How to format SSIS Expression for weekend

I have the following format to account for weekends while naming an excel output. How do I get only the date and not the full startime formatted like 20150429? ((DT_I4)DATEPART("weekday",@[System::StartTime]) ==2 ? Replace((DT_STR, 20, 1252)(DATEADD( "D", -3,@[System::StartTime])),":","") + ".xlsx" : Replace((DT_STR, 20, 1252)(DATEADD( "D", -1,@[System::StartTime])),":","") + ".xlsx")