I have two hosted feature services - point and polygon - that have date field (DateOpened and DateClosed). Both of these field are coded as date fields.
However, I can not make them time aware. Going through the regular process to make a hosted feature service time aware from the ArcGIS Online service property page does not enable the time settings.
While I'm not new to this process, I don't time enable layers all that often. However, the last time I tried it, several weeks ago, it worked just fine. Not so now. Perhaps something 'broke' with the 9 September update?