The Publication Date module adds a "Published on" date to each node, containing the date when it was first published.
Without this, the only dates available for nodes are their created date and changed date, but these are often insufficient to meet the needs of publishing workflows. Sorting nodes by their created date doesn't account for content which is held in draft form for a time while being readied for publication. Sorting nodes by their changed date fails to account for content needing minor corrections after being published.
The "Published on" date can be used in any templates, or views. Publication date also incudes integration with the Display Suite module and has entity wrapper support, so it can integrate with modules like Rules that use the Entity API module.
By default, the "Published on" date is set automatically when a node is first published, but it can also be edited through the node edit form, allowing a custom published date to be set. This is useful when backdating migrated content from an older site. The module provides permisions for restricting this field, per content type. If a custom date is set prior to publication, it will be respected when the node is published. The publication date is also retained when a node is unpublished and then republished.
This will add a new $node->published_at field to nodes, containing the published date, or 'false' if no published date has been set. You can then use this field in any templates. Moreover, this field is available in the views parameters.
IMPORTANT: For nodes that were published BEFORE the installation of this module, we can not know the exact date of publication, so $node->published_at will initially contain the creation date.
Please pay special attention to the README and CHANGELOG, as there are some changes to the way the Published On date is handled. Notably in 2.x, once the date has been set for a node it will persist until manually changed, even if the node is unpublished and republished. In version 1.0 the date was lost when the node was unpublished.
Other modules can use hook_publication_date_alter() to change the publication date when it is set. See publication_date.api.php for more documentation.
The Publication Date module was originally written by Emmanuelle Gouleau and Tristan Marly from Clever Age (http://www.clever-age.org).
D7 version initially ported by joostvdl (https://drupal.org/user/115719) and maintained by dgtlmoon (https://drupal.org/user/25027).
Publication Date is currently maintained by jstoller (https://drupal.org/user/99012).