I think I get what you're looking for. You need a calculation that gracefully fails if one of the fields isn't there.
The best way it to use an If statement in the field calculation
If ( IsEmpty 'date of referral' ; "" ; 'interim hearing - date of referral' )
This will set the field to nothing if the referral date isn't known, otherwise it sets it to the calculation you specified. Obviously you need the true field names in there. Look up the If and IsEmpty statements for clarification.