Drafts already stores the creation date, it can be seen in the (i) detail screen, and it is available for use in actions when you output with the [[created]] tag (more on templates). So there may not be a need for you to clutter your text with that value.
If you do want it to appear in your text, there is no way to automatically add it. You could create new drafts using templates to have that value inserted, or use actions to insert the text when you start the draft.