unable to set AttributeObj.Datatype

sorry, i'm a noob to this. i've written the following attempting to add attributes to a physical model but below when i attempt:

MyAttribute.DefaultName = "timestamp now"

it doesn't work.

i'm tryin g to set the default value from the data dictionary defaults.

the code:

Sub Main

  Dim MyDiagram As Diagram
  Dim MyModel As Model
  Dim MyEntity As Entity
  Dim MyAttribute As AttributeObj

    Set MyDiagram = DiagramManager.ActiveDiagram
    Set MyModel = MyDiagram.ActiveModel

    If Not MyModel.Logical Then
      If MsgBox("This macro will add audit columns to all objects.", vbOkCancel) = vbOK Then
        For Each MyEntity In MyModel.Entities
          MsgBox(MyEntity.EntityName)
          Set MyAttribute = MyEntity.Attributes.Add("create_date", False)
          MyAttribute.Datatype = "TIMESTAMP" 
          MyAttribute.Definition = "The datetime the record was created."
          MyAttribute.NullOption = "NOT NULL"
          MyAttribute.DefaultName = "timestamp now" 'not working
          MsgBox(MyAttribute.Datatype)
          MsgBox(MyAttribute.Definition)
        Next

      End If
    Else
        MsgBox "This macro is designed to run in the Physical model. Please switch to Physical model first."
    End If


End Sub