The following code sorts completed tasks in reverse-chronological
order and incomplete tasks in chronological order. This makes it
easier to see the next action (top of list) and the last action (first
completed task).

(defun sacha/planner-sort-tasks-by-date ()
  "Sort tasks by date, status and priority."
  (skip-chars-forward "#ABC")
  (let ((ch (char-before))
        status)
    (skip-chars-forward "0123456789 ")
    (setq status (char-after))
    (goto-char (line-end-position))
    (skip-chars-backward "]) ")
    (format "%1c%1c%10s"
            (cond
             ((= status ?o) ?1)
             ((= status ?X) ?3)
             ((= status ?C) ?4)
             (t ?2))
            ch
            (let ((date (or (planner-task-date (planner-current-task-info))
                            planner-sort-undated-tasks-equivalent)))
              (if (or (= status ?X)
                      (= status ?C))
                  (sacha/planner-invert-date date)
                date)))))

(defun sacha/planner-invert-date (date)
  "Reverse the date in the sorting order."
  (mapconcat (lambda (ch)
               (if (string= ch ".")
                   ch
                 (number-to-string
                  (- 9 (string-to-number ch)))))
             (split-string date "" t) ""))

pos://../emacs/planner-config.el#35188