;; format-time-string is a function, not a variable
;; don't add hooks with setq, use add-hook
;; don't add lambdas to hooks as a general rule, it can get messy later.

(defun html-helper-deutsche-timestamp ()
  (insert (format-time-string "    Letzte Änderung:  %d. %m. %Y")))

(add-hook 'html-helper-timestamp-hook 
	  'html-helper-deutsche-timestamp)