You can simply use datetime diff and format for calculating difference.
<?php
$datetime1 = new DateTime('2009-10-11 12:12:00');
$datetime2 = new DateTime('2009-10-13 10:12:00');
$interval = $datetime1->diff($datetime2);
echo $interval->format('%Y-%m-%d %H:%i:%s');
?>
For more information OF DATETIME format, refer: here
You can change the interval format in the way,you want.
Here is the working example
P.S. These features( diff() and format()) work with >=PHP 5.3.0 only