(defun insert-date (prefix)
    "Insert the current date. With prefix-argument, use ISO format. With
   two prefix arguments, write out the day and month name."
    (interactive "P")
    (let ((format (cond
                   ((not prefix) "%d.%m.%Y")
                   ((equal prefix '(4)) "%Y-%m-%d")
                   ((equal prefix '(16)) "%A, %d. %B %Y")))
          (system-time-locale "de_DE"))
      (insert (format-time-string format))))
(global-set-key (kbd "C-c d") 'insert-date)