You can absolutely pass the existing value from one date field to another date field using the action framework. If the field “Date__c” is in the model that provides context (to a page title or table) you can use the syntax you describe above {{Date__c}} to send the value for the row in context to the new field. If you field is in another model you can use the global merge syntax… {{$Model.ModelName.data.0.Date__c}} (changing the ModelName obviously…)
