Calculate a due date in PHP

One of my co-workers recently installed an open-source script into our billing system. The purpose is to re-calculate the next due date for a service when the customer's service is paid for late. Example being service is due 11/20/2012, is suspended the next day, and then they pay for it 11/25/2012. This script is supposed to update the next due date to 12/25/2012, but is malfunctioning and changing (monthly) dates that should be 12/25/2012 to 1/25/2013 for some reason.

I think the issue is with the way the $month variable is being handled, but can't quite figure out where the issue is.

function calculate_postpone_due_date($billingcycle)
{
switch($billingcycle)
{
case "Monthly": $months = 1; break;
case "Quarterly": $months = 3; break;
case "Semi-Annually": $months = 6; break;
case "Annually": $months = 12; break;
case "Biennially": $months = 24; break;
case "Triennially": $months = 32; break;
default: $months = 0; break;
}

//we return FALSE for any other billing cycles: "One Time", "Free Account" etc, they are not recurring
if ($months == 0)
return FALSE;

//a bit complex calculation based on day of the month
//exactly like native whmcs logic do
$year = date("Y");
$month = date("m");
$day = date("d");
for ($i=1; $i<=$months; $i++)
{
$month++;
if ($month == 13)
{
$month = 1;
$year++;
}
}

if (checkdate("$month", $day, $year))
{
return "$year-$month-$day";
}
else
{
//getting last day of the month
$last_day = date("t", mktime(0, 0, 0, $month, 1, $year));
return "$year-$month-$last_day";
}
}

Try this (not tested):

function calculate_postpone_due_date($billingcycle)
{
switch($billingcycle)
{
case "Monthly": $months = 1; break;
case "Quarterly": $months = 3; break;
case "Semi-Annually": $months = 6; break;
case "Annually": $months = 12; break;
case "Biennially": $months = 24; break;
case "Triennially": $months = 32; break;
default: return FALSE;
}

$expires = new DateTime('plus '.$months.' months');
$expires->modify('last day of this month');
return $expires->format('Y-m-d');
}

function calculate_postpone_due_date($billingcycle)
{
switch($billingcycle)
{
case "Monthly": $months = 1; break;
case "Quarterly": $months = 3; break;
case "Semi-Annually": $months = 6; break;
case "Annually": $months = 12; break;
case "Biennially": $months = 24; break;
case "Triennially": $months = 36; break;
default: $months = 0; break;
}


if ($months == 0)
return FALSE;

$today = date('Y-m-d');
$next_due_date = strtotime($today.' + '.$months.' Months');
return date('Y-m-d', $next_due_date);

}

function countDueDate($policy_start_date,$months){
$next_due_date = strtotime($policy_start_date.' + '.$months.' Months');
if($next_due_date<time()){
return countDueDate(date('Y-m-d', $next_due_date), $months);
}else{
return date('Y-m-d',$next_due_date);
}
}
function getModeMonth($premium_mode){
switch ($premium_mode){
case 'yearly': $q=12;break;
case 'monthly': $q=1;break;
case 'quarterly': $q=3;break;
case 'half year': $q=6;break;
default : $q=12;break;
}
return $q;
}
$date=countDueDate(date('Y').'-'.date('m-d',strtotime($policy_start_date)), getModeMonth($premium_mode));