In Sitefinity 4.2.1679 we added a feature to the Migration Module that allows the change of a publication date for some content item. In order to use it include the following namespace:
and then call
where manager is the appropriate content manager (e.g. BlogsManager) and publicationDate is the new publication date.
In 4.2 SP1 we added a feature that allows you to set publish/unpublish date in the future. When you create a blog post click on More actions -> Publish/Unpublish on Specific Date. I attached a screenshot with the dialog.Kind regards,