$first_date = "2014-02-14 10:21:02";
$second_date = "2014-02-14 08:21:02";

$first_time = strtotime($first_date);
$second_time = strtotime($second_date);

$rand_time = rand($first_time, $second_time);
$rand_date = date('Y-m-d g:i:s', $rand_time);

echo $rand_date;