You can't enable time on a shapefile added to a map. You need an ArcGIS Online subscription, along with a hosted feature service to do this. Or alternatively, if you have your own ArcGIS Server, you can publish a time aware service.
As far as the description, that only applies to layers derrived from items, for example, in your My Content. It does not apply to data added directly to the map.
Thanks,
Mike