[O] modify postamble in html export

[pw]

Hi,

I want to have a postamble in html with just the date (and without hours).

I already delete other informations in the postamble with these 
variables into my .emacs :
 '(org-export-author-info nil)
 '(org-export-creator-info nil)
 '(org-html-validation-link nil)

Now I have only the date left in the postamble but I want to change the 
format.

The default is : "Created: 2013-09-19 jeu. 14:09" and I want "Last 
update : 19 sept. 2013"

To do this I tried to set this variable :
 '(org-export-date-timestamp-format "%d %h %y")

But it is not working (and I have no idea how to put the "Last update :").

Could you indicate me how to achieve this. An issue is also I don't want 
to put this date format in all org-mode (I want to keep the default 
format for task and other stuff).

bye
pw