test.el

(defun org-late-todo (n)
  "Switch todo assuming an old date [n days ago]"
  (interactive "nDays: ")
  (let* ((delta-time (days-to-time n))
         (now (time-subtract (current-time)
                             delta-time)))
    (letf ((symbol-function 'current-time) (lambda ()
                                             now))
      (org-agenda-todo))))