adding date to filename

I'm trying a to automatically add the date to an items name in the render queue.

I have the following.

var x = app.project.renderQueue.item(1).outputModule(1);  

alert (x.file.name);

This returns the name of the file that will be output but I'm unable to change this value. The scripting guide says this attribute is read/write but I can't figure out how to alter it.

Thanks.