export back-ends in contrib: Handle DATE keywords the same way

* contrib/lisp/ox-deck.el (org-deck--build-meta-info): Use
  `org-export-get-date'.
* contrib/lisp/ox-groff.el (org-groff--mt-head): Use
  `org-export-get-date'.
* contrib/lisp/ox-koma-letter.el (org-koma-letter-template): Use
  `org-export-get-date'.

contrib/lisp/ox-deck.el

@@ -463,7 +463,7 @@ INFO is a plist used as a communication channel."
                       (let ((auth (plist-get info :author)))
                         (and auth (org-export-data auth info)))))
          (date (and (plist-get info :with-date)
-                    (let ((date (plist-get info :date)))
+                    (let ((date (org-export-get-date info)))
                       (and date (org-export-data date info)))))
          (description (plist-get info :description))
          (keywords (plist-get info :keywords)))

contrib/lisp/ox-groff.el

@@ -622,8 +622,8 @@ See `org-groff-text-markup-alist' for details."
 
    ;; 5. Date.
    (when (plist-get info :with-date)
-     (let ((date (org-export-data (plist-get info :date) info)))
-       (and date (format ".ND \"%s\"\n" date))))
+     (let ((date (org-export-data (org-export-get-date info) info)))
+       (and (org-string-nw-p date) (format ".ND \"%s\"\n" date))))
 
    ;;
    ;; If Abstract, then Populate Abstract

contrib/lisp/ox-koma-letter.el

@@ -206,7 +206,7 @@ holding export options."
            (org-export-data (plist-get info :email) info))
    (format "\\setkomavar{fromphone}{%s}\n" (plist-get info :phone-number))
    ;; Date.
-   (format "\\date{%s}\n" (org-export-data (plist-get info :date) info))
+   (format "\\date{%s}\n" (org-export-data (org-export-get-date info) info))
    ;; Letter Class Option File
    (format "\\LoadLetterOption{%s}\n" (plist-get info :lco))
    ;; Letter start.