After setting the date format in the Preferences on the Home page in the Symantec Endpoint Protection Manager (SEPM) to something other than MMDDYY, the definition dates still display in the MMDDYY format.
English SEPM 14 build 1904
The SEPM also factors in the OS date format and region, if these are set differently the formats of some items will not change.
Set the correct region format and date format in the Windows Control Panel -> Region settings. If the desired format is D/M/Y then please ensure it's set to DD/MM/YYYY on the Date tab in the Additional Settings as well.