org-element: Allow timestamps in parsed keywords (i.e. DATE)

* lisp/org-element.el (org-element-object-restrictions): Allow
  timestamps in parsed keywords (i.e. DATE).

lisp/org-element.el

@@ -314,7 +314,7 @@ a secondary string.")
     (item export-snippet footnote-reference inline-babel-call latex-or-entity
 	  link macro radio-target sub/superscript target text-markup)
     (keyword inline-babel-call inline-src-block latex-or-entity link macro
-	     sub/superscript text-markup)
+	     sub/superscript text-markup timestamp)
     (link export-snippet inline-babel-call inline-src-block latex-or-entity link
 	  sub/superscript text-markup)
     (paragraph export-snippet footnote-reference inline-babel-call