Use
  ActiveDocument.CustomDocumentProperties("MyFormattedDate").Value
to both read and write the value.

You could add some testing code to make sure the variables are in the
document if there's a possibility that they might not be, eg

Dim docProp As DocumentProperty
Dim blnFound As Boolean

blnFound = False

For Each docProp In ActiveDocument.CustomDocumentProperties
   If docProp.Name = "MyFormattedDate" Then
       blnFound = True
   End If
Next docProp

With ActiveDocument

If blnFound = False Then
   .CustomDocumentProperties.Add _
       Name:="MyFormattedDate", _
       Value:="MyValue"
Else
   .CustomDocumentProperties("MyFormattedDate").Value = "x/x/xx"
End If

End With