This is a UNIX epoch datetime format i.e. number of seconds elapsed since 1st Jan, 1970. To take average of the two dates, it is similar to any other numeric average.
$avg_date = $site_calls->call_date + (($site_calls->call_date2 - $site_calls->call_date)/2);
$call_date = date("m/d/y",$avg_date);