How Leap Years Are Determined: The Rules, and How People Born on February 29 Age

The three rules that decide a leap year

In the Gregorian calendar used today, you check the following in order. Apply them from the top and stop as soon as one gives an answer.

1. 1

   Is it divisible by 4?

   If not, it is a common year. This settles most years.

2. 2

   Is it divisible by 100?

   A year divisible by 4 that is also divisible by 100 goes back to being a common year. 1900 and 2100 are examples.

3. 3

   Is it divisible by 400?

   A year divisible by 100 that is also divisible by 400 is a leap year. 2000 is an example.

Why once every four years isn't enough

The Earth takes about 365.2422 days to go around the Sun once. If a year is 365 days, it falls short by about 0.2422 days every year. Over four years that adds up to about 0.97 days, so adding one day every four years almost balances the books.

But adding a day every four years without exception overshoots. Four times 0.2422 doesn't quite reach 1.0, and the excess builds up to about 0.78 days per century. So one leap year is dropped every 100 years, and since that removes slightly too much, one is put back every 400 years.

• Every four years only: the average year is 365.25 days, about 0.0078 days too long
• Add the 100-year rule: the average year is 365.24 days, now about 0.0022 days too short
• Restore with the 400-year rule: the average year is 365.2425 days, about 0.0003 days off

The error that remains is about 26 seconds per year, so it takes roughly 3,000 years to drift by a whole day. The leap year exceptions are fine-tuning to keep dates from drifting away from the seasons.

Upcoming leap years

From 2000 to 2096, every year divisible by 4 is a leap year. The first exception is 2100, so until then checking divisibility by 4 is all you need. In everyday calculations, the 100-year and 400-year rules only matter when you go back to past dates.

When does someone born on February 29 age in a common year?

In Japan, age is counted under the Act on Age Calculation. Under this law, the day of birth is the starting day, and you become a year older when the day before the corresponding date in each following year ends. That is why people say your age goes up the day before your birthday.

Someone born on February 29 has no corresponding date in a common year. In that case the year is added at the end of February 28. So even in a common year, they have reached their new age on February 28. Their birthday is not moved to March 1.

How to check your age and days elapsed

1. 1

   Enter your date of birth

   Enter a date of birth in the age & anniversary counter, and your age is shown in the form “X years, Y months, Z days”.

2. 2

   Change the reference date

   Changing the reference date shows your age as of that day. Enter February 28 and then March 1 to see exactly when the year is added.

3. 3

   Look at the next birthday

   For someone born on February 29, the next birthday is shown as February 28 in common years and as February 29 in leap years.

4. 4

   Check the total days and milestones

   The number of days since birth and milestones such as day 10,000 are shown too. For someone whose birthday only comes once every four years, these can be easier occasions to celebrate.

If you are unsure about the age field on a form or a procedure with an age limit, check using that day as the reference date. Conditions such as “the fiscal year in which you turn X” can be off by a day if you don't know the principle that the year is added the day before your birthday.

Small facts about leap years

• Summer Olympic years often coincide with leap years, but the Games aren't held because it is a leap year
• February 29 is the 60th day of the year. In a leap year, every day from March onward falls one day later in the year than in a common year
• A leap year has 366 days, so when salaries or interest are prorated by day, the daily rate differs slightly from a common year
• For one-year contracts that include February 29, the end date may be governed by specific rules. Check the wording of the contract
• In software development, bugs caused by not accounting for February 29 keep turning up

Date calculation is error-prone by hand because months have different lengths and leap years add an exception on top. When counting days or ages, don't rely on mental arithmetic or counting on your fingers; keeping a record of the result makes it easier to double-check. How to count the days between two dates, leap years included, is covered in how to calculate the days between dates, and the specific steps for counting age are in how age is counted. Japan's spring and autumn equinox holidays, whose dates are also set by astronomical calculation, are covered in a separate guide to Japanese public holidays.

FAQ

How do you decide whether a year is a leap year?

A year divisible by 4 is a leap year. However, a year also divisible by 100 goes back to being a common year, and a year divisible by 400 is a leap year again. Applying these in order, 1900 is a common year, 2000 is a leap year and 2100 is a common year. From 2000 to 2096, divisibility by 4 alone is enough.

When is the next leap year?

2028. After that come 2032, 2036 and 2040, every four years. The first exception is 2100: it is divisible by 4 but also by 100, and not by 400, so it is a common year.

When do people born on February 29 age in a common year?

Even in a common year, they become a year older when February 28 ends. Japanese age calculation adds the year at the end of the day before the birthday, so in years without a matching date, February 28 is the day the year is added. It does not move to March 1. In the age & anniversary counter, compare the reference dates February 28 and March 1 to see this.

Why is a leap year skipped once every 100 years?

Because a year is actually 365.2422 days long, and adjusting only once every four years adds too much. With the four-year rule alone, the average year is about 0.0078 days too long, which adds up to about 0.78 days per century. Dropping one leap year every 100 years and restoring one every 400 years cuts the error to about 26 seconds per year.