Shipping_date__c
    IF(ISBLANK(Date_Shipped__c),
DATE(VALUE(mid(Date_Returned__c,1,4)),VALUE(mid(Date_Returned__c,6,2)),VALUE(mid(Date_Returned__c,9,2))),
DATE(VALUE(mid(Date_Shipped__c,1,4)),VALUE(mid(Date_Shipped__c,6,2)),VALUE(mid(Date_Shipped__c,9,2))))
    false
    IF(ISBLANK(Date_Shipped__c),
DATE(VALUE(mid(Date_Returned__c,1,4)),VALUE(mid(Date_Returned__c,5,2)),VALUE(mid(Date_Returned__c,7,2))),
DATE(VALUE(mid(Date_Shipped__c,1,4)),VALUE(mid(Date_Shipped__c,5,2)),VALUE(mid(Date_Shipped__c,7,2))))
    BlankAsZero
    
    false
    false
    Date