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

On Thu, Dec 10, 2015 at 10:45 AM, Rasmus <rasmus@gmx.us> wrote:

> Hi,
>
> York Zhao <gtdplatform@gmail.com> writes:
>
> > Thanks for your help. However, adding the :EXPORT_DATE: property doesn't
> > work. Firstly, there's an error: "PDF file produced with errors.".
>
> You have to add an address, e.g.
>
> * 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
>
> > Secondly, even though it still produced a PDF output, the date is always
> > today's date which is wrong.
>
> I cannot reproduce.  I get this as part of exporting the first subtree
> (I click: C-c C-e C-s k L)
>
>          \date{2015-10-26 Monday}
>
> Hope it helps,
> Rasmus
>
> --
> Enough with the bla bla!
>
>
>