Publish and unpublish date are used by cronjob to create or remove objects ( including all nodes ), but you have to enable it, because it's not enabled by default.
Thank you for your replay Thiago. I have a doubt , Suppose I have referred a node to blocks in frontpage , the time when node become unpublished (by specifying the time in unshedule attribute), automatically the reference will be removed or node will not be available in the frontpage.
Any idea will be helpfull . Thanks in advance. Romeo