By default, the Publish At date/time of an article will be set when the publish checkbox is checked.

A user can input a Publish At time and date in the future or past. This date will not be overwritten by checking the publish box.

You can also configure your site to set the Publish At date/time of a new article when the article is created. Set the system setting default.post.date.enabled to 'true' if you would like to enable the default value.