Control time of addition of posts to RSS?

Your question is pretty minimal but I will give it a shot.

1. You would want to add a meta box for your value.
2. On save_post save the data to $wpdb->postmeta
3. Filter your feed using pre_get_posts and/or posts_where as
   needed.

A very basic code outline:

function generic_box() {
    add_meta_box(
        'generic_box', // id, used as the html id att
        __( 'Generic Title' ), // meta box title
        'generic_cb', // callback function, spits out the content
        'post', // post type or page. This adds to posts only
        'side', // context, where on the screen
        'low' // priority, where should this go in the context
    );
}
function generic_cb() {
  echo 'generic content';
}
add_action( 'add_meta_boxes', 'generic_box' );

function generic_save($post_id) {
  // use $post_id and the global $_POST variable to process information
  // use update_post_meta() to save information
}
add_action('save_post','generic_save');

function generic_pre_get_posts($qry) {
  if (is_feed()) {
    // alter your query
  }
}
add_action('pre_get_posts','generic_pre_get_posts');