と、

これを書こうとしたら emacs に

Use M-x make-dir RET RET to create the directory

と言われたので月が変わったことを知る。

alias today='mule -nw ~/diary/`date +%Y/%m/%d`.tdf'

でも、

(defun today ()
  "open today's diary"
  (interactive)
  (let
      ((format-time-month-list
	'(("Zero" . 0)
	  ("Jan" . 1) ("Feb" . 2) ("Mar" . 3) ("Apr" . 4) ("May" . 5)
	  ("Jun" . 6) ("Jul" . 7) ("Aug" . 8) ("Sep" . 9) ("Oct" . 10)
	  ("Nov" . 11) ("Dec" . 12)))
       (time-string (current-time-string))
       year month day path)
    (setq year
	  (string-to-int
	   (substring time-string 20 24)))
    (setq day
	  (string-to-int
	   (substring time-string 8 10)))
    (setq month
	  (cdr
	   (assoc
	    (substring time-string 4 7)
	    format-time-month-list)))
    (setq path
	  (format "%s/diary/%4d/%02d/%02d.tdf"
		  (expand-file-name "~") year month day))
    (find-file path)))

もいい。