For me the simplest way would be to have nothing in the ControlSource property of the text box. Then in the LostFocus event I would put

If This.Value = "X"
Select table
Replace dateField With date
Endif

Alternatively, if you really want to use a data aware control have two text boxes. The one with the X in it is visible and again has no ControlSource. The second box has a ControlSource of the data field but it its Visible property is False. Then in the LostFocus of the X box

If This.Value = "X"
Thisform.txtRealBox.Value = date
Endif

Generally, if you are doing a like for like conversion from FP26 to Visual, where in FP26 you have an input control based on m.something you can have a VFP control with no ControlSource and use its Value property as if it were the m.something.

Hope this solves your problems 9or at least some of them)

Andrew