Found the answer: the time slider doesn't work if you add a time enabled layer (i.e. server.com/ArcGIS/rest/services/folder/MapServer/2) it does work if you add the entire map service (i.e. server.com/ArcGIS/rest/services/folder/MapServer). It would be nice to be able to keep time enabled when just adding a single layer, but at least there's an answer.