org-set-created.el

(defvar ct/org-created-property-name "CREATED"
  "The name of the org-mode property that stores the creation date of the entry")

(defun ct/org-set-created-property (&optional active NAME)
  "Set a property on the entry giving the creation time.

By default the property is called CREATED. If given the `NAME'
argument will be used instead. If the property already exists, it
will not be modified."
  (interactive)
  (let* ((created (or NAME ct/org-created-property-name))
         (fmt (if active "<%s>" "[%s]"))
         (now (format fmt (format-time-string "%Y-%m-%d %a %H:%M"))))
    (unless (org-entry-get (point) created nil)
      (org-set-property created now))))