Get Age from BuddyPress datebox field data

bpdev_get_age_from_dob

/**
 * Get Age from BuddyPress date of Birth
 * @param string $dob_field_name :name of the DOB field in xprofile, like Dob or Date of Birth
 * @param int $user_id : the user for which you want to retrieve the age
 * @param string $format: the way you want to print the difference, look t <http://php.net/manual/en/dateinterval.format.php> for the acceptable agrs 
 * @return string :the formatted age in year/month
 */

function bpdev_get_age_from_dob($dob_field_name,$user_id=false,$format="%y Years, %m Month(s), %d days"){
    if(!$user_id)
        $user_id=bp_displayed_user_id ();
    
    $dob_time=xprofile_get_field_data($dob_field_name, $user_id);//get the datetime as myswl datetime
    
    $dob=new DateTime($dob_time);//create a DateTime Object from that
       
    $current_date_time=new DateTime();//current date time object
    //calculate difference
    $diff= $current_date_time->diff($dob);//returns DateInterval object
    //format and return
    return $diff->format($format);
    
  
    
}



/*usage

echo "Your Age".bpdev_get_age_from_dob("Dob",bp_loggedin_user_id());
//and so on

*/

I got an error when put in echo:
Fatal error: Call to undefined method DateTime::diff() in /home/dom/public_html/wp-content/plugins/buddypress/bp-themes/bp-default/functions.php on line 377

Line 377 is this – $diff= $current_date_time->diff($dob);//returns DateInterval object

Any idea how to correct that?
Thanks.