org-archive-subtree-defadvice.el

;(setq org-archive-location "%s_archive::date-tree")
(defadvice org-archive-subtree
  (around org-archive-subtree-to-data-tree activate)
  "org-archive-subtree to date-tree"
  (if 
      (string= "date-tree" 
	       (org-extract-archive-heading 
		(org-get-local-archive-location)))
      (let* ((dct (decode-time (org-current-time)))
	     (y (nth 5 dct))
	     (m (nth 4 dct))
	     (d (nth 3 dct))
	     (this-buffer (current-buffer))
	     (location (org-get-local-archive-location))
	     (afile    (org-extract-archive-file location))
	     (org-archive-location 
	      (format "%s::*** %04d-%02d-%02d %s" afile y m d
		      (format-time-string "%A" (encode-time 0 0 0 d m y)))))
	(message "afile=%s" afile)
	(unless afile
	  (error "Invalid `org-archive-location'"))
	(save-excursion
	  (switch-to-buffer (find-file-noselect afile))
	  (org-datetree-find-year-create  y)
	  (org-datetree-find-month-create y m)
	  (org-datetree-find-day-create   y m d)
	  (widen)
	  (switch-to-buffer this-buffer)
	  )
	ad-do-it)
    ad-do-it))

I think this hack is obsolete now that there is the datetree/ option for org-archive-location - see: http://orgmode.org/worg/doc.html#org-archive-location