What is the rule for leap years?

A year is a leap year if it divides evenly by 4, except that years divisible by 100 are excluded unless they also divide by 400. That gives 97 leap years every 400 years, so 2024 qualifies, 1900 did not, and 2000 did.

When do people born on 29 February celebrate?

In common years leaplings usually mark the day on 28 February or 1 March as they prefer. Legal systems have to be firmer, and for statutory purposes such as reaching the age of majority the United Kingdom generally treats the anniversary as 1 March.

How often does a leap year happen?

Almost every four years, but not quite. Three century years in four are skipped, giving 97 leap years in each 400-year cycle rather than 100. That brings the average calendar year to 365.2425 days, very close to the true tropical year.

Leap Year Explained | datetoday.now

Q: Why was 1900 not a leap year?

Because it is divisible by 100 but not by 400, so the century exception applies. The exception exists because adding a day every four years slightly overcorrects the calendar, and dropping three leap days per four centuries brings the average year length back into line.

Almost everyone knows that a leap year comes round every four years and adds 29 February to the calendar. Rather fewer know that the rule has an exception, and that the exception has an exception of its own. The gap matters: 1900 was not a leap year, 2000 was, and a surprising amount of software written in the intervening century got that wrong because its authors stopped learning the rule after the first clause.

This article gives the complete leap year rule, explains the astronomical problem it solves, works through the 1900 and 2000 cases, and covers what the extra day does to birthdays and to the rest of the calendar. The today's date page shows whether the current year is a leap year, along with the year's length.

What Is The Full Leap Year Rule?

A year is a leap year if it is divisible by 4, except that years divisible by 100 are not, unless they are also divisible by 400. So 2024 and 2028 are leap years, 1900 and 2100 are not, and 1600 and 2000 are.

Applied in order, the three clauses are easy to check by hand:

Divisible by 4: the year is a candidate. This catches 2024, 2028, 2032 and so on.
Divisible by 100: the candidate is rejected. This removes 1700, 1800, 1900 and 2100.
Divisible by 400: the rejection is overturned. This restores 1600 and 2000.
Anything else: a common year of 365 days, which is roughly three years in every four.

The result is 97 leap years in every 400 years rather than the 100 that a plain four-year rule would give. Three leap days are dropped per four centuries, and that small subtraction is what keeps the calendar honest.

Why Does The Leap Year Correction Exist?

Because the earth's orbit does not take a whole number of days. A tropical year, the interval between successive spring equinoxes, is about 365 days, 5 hours, 48 minutes and 45 seconds, so a fixed 365-day calendar loses almost six hours annually.

Left uncorrected, that shortfall moves the seasons through the calendar: after roughly 700 years the spring equinox would fall in what the calendar called midwinter. Adding one day every four years overcorrects slightly, because four times the surplus is a little under a day. The Julian calendar, in use across Europe from Roman times, did exactly that, assuming a year of 365.25 days and therefore running about eleven minutes fast each year. Eleven minutes is nothing in a lifetime and about a day every 128 years across centuries, and by the sixteenth century the calendar had drifted some ten days out of step with the sky. The century exception is the fix: it brings the average year to 365.2425 days, within about half a minute of the true figure, which is close enough to keep the equinox in place for thousands of years.

The 1582 Reform And Pope Gregory XIII

The correction was introduced in 1582 by Pope Gregory XIII, whose reform both changed the leap year rule and removed the accumulated error in one step. In the countries that adopted it immediately, 4 October 1582 was followed directly by 15 October, deleting ten days that had crept in under the Julian calendar.

The motive was ecclesiastical as much as astronomical. The date of Easter is fixed with reference to the spring equinox, and the drift had pushed the calendar equinox well away from the real one. Adoption was slow and political: Catholic states moved first, Protestant and Orthodox countries much later. Britain and its colonies did not change until 1752, by which time eleven days had to be dropped, so 2 September was followed by 14 September. This is why historical dates before the local changeover need care, and why astronomers prefer the continuous day count described in day of year and Julian day, which is indifferent to calendar reform.

Why Was 1900 Not A Leap Year But 2000 Was?

Both are divisible by 4 and both are divisible by 100, so the century exception rules both out at the second step. Only 2000 survives the third clause, because 2000 divides evenly by 400 and 1900 does not.

The consequence is that anyone whose working life spanned the turn of the millennium saw a century year behave normally, which is misleading, since three century years in four do not. Software written with the simple four-year rule happened to give the right answer in 2000 and will give the wrong one in 2100. The same shortcut caused genuine failures in systems that skipped the 400-year clause and treated 2000 as a common year, producing off-by-one errors in date arithmetic throughout that year. If you are calculating spans across a century boundary, the date calculator applies the full rule rather than the folk version.

29 February And Leap Day Birthdays

The extra day is inserted as 29 February, making February 29 days long and the year 366. It sits at day 60 of the year, which pushes every subsequent date one place later in the ordinal count.

People born on 29 February, sometimes called leaplings, have a true birthday only once every four years, and the odds of being one are roughly 1 in 1,461, the number of days in a four-year cycle. In common years they celebrate on 28 February or 1 March according to preference, though legal systems have had to be more decisive: for statutory purposes, such as reaching the age of majority, the United Kingdom generally treats the anniversary as falling on 1 March. Contracts and insurance policies dated 29 February raise the same question, and well-drafted ones say explicitly which day they mean in common years.

What A Leap Year Changes On The Calendar

The extra day shifts far more than February. Every date from 1 March onward moves forward one weekday relative to the previous year instead of the usual one, so a leap year advances the calendar by two weekdays across its span. The day of year for any date after February increases by one, and the count of days left in the year adjusts accordingly, as set out in how many days are left in the year.

Anything measured in days rather than months feels it: interest accruals, subscription periods, payroll on daily rates, and any gap that spans the end of February, which is the classic source of the errors described in days between two dates. The ordinal shift is shown live on the day of year page.

Conclusion

A leap year is any year divisible by 4, unless it is divisible by 100, unless it is also divisible by 400. That awkward three-part rule exists because the tropical year is about 365.2422 days long, and the 1582 reform under Pope Gregory XIII adopted it to stop the seasons drifting. 1900 failed the century test, 2000 passed the 400-year one, and 2100 will fail again. Check whether the current year has 366 days on the today's date page, or see the rest of the calendar tools at datetoday.now.

Leap Year Explained: The Rule, The Reason, The Exceptions