ox-html: Fix %d placeholder in pre/postamble string

* lisp/ox-html.el (org-html-format-spec): Correctly apply
  `org-html-metadata-timestamp-format' on %d placeholder.

Reported-by: Pavel Panchekha <me@pavpanchekha.com>
<http://permalink.gmane.org/gmane.emacs.orgmode/110374>

lisp/ox-html.el

@@ -1875,25 +1875,24 @@ INFO is a plist used as a communication channel."
 	    (setq template (replace-match val t t template))))))))
 
 (defun org-html-format-spec (info)
-  "Return format specification for elements that can be
-used in the preamble or postamble."
-  `((?t . ,(org-export-data (plist-get info :title) info))
-    (?s . ,(org-export-data (plist-get info :subtitle) info))
-    (?d . ,(org-export-data (org-export-get-date info) info))
-    (?T . ,(format-time-string
-	    (plist-get info :html-metadata-timestamp-format)))
-    (?a . ,(org-export-data (plist-get info :author) info))
-    (?e . ,(mapconcat
-	    (lambda (e)
-	      (format "<a href=\"mailto:%s\">%s</a>" e e))
-	    (split-string (plist-get info :email)  ",+ *")
-	    ", "))
-    (?c . ,(plist-get info :creator))
-    (?C . ,(let ((file (plist-get info :input-file)))
-	     (format-time-string
-	      (plist-get info :html-metadata-timestamp-format)
-	      (when file (nth 5 (file-attributes file))))))
-    (?v . ,(or (plist-get info :html-validation-link) ""))))
+  "Return format specification for preamble and postamble.
+INFO is a plist used as a communication channel."
+  (let ((timestamp-format (plist-get info :html-metadata-timestamp-format)))
+    `((?t . ,(org-export-data (plist-get info :title) info))
+      (?s . ,(org-export-data (plist-get info :subtitle) info))
+      (?d . ,(org-export-data (org-export-get-date info timestamp-format)
+			      info))
+      (?T . ,(format-time-string timestamp-format))
+      (?a . ,(org-export-data (plist-get info :author) info))
+      (?e . ,(mapconcat
+	      (lambda (e) (format "<a href=\"mailto:%s\">%s</a>" e e))
+	      (split-string (plist-get info :email)  ",+ *")
+	      ", "))
+      (?c . ,(plist-get info :creator))
+      (?C . ,(let ((file (plist-get info :input-file)))
+	       (format-time-string timestamp-format
+				   (and file (nth 5 (file-attributes file))))))
+      (?v . ,(or (plist-get info :html-validation-link) "")))))
 
 (defun org-html--build-pre/postamble (type info)
   "Return document preamble or postamble as a string, or nil.