The post_date
and post_date_gmt
serves as the date that the post was created. For scheduled posts this will be the date on which the post is scheduled to be published.
There is no reliable native method to determine the date when a scheduled post was added. For scheduled posts, you can try the post_modified
or post_modified_gmt
dates as this will correspond when the post was first added. You have to remember, this is unreliable as this date will change when a post is modified
Another (unreliable as well) method, is to use revisions. If you have revisions enabled, when a post is scheduled or published, a post revision is saved. That revision, which will be saved after the scheduled post when the scheduled button is clicked, will hold the current post date on which the scheduled button is clicked. Check the pic below
If you need a reliable way of doing this, you will need to write your own custom function to save the exact time when the scheduled button is clicked to schedule a post