A guess, try: 
x = Array("", "", "[Report Date].[Date].[Day].&[2018-10-01T00:00:00]", "[Report Date].[Date].[Day].&[2018-10-02T00:00:00]", "[Report Date].[Date].[Day].&[2018-09-01T00:00:00]", "[Report Date].[Date].[Day].&[2018-09-02T00:00:00]")
ActiveWorkbook.Sheets("MTD").PivotTables("PivotTable4").PivotFields("[Report Date].[Date].[Day]").VisibleItemsList = x