with this macro I can't insert the file format xlsx, only xls but I need format xlsx

Please have a look to the following code.
It works but if I type xlsx instead xls, I get an error.

Option Explicit
Sub save()
Dim Pfad, Tag

With ActiveWorkbook
Pfad = "C:\test\"

Tag = Format(Now, "yymmdd")
.save
.SaveAs Pfad & Left(.Name, Len(.Name) - 21) & _
"" & Tag & ".xls"
.Close
End With

End Sub