org-table.el: return correct duration string for Calc formulas.

lisp/org-table.el

@@ -2517,7 +2517,9 @@ not overwrite the stored one."
 	  (or (fboundp 'calc-eval)
 	      (error "Calc does not seem to be installed, and is needed to evaluate the formula"))
 	  (setq ev (calc-eval (cons form modes)
-			      (if numbers 'num))))
+			      (if numbers 'num))
+		ev (if duration (org-table-time-seconds-to-string 
+				 (string-to-number ev)))))
 
 	(when org-table-formula-debug
 	  (with-output-to-temp-buffer "*Substitution History*"