OK... Take 3:

Sub AddMonth()
For i = 1 To Sheets.Count
Sheets(i).Select
Range("A1").Formula = "=today()"
Range("A1").Value = Range("A1").Value
Range("A1").NumberFormat = "mmmm yyyy"
Range("A1").Select 'Not necessary, but you might like it...
Next i
Sheets(1).Select 'Not necessary, but again, you may find it useful
End Sub