So if you are ever unable to add a "date" type custom property, it may be your Windows date format.

We had ours set to "ddMMMyy".  When adding a custom property, the date showed up as 01Jan01 in the evaluated box, and appeared fine.  But when you click OK, the date custom property is deleted.

Issue went away when I changed the date format to "dd MMM yy".

This also applies to parts and assemblies.