Can someone help me with some vba code for the date function?

what i'm trying to accomplish is to convert the string 20050401 to
04/01/2005.

The formula =DATE(LEFT(E7,4),MID(E7,5,2),RIGHT(E7,2)) works fine for that.
I would like to put it in vba. I've written:

Function new_date(last_done_date) As String
'returns New last_done_date
new_date = Application.Date(Left(last_done_date, 4), Mid(last_done_date, 5,
2), Right(last_done_date, 2))
End Function

But it returns a value error.

last_done_date is the range of cells that contain the date string - 20050401

Thanks,

Brian