Index and Match based on expiry dates in multiple sheets

Sub Test()
For Each Sht In ActiveWorkbook.Sheets
    If Right(Sht.Name, 3) = "SCL" Then
        Sht.Activate
        For Each Cl In Range(Range("F4", Range("F" & Rows.Count).End(xlUp)).Address)
            If Cl < Now Then
                a = a + 1
                mystr = mystr & Cl.Offset(0, -3).Value & ","
            End If
        Next Cl
    End If
Next Sht
If mystr <> "" Then MyArr = Split(mystr, ",")
Sheets("Index").Range("A3:A" & Rows.Count).Clear
Sheets("Index").Range("A3").Resize(UBound(MyArr), 1) = MyArr
End Sub