// find the time difference between now and the expiry date
$expiry_date = '2017-04-20 09:02:23';
$then = new DateTime($expiry_date);
$now = new DateTime();
$remaining_time = $then->diff($now);
// I want to add two weeks to the new date.
$two_weeks = $now->add(new DateInterval('P2W'))->format('Y-m-d H:i:s');
// new date that combines the remaining time from above and adds 2 weeks to create a new date. This is the issue here. How do I combine these two dates properly? The new date has to be in this format ('Y-m-d H:i:s')
$new_date = $remaining_time + $two_weeks;