WordPress does not require $post_date or $post_date_gmt to be set if the status of the post is ‘draft’. The problem is the database doesn’t distinguish between post types and will not accept an empty value. Simply removing if ( 'draft' != $post_status ) from within the if statements that set the $post_date and $post_date_gmt variables solves the problem. After the change, regardless of the post status, the timestamps are created and stored.
The file that needs changed is functions-post.php found in the wp-includes folder. Hope that helps.
Ryan
