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.
Great Find Terry! I've been fighting with this exact issue for a long time.
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