Any ideas on how to force [Case].Due_Date__c to match the case's milestone target date? 

I keep getting a syntax error in Process Builder that [CaseMilestone].TargetDate is an incorrect object reference when attempting to update custom field [Case].Due_Date__c to match.

Else is there a way to set a case Due Date field based on SLA taking into account company business hours? It just seemed to me that the Milestone tracker already has that value, and if there was a way to access it and update a custom field to match it, I might save having to reinvent the wheel.  Any ideas?