Check this post
Because you are passing string whereas date_diff expects datetime object,
$date_expire = '2014-08-06 00:00:00';
$date = new DateTime($date_expire);
$now = new DateTime();
echo $date->diff($now)->format("%d days, %h hours and %i minuts");
I’d suggest you check the values you’ve got stored. My hunch is $datetime2 isn’t the right format