Avarage time duration

Hello, Im using this:     
                                   Max(${<[TO shipment]>}[Over Date Time]) - Min(${<[TO shipment]>}[Conf.tDate&Time])

to get time duration. Now I would like use result time to get Avarage time for [TO shipment] in my selected data. I tried:

                                   avg (aggr (Max(${<[TO shipment]>}[Over Date Time])-Min(${<[TO shipment]>}[Conf.tDate&Time]),))

but this one dont work. Result in my tab is " - ".

Please, can anybody help me? Sorry, for my english. Thank you for your help.