I am pretty sure you can use a mysql stored procedure to help do this, but I have used a quick and dirty method in the past.
Keep the timestamp field for when the record is updated, with ON_UPDATE Current Timestamp as the field attribute.
Set the dateadded field as date time.
on the php page, set a php variable to the current time
<?php $dateandtime = date("Y-m-d H:i:s");?>
use the $dateandtime variable in your insert record server behavior