No, this still doesn't work. I just tested with my .emacs having only two lines:
#+BEGIN_SRC emacs-lisp
(require 'org "/usr/local/share/emacs/site-lisp/org/org")
(require 'ox-koma-letter
(locate-user-emacs-file "path/to/org-mode/contrib/lisp/ox-koma-letter"))
#+END_SRC
the org-mode version is from git commit "a198d81". M-x org-version gives:
Org-mode version 8.3.2 (release_8.3.2-378-ga198d8 @
/usr/local/share/emacs/site-lisp/org/)
I then copied the text from your email exactly as:
#+BEGIN_EXAMPLE
* letter 1
:PROPERTIES:
:EXPORT_DATE: 2015-10-26 Monday
:END:
** to :to:
The Org list
** To whom it may concern,
txt
* letter 2
:PROPERTIES:
:EXPORT_DATE: 2015-12-03 Thursday
:END:
** to :to:
list
** To whom it may concern,
txt
#+END_EXAMPLE
I selected the "letter 1" subtree, pressed C-cekp, opened the pdf file. But the
date is still "Dec 12, 2015".
Hope this helps,\\
York