Allowing role permissions to a content type based on the date I'd like to allow a certain role can add new content of a certain content-type ONLY between two dates. Before or after these dates the role shouldn't be able to create a new content of this content-type.

Is it possible through the Drupal core or any module? I haven't found anything about it.
You can implement hook_node_access() (This is now a real hook in Drupal 7) and implement whatever you want.