Rules content unpublish in 30 days I've followed this thread https://groups.drupal.org/node/240678#comment-782543, but that's based on a date field and it has a different scenario.

My idea is: When a new content is created, after 30 days will be automatically unpublished. 

How can I achieve this without using additional date fields?
Instead use Auto Expire module, Using this module you can set the node to unpublish after 30 days.



For your case set zero in purge field, otherwise it will automatically delete from database.


  This module allows site administrators to set expiration times for
  content types.
  
  End users creating content that has auto expiry set cannot control the
  expiry time. They could be allowed to renew the content for one or
  more cycles.