Can anybody tell me, how I can put some result from one cell into another cell?

My experssion in Cell1: =IIf(Previous(Fields!VALUE.Value)>0,DateDiff("s",Previous(Fields!TIMESTAMP.Value),Last(Fields!TIME STAMP.Value)),0)

I have some result and if I use(in Cell2) =ReportItems!Cell1.Value, that it is not working.

I have results form timestamp(it means times in some state), and I want do Gantt diagrem.

Thank you