best way to change defconst value for a particular session? (emacs)

Question

Hi, I want to change the value of

(defconst org-time-stamp-formats '("<%Y-%m-%d %a>" . "<%Y-%m-%d %a %H:%M>")
  "Formats for `format-time-string' which are used for time stamps.
It is not recommended to change this constant.")

to

'("<%Y-%m-%d %a>" . "<%H:%M>")

Not always, but for particular org-mode sessions, even while knowing that defconst values are not really meant to be changed. I wonder if there's a good way to do this?

Thanks...

Edit: I guess my main goal is to be able to be able to insert a time-stamp which contains hours and minutes only, which could similarly be accomplished by defadvice on org-time-stamp, or some other means?

Answer 1

You can use advice to solve this problem.

(defadvice org-time-stamp (around org-time-stamp-new-format activate)
  "change the org time-stamp when desired"
  (let ((org-time-stamp-formats '("<%Y-%m-%d %a>" . "<%H:%M>")))
    ad-do-it))
;; control whether it is active via
;; M-x ad-activate org-time-stamp
;; M-x ad-deactivate org-time-stamp

Or, you can set a variable that controls the behavior:

(defvar use-new-org-timestamp t)
(defadvice org-time-stamp (around org-time-stamp-new-format activate)
  "change the org time-stamp when desired"
  (let ((org-time-stamp-formats (if use-new-org-timestamp
                                    '("<%Y-%m-%d %a>" . "<%H:%M>")
                                  org-time-stamp-formats)))
    ad-do-it))