'Rosario Peña 13/04/18
Private Sub Worksheet_Change(ByVal Target As Range)
    Dim KeyCells As Range

    ' The variable KeyCells contains the cells that will
    ' cause an alert when they are changed.
    Set KeyCells = Range("B7")
    
    If Not Application.Intersect(KeyCells, Range(Target.Address)) _
           Is Nothing Then

        ' Display a message when one of the designated cells has been
        ' changed.
        ' Place your code here.
        ' MsgBox "Cell " & Target.Address & " has changed."
        id_muestreo
       
    End If
End Sub

Sub id_muestreo()
    Dim fecha_Default As Date
    Dim fecha_Campo As Date
    
    fecha_Default = CDate("01/04/3000")
    fecha_Campo = Range("B7").Value
    
    If fecha_Default <> fecha_Campo Then
        If Range("M1").Value = "" Then
            Range("M1").Value = Application.WorksheetFunction.RandBetween(1000, 10000)
            'MsgBox Application.WorksheetFunction.RandBetween(1000, 10000)
        End If
    End If
End Sub