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The calendar

Why we have leap years, why September is not the seventh month, and the year Britain lost eleven days.

4 min read Last updated 3 Nov 2026
A calendar with a seasonal year wheel and a hand adding an extra yellow day tile.

The short version

The Earth takes about 365 and a quarter days to go round the Sun, so a calendar of exactly 365 days drifts out of step with the seasons. Leap years fix it. Our current calendar, the Gregorian one, is the result of two corrections made 1,600 years apart, and Britain only adopted it in 1752.

I always assumed a year was exactly 365 days.

It is closer to 365.2422 days.

That little extra quarter is why we have a leap day.

The problem: a year is not a whole number of days

A day is one turn of the Earth. A year is one trip round the Sun. There is no reason for the two to fit neatly, and they don't. A year is about 365.2422 days long, which is 5 hours, 48 minutes and 46 seconds more than 365 days.

If a calendar ignored the extra time, it would slip by about one day every four years. After a few hundred years, midsummer would fall in what the calendar called autumn. For more about why we have seasons, read Seasons.

The first fix: Julius Caesar

In 45 BC Julius Caesar introduced the Julian calendar. It had 365 days and added one extra day every fourth year. Averaged out, that is 365.25 days, close enough to the real figure for people to use for centuries.

But 365.25 is slightly too long. The calendar gains about 11 minutes a year, which adds up to a whole day roughly every 128 years.

The second fix: Pope Gregory

By 1582 the Julian calendar had drifted about ten days away from the seasons, which mattered because the date of Easter depends on the spring equinox. Pope Gregory XIII issued a new calendar, the Gregorian calendar, with a better rule for leap years:

  1. A year divisible by 4 is a leap year.
  2. Except a year divisible by 100, which is not a leap year.
  3. Except a year divisible by 400, which is a leap year again.

So 1900 was not a leap year, but 2000 was. Averaged out, the year is 365.2425 days long, very close to the real figure.

How Britain got here

  1. 45 BC Julius Caesar introduces the Julian calendar.
  2. 1582 Pope Gregory XIII introduces the Gregorian calendar; Spain, Portugal and parts of Italy switch at once.
  3. 1752 Britain and its colonies switch; 2 September is followed by 14 September.
  4. 1752 New Year's Day moves from 25 March to 1 January in Britain.
  5. 2000 A leap year, using the rule for years divisible by 400.

11 days

The number of days that disappeared from the British calendar in September 1752, when Wednesday 2 September was followed by Thursday 14 September.

Why are the months named so oddly?

The old Roman year began in March. That makes sense of September, October, November and December: the Latin words for seven, eight, nine and ten. Later, January and February were added to the start of the year, and the numbers slipped two places.

July is named after Julius Caesar and August after the emperor Augustus.

August has 31 days because the emperor Augustus took a day from February so his month was as long as July's.

This is a popular story, but it first appears in a medieval textbook, centuries after Augustus. Historians find no ancient evidence for it.

Why not just fix it once and for all?

The year is not constant. The Earth's orbit and spin change very slightly, and the day is getting longer by a tiny amount each century. The Gregorian calendar will be about a day out after roughly 3,000 years.

That is far in the future for everyday use. Separately, clocks are sometimes adjusted by a "leap second" to keep them in step with the Earth's spin. That is decided by international scientists, not by the calendar rules.

Sources: Royal Museums Greenwich, The calendar; Calendar (New Style) Act 1750. Dates and the year length are approximate.

Key takeaways

  • A year is about 365.2422 days.
  • Leap years keep the calendar in step.
  • The Julian calendar was a little too long.
  • The Gregorian rule skips some century years.
  • Britain switched in 1752 and lost eleven days.

Now you know.

The Earth's year is not a whole number of days.

Leap years correct the difference.

And Britain once skipped eleven days.

Last reviewed 3 Nov 2026

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