protected function getYears()
    {
        $intSecondsPerYear = ($this->isPositive()) ? DateTimeSpan::SECONDS_PER_YEAR : ((-1) * DateTimeSpan::SECONDS_PER_YEAR);
        $intYears = floor($this->intSeconds / $intSecondsPerYear);
        if ($this->isNegative()) {
            $intYears = (-1) * $intYears;
        }
        return $intYears;
    }