How to Calculate Leap Years

Some software must know if the current or other specific year is a leap year. Not just calendar software, but any that requires knowing how many days are in a year or how many days are in a particular month of February.

This tutorial presents steps to do the calculations manually. Following that is code for doing it with PHP, JavaScript, and Perl.

There are 3 rules for doing it manually. The rules need to be done in order.

With those rules, code can be written with pretty much any computer programming language to determine whether or not a specific year number is a leap year. Code for PHP, JavaScript, and Perl are further below.

For the three represented programming languages, each has a function coded that is named IsLeapYear and returns either true (or the digit 1) or false (or the digit 0). Each step of the function might return true or false. If a step does not return, then the next step is consulted.

This is the logic implemented in the functions:

1. If there is a remainder when dividing the year by 4, then it is not a leap year. The function returns false;

   But if there is no remainder, it might be a leap year and the next step runs.

2. If there is a remainder when dividing the possible leap year by 100, then it is a leap year. The function returns true;

   But if there is no remainder, it might still be a leap year and the next step runs.

3. If there is a remainder when dividing the likely leap year by 400, then it is not a leap year. The function returns false;

   But if there is no remainder, it is a leap year and the next step runs.

4. The function returns true because it is indeed a leap year.

The above is fairly straight-forward for logic applicable to all programming languages represented in this article.

I've seen one-line leap year code, and some that use language built-in function names. But I wanted a logic that can be implemented with pretty much any programming language.

Determining Leap Year with PHP

Here is the PHP-coded version of the IsLeapYear function.

function IsLeapYear($year)
{
   if( $year % 4 ) { return false; }
   if( $year % 100 ) { return true; }
   if( $year % 400 ) { return false; }
   return true;
}

Example of use, assigning the number of days in February:

$February = IsLeapYear(2024) ? 29 : 28;

Determining Leap Year with JavaScript

Here is the JavaScript-coded version of the IsLeapYear function.

function IsLeapYear(year)
{
   if( year % 4 ) { return false; }
   if( year % 100 ) { return true; }
   if( year % 400 ) { return false; }
   return true;
}

Example of use, assigning the number of days in February:

var February = IsLeapYear(2024) ? 29 : 28;

Determining Leap Year with Perl

Here is the Perl-coded version of the IsLeapYear function (or subroutine, as it is called in Perl).

sub IsLeapYear
{
   my $year = shift;
   return 0 if $year % 4;
   return 1 if $year % 100;
   return 0 if $year % 400;
   return 1;
}

Example of use, assigning the number of days in February:

my $February = IsLeapYear(2024) ? 29 : 28;

When you implement one of the above in your own software, here is a list of the current and next 20 years you can use to compare the output of your implementation. The leap years are marked with a √ check mark.

The same logic as used with the three represented programming languages is likely to be possible for other programming languages.

(This article first appeared with an issue of the Possibilities newsletter.)

Will Bontrager