This guide covers What Is a Leap Year? | 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a count — it is computed from the IANA Time Zone Database for the date stated, not copied from a table that may be out of date.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 leap year is a year with 366 days instead of 365, with the extra day added as 29 February. The rule is: a year is a leap year if it is divisible by 4, except for years divisible by 100, which are leap years only if they are also divisible by 400. So 2024 was a leap year, 2100 will not be, and 2000 was.

The reason is astronomy. The Earth takes about 365.2422 days to orbit the Sun, not 365. If the calendar ignored that quarter-day, the seasons would drift: after 100 years, the calendar would be 24 days out, and after 700 years, summer would fall in December. The leap day corrects most of the drift, and the century rule corrects the rest — the Gregorian calendar is accurate to about one day in 3,300 years.

The extra day goes in February because February is the shortest month, and because in the Roman calendar February was the last month of the year. The Romans added the leap day after 23 February, and the tradition stuck.

Leap years matter for calculations. Age calculators, days-between-dates tools and countdowns all have to handle 29 February correctly — and the common mistakes are to forget the century rule (1900 was not a leap year, 2000 was) and to forget that a person born on 29 February has a birthday only every four years in the usual sense.

In practice

The rule has three parts, and the third is the one people forget. A year is a leap year if it is divisible by 4 — except if it is divisible by 100, in which case it is not a leap year — except if it is divisible by 400, in which case it is. So 2024 was a leap year, 2100 will not be, and 2000 was. The rule is precise, and it is why the calendar stays in step with the seasons for thousands of years.

The reason the rule exists is that the Earth's year is not 365 days. It is 365 days, 5 hours, 48 minutes and 45 seconds — just under 365 and a quarter days. If we ignored the extra quarter day, the calendar would drift by a day every four years, and after a century the seasons would be a month out. The leap day is the correction, and the century rule is the correction to the correction.

The extra day goes in February because February is the shortest month, and the tradition dates back to the Romans, who added the extra day at the end of February. Julius Caesar introduced the leap year in 45 BC, and Pope Gregory XIII refined the rule in 1582, when the calendar had drifted 10 days out of step. The Gregorian calendar — the one the whole world uses today — is the result of that refinement.

The numbers: the year 2026

2026 is not a leap year: it has 365 days, 52 full weeks and one extra day, and 53 ISO weeks (because it starts on a Thursday). There are 261 weekdays in the year and 104 weekend days. The next leap year is 2028, and the one after that is 2032; the century year 2100 will not be a leap year, because it is divisible by 100 but not by 400. 1 January 2026 was a Thursday, and 31 December 2026 will be a Thursday.

The extra day is the reason the calendar shifts: because 365 days is 52 weeks and one day, every date falls one weekday later the following year — two days later after a leap year. That is why your birthday is on a different weekday almost every year, and why the calendar repeats every 28 years in most cases. The week number finder gives the ISO week for any date, the date calculator works out what date it is a number of days from now, and the days between dates calculator counts the days between any two dates.

2027 is also not a leap year: it has 365 days, 52 ISO weeks, and 261 weekdays. It starts on a Friday and ends on a Friday. The next leap year after 2026 is 2028, which will have 366 days and 262 weekdays. The business days calculator counts the business days in any year, with the public holidays subtracted.

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 leap year?

A year with 366 days instead of 365, with an extra day — 29 February. It happens roughly every four years, to keep the calendar in step with the Earth's orbit.

How do I know if a year is a leap year?

It is a leap year if it is divisible by 4 — except if it is divisible by 100, in which case it is not — except if it is divisible by 400, in which case it is. So 2024 was a leap year, 2100 will not be, and 2000 was.

Why do we need leap years?

The Earth's year is 365 days, 5 hours, 48 minutes and 45 seconds — just under 365 and a quarter days. Without the extra day, the calendar would drift by a day every four years.

When is the next leap year?

Leap years come every four years: 2024, 2028, 2032. The next century year, 2100, will not be a leap year, because it is divisible by 100 but not by 400.

Who invented the leap year?

Julius Caesar introduced it in 45 BC, and Pope Gregory XIII refined the rule in 1582, when the calendar had drifted 10 days out of step.

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.