PHP使用Laravel 5.2和Carbon没有正确添加月份

我创建了一个计算贷款申请摊销时间表的方法。我使用Carbon为用户每次支付贷款增加一个月，下面是我的代码片段：

 public function amoritization($amount,$interest_rate,$repaid_months,$start_date)
{
    $beginning_balance = $amount;
    $interest = $interest_rate/100/12;
    $payback = $repaid_months;
    $x = pow(1 + $interest,$payback);
    $monthly_payment = ($beginning_balance*$x*$interest)/($x-1);
    $payment_date = new Carbon($start_date) 

    $info = (object)[]; //convert empty array to an object
    $amoritization = [];
    $count = 0;
    while($payback > 0){
        $total_interest = $interest * $beginning_balance; //interest
        $principal_payment = $monthly_payment - $total_interest; //principal
        $ending_balance = $beginning_balance - $principal_payment; //ending balance
        $info->beginning_balance = (float)$beginning_balance;
        $info->total_interest = $total_interest;
        $info->payment = $monthly_payment;
        $info->principal_payment = $principal_payment;
        $info->ending_balance = $ending_balance;
        $info->payment_date  = $payment_date->addMonths($count++);
        array_push($amoritization,$info);
        $beginning_balance = $ending_balance;
        $info = (object)[];
        $payback--;
    }
    return $amoritization;
}

这个方法在我的控制器中调用如下：

  return $this->calculation->amoritization($data['amount'],$data['interest_rate'],$data['loan_term'],$data['start_date']);

这是一个问题，例如，$start_date变量存储了这个值2016-04-14 10:34:56，$return变量的值为6。在while循环语句中，$info对象添加了payment_date的属性，该属性存储添加的月份：

 $info->payment_date  = $payment_date->addMonths($count++);

当while循环语句循环一次时，我应该得到的值是2016-05-14。然而，这是我从回复中得到的：

payment_date :{date: "2016-10-14 10:34:56.247000", timezone_type: 2, timezone: "Z"}

此外，当我在循环中这样做时：

return    $info->payment_date;

我找回了价值2016-05-14。这可能是什么原因造成的？提前感谢您的帮助，