You have probably heard of org mode, the tool which helps keep you organized. It is really great, and you should check it out. One of its less widely-known features is habit tracking. Normally, org mode gives you the ability to mark org entires as TODO items, change them to done, etc. This works really well, but it somewhat falls apart for things that we want to do regularly.

The org-habits module allows you to make TODO items into habits. When you set one of these items to DONE, a log entry gets created, and the habit due date gets moved forward in time based on the repeater interval.

The problem, though, is that I often don't get around to setting the TODO item to done when I actually do it. I am not always at my computer, after all.

Dynamic binding to the rescue! We could handle this in a much more complicated way, but the easiest solution is to just "pretend" that it is actually yesterday while the code that changes the TODO is run.
Because of dynamic binding, we can be make the change really easily.

The code below that implements this uses some rather specific elisp features, so I heavily commented it to help, but the part that is important is at the end.

(defun org-todo-toggle-yesterday ()
  ;; this function is interactive, meaning a "command" that we call
  ;; as an emacs user (allows us to do "M-x org-todo-toggle-yesterday")
  (interactive)

  (let ((time-in-question (decode-time))) 
    ;; time-in-question is the current time, decoded into convenient fields

    ;; decrease the field by one which represents the day -- make it "yesterday"
    (decf (nth 3 time-in-question))

    ;; now, re-encode that time
    (setq time-in-question (apply 'encode-time time-in-question))

    (flet ((current-time () time-in-question))
      ;; flet temporarily binds current-time to this version, which
      ;; returns the time from yesterday 

      (org-todo)
      ;; toggles the todo heading
      )))

Most of this code just helps with getting a date for the previous day. The important part is the last flet, which temporarily changes the definition of current-time for those let body calls.

Okay, hope this helps anyone who uses org mode and the habits module. Dynamic binding lets this solution work well, and so is not nearly as problematic as could be seen.