Changing TODO status on non-current date

[Marcin Borkowski <mbork@mbork.pl>]

Hi all,

assume that I have a habit (or other TODO headline where the /date/ of
converting to DONE is important), and I completed it, say, at 23:30 and
forgot to change the status immediately.  Is there any way to tell Org
that the change was not today, but e.g. yesterday or two days ago etc.?
This is especially important with habits, since the date information is
then recorded in two places, so changing it manually is a nuisance.

I assume that what I'm after could be done with advice and cl-letf-ing
current-time and/or current-time-string, but I'm wondering whether
someone has it solved already.  OTOH, org-todo already has a bunch of
prefix argument options (up to triple C-u), so I'd have either to resort
to /quadruple/ C-u (not good UI) or something else completely - any
ideas?  Maybe a /negative/ argument, or just C-- C-c C-t?  (Of course,
I'd also need doing this from the agenda, so org-agenda-todo might also
need some tweaking.)

TIA,

-- 
Marcin Borkowski
http://octd.wmi.amu.edu.pl/en/Marcin_Borkowski
Faculty of Mathematics and Computer Science
Adam Mickiewicz University