Working code omend to take all used range in column

HI Could some look at below code just need it to be able to take all used range in column N.

I thought I could easily do that, but it always give an error. Please have a look.

Thanks

Sincerely
Abjac

Sub DDMMYY()
Sheets("AT").Activate
    Dim myValue As Date
    myValue = Range("N9")
    Range("N9").NumberFormat = "@"
    Range("N9") = Format(myValue, "DDMMYY")

End Sub