sub leapYear {

    my $year = shift;

    if ( $year % 400 == 0 ) {

        #  1. If it's divisible by 400: it's a leap year.
        return 1;        
    }
    else {
        if ( $year % 100 == 0 ) {

            #  2. If it's divisible by 100: it's not a leap year.
            return 0;    
        }
        else {

            #  3. Simple case: it's a leap year if it's divisible by 4
            return ( $year % 4 ) ? 0 : 1;
        }
    }
}