I’m now an official contributor and my part of ERC can be merged into GNU Emacs eventually. Yay!

(defun sacha/planner-diary-schedule-task (time)
  "Add a diary entry for the current task at TIME."
  (interactive "MTime: ")
  (save-window-excursion
    (save-excursion
      (save-restriction
        (let ((info (planner-current-task-info)))
          (sacha/planner-diary-add-entry
           (planner-task-date info)
           (concat time " | " (planner-task-description info) "")))))))

(defun sacha/planner-diary-add-entry (date text &optional annotation)
  "Prompt for a diary entry to add to `diary-file'."
  (interactive
   (list
    (if (or current-prefix-arg
            (not (string-match planner-date-regexp (planner-page-name))))
        (planner-read-date)
      (planner-page-name))
    (read-string
     "Diary entry: ")))
  (save-excursion
    (save-window-excursion
      (make-diary-entry
       (concat
        (let ((cal-date (planner-filename-to-calendar-date date)))
          (calendar-date-string cal-date t t))
        " " text
        (or annotation
            (let ((annotation (run-hook-with-args-until-success
                               'planner-annotation-functions)))
              (if annotation
                  (concat " " annotation)
                "")))))
      (planner-goto date)
      (planner-diary-insert-diary-maybe))))

../../notebook/emacs/planner-config.el