For the UDF method, add this to a Module and use it as the comment demonstrates.
Code:
' =Month2(A1+180)
Function Month2(aDate As Date) As String
  Dim d() As String
  Application.Volatile False
  d() = Split("JA FE MR AL MA JN JL AU SE OC NO DE")
  Month2 = Year(aDate) & "/" & d(Month(aDate) - 1) & "/" & Day(aDate)
End Function