same thing, but the day does not. That is why good scheduling looks at the local time of day, not just the hour difference — a 3-hour gap that lands at 2am for one person is worse than a 10-hour gap that lands at 10am for both.

If you travel, your phone handles time zones automatically — but it is worth knowing how. Phones use the IANA Time Zone Database, a list of rules for every zone on Earth, and they apply the rules for the zone you are standing in. The common mistake is to set your phone's clock manually when you land: leave it on automatic and it will change itself, including for daylight saving, which starts on different dates in different countries.

Key facts

  • A normal year has 365 days and a leap year has 366, with the extra day on 29 February.
  • A year has 52 full weeks and one extra day — or two in a leap year.
  • The Gregorian leap year rule: divisible by 4, except centuries, except centuries divisible by 400.
  • A business day is a weekday that is not a public holiday — and the holidays depend on the country.
  • In the ISO week system, the week starts on Monday and the first week is the one containing the first Thursday.

Frequently asked questions

What is a time zone?

A time zone is a region where the same standard time is used. It exists because the Sun can only be overhead in one place at a time, so local time is set to match the Sun's position roughly.

How many time zones are there?

There are 24 theoretical time zones and 38 different UTC offsets in actual use, from UTC−12:00 to UTC+14:00. The IANA database lists about 350 named zones.

What is a UTC offset?

The difference between a place's local time and UTC. New York is UTC−05:00 in winter, London is UTC+0, Tokyo is UTC+09:00. Add the offset to UTC to get local time.

Why do time zones follow borders instead of longitude?

Because governments draw them. China spans five geographical zones and uses one; Spain sits on the Greenwich meridian and uses Central European Time.

Do all countries have one time zone?

No. The United States has six, Russia has eleven, Australia has three main zones plus half-hour zones, and France has the most worldwide because of its overseas territories.

How many weekdays are there in 2026?

261 weekdays and 104 weekend days, out of 365 days in total. 2026 is not a leap year. The business days calculator counts the business days, with the public holidays subtracted.

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.