Increment letter in a cell

[sgf0568]

I'm looking for code to increment the letter in a cell. I'm currently using some code I found on this forum that works well (see below), but when "Z" is reached it goes back to "A". I need it to go from "Z" to "AA", "AB", "AC", etc.

Dim rng As Range
    Set rng = Range("J1")
    With rng
    If .Value = "Z" Then
    .Value = "A"
    Else
    .Value = Chr(Asc(.Value) + 1)

Does anybody have any ideas?