- floor: documentation ( source)
- date: documentation ( source)
- strtotime: documentation ( source)
<?php
$cadastro = new DateTime( date('2014-04-01') );
$expira = $cadastro->modify('+182 day');
$today = date('m/d/Y');
$today = strtotime($today);
$finish = strtotime($expira);
//difference
$diff = $finish - $today;
$daysleft=floor($diff/(60*60*24));
echo "$daysleft days left";
?>