THE SUN’S YEAR
365 days and a bit
One trip round the Sun, season to season, takes 365 days, 5 hours, 48 minutes and 46 seconds.
See the numbersThe Earth does not go round the Sun in a whole number of days.
Every calendar has to cope with that leftover quarter day. Ours does it with a 400-year rule, after two reforms that made whole days disappear. Here is how it works, in plain English.
Plain English, with the sources linked.
- Every 4 years
- Not every 100
- But every 400
- 97 leap days in 400 years
THE SHORT ANSWER
A quarter day,
almost.
The year of the seasons is about 365.2422 days.
The calendar has to round it, and keep correcting.
THE RULE
Every fourth year, mostly
Leap years fall every four years, except century years not divisible by 400. So 2000 was one; 2100 will not be.
Try a year
THE CATCH-UP
Days that never happened
Fixing centuries of drift meant skipping whole days: ten in 1582, eleven in Britain in 1752.
The two reforms
DATE TIPS
Four habits
for clearer dates.
Write dates year first
The international standard ISO 8601 writes dates as year, month, day. A date like 2026-10-13 cannot be misread as American or British order, and it sorts correctly.
Do not assume every fourth year
If you plan far ahead or build a spreadsheet, remember that 2100 will have no 29 February, even though it is divisible by 4.
Read old British dates with care
Before 1752 the year changed on 25 March, so a date in January to March may be recorded under the previous year. Check before you count.
Mind the gap in September 1752
There were no dates from 3 to 13 September 1752 in Britain. Working out how long ago something happened across that gap needs eleven days’ correction.
TWO REFORMS WORTH KNOWING
The days
that vanished.
The old Julian calendar ran about 11 minutes a year too long.
Over centuries, that added up to whole days.
10 days
Days removed from October
Pope Gregory XIII reformed the calendar in 1582. Thursday 4 October was followed directly by Friday 15 October, and the new leap year rule kept the error from building up again.
ROME · 1582
11 days
Days removed from September
Britain and its colonies waited until 1752, by which time one more day of drift had built up. By the Calendar (New Style) Act 1750, the day after 2 September 1752 was 14 September.
GREAT BRITAIN · 1752
The weekdays carried on as normal: only the date numbers jumped, so in 1582 Thursday was still followed by Friday. The skipped dates simply never existed in those places.
A QUICK LEAP YEAR CHECK
Does this year
get a 29 February?
Pick a year. The answer follows the Gregorian rule: divisible by 4 is a leap year, unless it is a century year that is not divisible by 400.
The same rule was written into British law by the Calendar (New Style) Act 1750, which named 1800 and 1900 as common years and 2000 as a leap year.
CALENDAR FACTS
Five numbers that
run your year.
The Sun’s year is not 365 days
The tropical year, from one spring to the next, lasts about 365.2422 days: 365 days, 5 hours, 48 minutes and 46 seconds.
The Julian calendar ran slow
A leap day every fourth year gives an average of 365.25 days, about 11 minutes and 14 seconds longer than the tropical year. That is why the dates drifted.
97 leap years every 400
Dropping three leap days every four centuries gives 97 leap years in 400 years, and an average calendar year of 365.2425 days.
One day in about 3,300 years
The Gregorian year is about 0.0003 days too long. It will take roughly 3,300 years before the calendar is a whole day out of step with the seasons.
New Year used to be in March
Until 1752 the legal year in England began on 25 March, Lady Day. The 1750 Act moved the first day of the year to 1 January.
QUICK QUESTIONS
The calendar, answered.
Short answers to the questions people ask most.
Why do we have leap years at all?
The year of the seasons, the tropical year, lasts about 365.2422 days: 365 days, 5 hours, 48 minutes and 46 seconds. A calendar of exactly 365 days would slip by almost a quarter of a day each year, so an extra day is added from time to time to keep the dates in step with the seasons.
Is 2100 a leap year?
No. Under the Gregorian rule, years divisible by 4 are leap years, except century years, which must also be divisible by 400. 2100 is divisible by 100 but not by 400, so it will be a common year of 365 days. 2000 was a leap year because it is divisible by 400.
Why did Britain lose eleven days in 1752?
Britain kept the older Julian calendar until 1752, by which time it had drifted eleven days away from the Gregorian calendar used in much of Europe. The Calendar (New Style) Act 1750 ordered that the day after 2 September 1752 would be 14 September.
Did the year really use to start in March?
Yes. Before 1752 the legal year in England began on 25 March, Lady Day. The 1750 Act moved the start of the year to 1 January from 1752 onwards.
How accurate is the calendar we use now?
Very close. With 97 leap years every 400 years, the average Gregorian year is 365.2425 days, about 0.0003 days longer than the tropical year. That works out at roughly one day of drift in about 3,300 years.
Sources
Each figure on this page comes from the sources below.
- Calendar calculations: the Julian and Gregorian yearsNASA Glenn Research Center
- Leap years: the rule and the 3,300-year driftUS Naval Observatory
- Calendar (New Style) Act 1750, full textIrish Statute Book
- Calendar: 97 leap years in 400 yearsBEV, Austrian Federal Office of Metrology
- Lady Day, 25 March, and the calendar change of 1752Newcastle University
- ISO 8601: the date and time formatInternational Organization for Standardization