It all depends on what type of field you are inserting into... Just define the variable before you insert it... There are other methods including MySQL now() definition, but this could be how to do it in PHP.
$posted = time(); //Unix Timestamp format for current date
// OR
$posted = date("Y-m-d"); // YYYY-MM-DD format for current date
$result = mysql_query ("INSERT INTO feedback (`id`,``username`,'posted`,`comment`) Values('','$username','$posted','$comment)") or die("INSERT error: ".mysql_error());