Add new analytic page: Average time spent into each column

app/Helper/Dt.php

@@ -21,6 +21,10 @@ class Dt extends \Core\Base
      */
     public function duration($seconds)
     {
+        if ($seconds == 0) {
+            return 0;
+        }
+
         $dtF = new DateTime("@0");
         $dtT = new DateTime("@$seconds");
         return $dtF->diff($dtT)->format('%a days, %h hours, %i minutes and %s seconds');