How to calulate ther remaining months and days in PHP?

Question

I am trying to calculate the remaining months and days before a payment is due.

So if the last payment was received >= 2013-08-01 then I want to add 24 months from the last payment and then show how many months and days remaining before the next payment is due.

Here is what I have tried but the output is not correct:

$lastOB = new DateTime('2013-10-15');
$ny_min = new DateTime('2013-08-01');
$now = new DateTime('now');

if($lastOB > $ny_min){
    $due_date = $lastOB->add(new DateInterval('P24M'));
    $duedate_interval = $now->diff($due_date);
    //echo $duedate_interval->days;
    echo $duedate_interval->format('%m month, %d days');
} else {
    echo 'due now';
}

The current output is 2 months, 7 days.

The output should be the remaining of (2013-10-15 + 24 MONTHS) - TODAY's date.