Planner uses timeclock.el, which is part of GNU Emacs. To make sure that all
of the relevant libraries are loaded when you start Emacs, add the following
to your ~/.emacs:

   (require 'planner-autoloads)
   (require 'planner-timeclock)

To start the clock on a task, type C-c C-i (planner-task-in-progress).
The task will be marked as “in progress” and the timer will start. To
clock out, mark the task as done (C-c C-x, planner-task-done) or clock
out manually with C-c C-o (timeclock-out).

To see a summary of your time use for any day, call M-x
planner-timeclock-summary. To see a summary of your time use for a
range of days, call M-x planner-timeclock-summary-show-range. The
range function is particularly useful for weekly time reports, which I
find myself now doing.

For even more fun, call M-x
planner-timeclock-summary-show-range-filter, which filters plan pages
and descriptions using the specified regular expression. For example,
to show only the time entries from the last seven days that included
the keyword “@work”, call M-x
planner-timeclock-summary-show-range-filter and specify “@work” as the
filter, “-7” as the start date, and “.” for the end date. You’ll get a
quick view of all your work-related time-clock entries.

If you use this often, you might want to create a shortcut to show the
timeclock summary for the current file. The following code defines a
function that displays the timeclock summary for the current page (day
or plan), and binds this function to C-c C-s in Planner mode files.

(require 'planner-timeclock-summary)
(require 'planner-timeclock-summary-proj)
(defun sacha/planner-timeclock-summary-show (page)
  "Show the timeclock summary for PAGE (defaults to the current page)."
  (interactive (list (planner-page-name)))
  (if (string-match planner-date-regexp page)
      ;; Day page
      (planner-timeclock-summary-show page)
    ;; Plan page
    (let ((data (planner-timeclock-proj-make-alist page)))
      (switch-to-buffer (get-buffer-create planner-timeclock-summary-buffer))
      (erase-buffer)
      (insert (planner-timeclock-proj-build-report data)))))
(define-key planner-mode-map (kbd "C-c C-s" 'sacha/planner-timeclock-summary-show))

This allows you to see how much time you’ve spent for the current day
or project.

If you like seeing this, you might want the timeclock information
automatically inserted into your Planner pages. To do so, add the following to your ~/.emacs:

(planner-timeclock-summary-proj-insinuate) ; For plan pages
(planner-timeclock-summary-insinuate)  ; For day pages
;; It's easier when both of them overwrite the same report
(setq planner-timeclock-summary-section
      "Timeclock"

      planner-timeclock-summary-proj-section
      "Timeclock"

      ;; Modify this to include your other template sections
      planner-day-page-template
      "* Tasks\n\n\n* Schedule\n\n\n* Timeclock\n\n\n* Notes\n\n\n"

      planner-plan-page-template
      planner-day-page-template)

The sections are updated only if they exist, so you can remove the
sections or add them to your Planner files any time you want.

CAUTION: Because the time information is stored in a separate file,
Planner gets confused if you edit the task details. Make sure you use
the built-in functions for editing the task description
(planner-edit-task-description) or changing the project associated
with a task (planner-replan-task).

Random Emacs symbol: planner-expand-name – Function: Expand the given NAME to its fullest form.