PHP添加方法工作不正确

我正在学习PHP的过程中，我遇到了一些麻烦。我的功能是返回"里程碑"与他们插入相同的日期。我认为我错误地使用了add()方法。谢谢。

PHPplayground: http://www.tehplayground.com/cARB1wjth

$milestones = null;
$milestones = createMilestone($milestones, true, 10, "15-1-1", "birthday" );
var_dump( $milestones );
function createMilestone($milestones, $forward, $days, $startDate, $milestoneName ){
    if ( is_string($startDate)){
        $date = DateTime::createFromFormat("Y-m-d", $startDate );
    }else if(is_array($startDate) ){
        $date = $startDate["date"];
    }else{
        $date = $startDate;
    };
    $daysInterval = DateInterval::createFromDateString($days);
    if ($forward){
        $date->add($daysInterval);
    }else{
        $date->sub($daysInterval);
    }

    $milestones[$milestoneName]['date'] = $date;
    return $milestones;
}