Use this:
$dateString = get_post_meta($post->ID,'dfd_ModificationTimestamp',true);
/* @var \DateTime $date */
$date = DateTime::createFromFormat('d/m/Y H:i:s', $dateString);
Now $date is DateTime object. You can modify it as you wish.
For example, you need 3 days ago from your date value(13/06/2015 11:02:18)
$date->modify('-3 days'); // $date becomes 10/06/2015 11:02:18
or if you need 2 months ago from your date value(13/06/2015 11:02:18)
$date->modify('-2 months'); // $date becomes 13/04/2015 11:02:18
To print $date value after modify, use:
echo $date->format('d/m/Y H:i:s');
See http://php.net/manual/en/class.datetime.php for more custom information.