# 2017 is not a leap year, it is a common year.

## February 29th, 2017 is not a valid date.

The last leap year before 2017: 2016.

The next leap year after 2017: 2020.

### Each year that is evenly divisible by 4 (remainder is zero when divided by 4) is a leap year, unless it is also evenly divisible by 100 - but even in this case it is a leap year if it is also evenly divisible by 400. A maximum of three steps to check.

## Details below.

## The leap year algorithm. The three conditions to check in order to know whether a year is a leap or a common one.

#### Briefly:

#### Each year that is evenly divisible by 4 (remainder is zero when divided by 4) is a leap year, unless it is also evenly divisible by 100 - but even in this case it is a leap year if it is also evenly divisible by 400. A maximum of three steps to check.

#### See the three steps to check, below, with explanations:

### 1) A leap year is evenly divisible by 4

(remainder is zero when divided by 4):

#### 2017 is not evenly divisible by 4.

#### We could have stopped at this step.

#### Our year is not a leap year but a common one.

#### It does not meet this condition.

### 2) Except if it is evenly divisible by 100 then it isn't a leap year:

#### 2017 is not evenly divisible by 100.

### 3) Except if it is evenly divisible by 400, then it's a leap year:

#### 2017 is not evenly divisible by 400.

## 2017 is not a leap year, it is a common year.

What is a leap year?

#### A leap year has 366 days.

2017, being a common year, has 365.

#### In a leap year month February has 29 days.

February 29th is a valid date.

In a common year month February has 28 days.

February 29th, 2017 does not exist.

### How often do the leap years occur?

#### Nearly once every four years is a leap year. More exactly, leap years occur 97 times in every cycle of 400 years.

### Why do we need leap years in our calendar?

#### Leap years keep the calendar year in sync with the astronomical year, preventing us, for example, from celebrating Christmas in the month of November.

### Note

#### The first year in the actual calendar (also called Gregorian, modern, civil) was 1582. Before this year another calendar (Julian) was in use, with different rules. Not all the countries adopted the Gregorian calendar at the same time, in 1582. The adoption process took hundreds of years and is not even now complete. The actual calendar (also called Gregorian, modern, civil) will get out of sync by 1 day with the astronomical calendar around the year of 4818 (after ≈ 3236 of years from its creation in 1582).

## More operations of this kind: