Item number with current month

[SamT]

Private Sub Worksheet_Change(ByVal Target As Range)
If Not Intersect(Target, Range("A:A) is Nothing Then _
       CheckMonthOfItem Intersect(Target, Range"A:A")
End Sub

Private Sub CheckMonthOfItem(Rng As Range)
Dim Cel As Range

Application.EnableEvents = False
   For Each Cel in Rng
     If Cel.Address = "A1" Then 
       Cel = "'1/" & Month(Date) 'Special Case

On Error GoTo CelNext
     ElseIf CInt(Split(Cel.Offset(-1), "/")(1)) = Month(Date) Then    
       Cel = CStr(CLng(Split(Cel.Offset(-1), "/")(0) + 1) & "/" & Month(Date)) 'Typical
     ElseIf  CInt(Split(Cel.Offset(-1), "/")(1) = Month(DateAdd( "m", -1, Date) Then 
       Cel = "'1/" & Month(Date) 'New Month
     End If
CelNext: Err = 0
   Next Cel
Application.EnableEvents = True
End Sub