Fix issue with changing the rotating file handler filename format, refs #267

src/Monolog/Handler/RotatingFileHandler.php

@@ -65,6 +65,8 @@ class RotatingFileHandler extends StreamHandler
     {
         $this->filenameFormat = $filenameFormat;
         $this->dateFormat = $dateFormat;
+        $this->url = $this->getTimedFilename();
+        $this->close();
     }
 
     /**