Maybe because its 2013-01-29 so +1 month would be 2013-02-29 which doesn't exist so it would be 2013-03-01
You could try
date('m/d/y h:i a',(strtotime('next month',strtotime(date('m/01/y')))));
from the comments on http://php.net/manual/en/function.strtotime.php
