Se você tiver constantes (não fórmulas) na coluna A , tente esta macro curta:
Sub INeedADate()
Dim Kolumn As Long, rng As Range, r As Range
Dim u As Long
Kolumn = 1
Set rng = Columns(Kolumn).Cells.SpecialCells(2)
For Each r In rng
u = UBound(Split(r.Text, "/"))
If u = 2 Then
r.NumberFormat = "mm/dd/yy"
End If
Next r
End Sub