function daz_custom_field_on_publish( $new, $old, $post ) {

	$unixtime = date(strtotime("+1 week"));
  $now = date("m/d/Y",$unixtime);

    if ( $new == 'publish' && $old == 'pending') {
             update_post_meta( $post->ID, '_post_expiration_date', $now );
    }
}
add_action( 'transition_post_status', 'daz_custom_field_on_publish', 10, 3 );