I often update my task descriptions. We haven’t found a neat way to do
this in-buffer, so I use planner-edit-task-description. However, if I
update the task description or replan a task, my timelog data gets out
of date. This code snippet updates all matching tasks in the timelog,
and can serve as an example for code that updates things after a task
is edited.

(defadvice planner-replan-task (around sacha/planner-timeclock activate)
  "Update the timelog as well. Warning! Do not have duplicate tasks!"
  (let ((info (planner-current-task-info)))
    ad-do-it
    (with-current-buffer (find-file-noselect timeclock-file)
      (goto-char (point-min))
      (while (re-search-forward
              (concat
               "^. [^ ]+ [^ ]+ "
               "\\("
               (regexp-quote (planner-task-plan info))
               "\\)"
               ": "
               (regexp-quote (planner-task-description info))
               "$")
              nil t)
        (replace-match (ad-get-arg 0) t t nil 1))
      (save-buffer)
      (kill-buffer (current-buffer)))))

(defadvice planner-edit-task-description (around sacha/planner-timeclock activate)
  "Update the timelog as well. Warning! Do not have duplicate tasks!"
  (let ((info (planner-current-task-info)))
    ad-do-it
    (with-current-buffer (find-file-noselect timeclock-file)
      (goto-char (point-min))
      (while (re-search-forward
              (concat
               "^. [^ ]+ [^ ]+ "
               (regexp-quote (planner-task-plan info))
               ": "
               "\\("
               (regexp-quote (planner-task-description info))
               "\\)"
               "$")
              nil t)
        (replace-match (ad-get-arg 0) t t nil 1))
      (setq planner-timeclock-current-task (ad-get-arg 0))
      (save-buffer)
      (kill-buffer (current-buffer)))))