You have enrolled your Symantec Endpoint Protection Manager (SEPM) into the Endpoint Protection 15 Cloud console.
Some Devices may show up with an "Invalid Date" in the Last Updated column, or no date at all on the Device details page. These are all running a Windows Server Operating system version.
Other Devices have valid Last Updated times. These are running a Windows Workstation Operating system version.
Symantec is aware and currently investigating this issue. This document will be updated when more information is available.