Personally, I seldom use MySQL's date & time types. You can, of course. I use a "bigint" field that holds a UNIX timestamp, and do all my date conversion(s) via PHP.
To do it your way, probably this should be close (as an example):
$posted=date("YmdHis");
$success=mysql_query("update mytable set date_added='$posted' where whatever is foo");
In PHP, I just do:
$now=time();
// then insert $now into a bigint field during the db stuff
HTH,