Hi
you may use a macro for this (AFAIK there's no setting to do this
automatically):
Sub CommentAddOrEdit()
'adds new plain text comment or positions
'cursor at end of existing comment text
Dim cmt As Comment
Set cmt = ActiveCell.Comment
If cmt Is Nothing Then
ActiveCell.AddComment text:= format(now,"YYYY-MM-DD")
End If
SendKeys "%ie~"
End Sub