This is working, but how???

I have a controller action for a post:

[AcceptVerbs(HttpVerbs.Post )]
public ActionResult Edit(Person person)
{
   bool isvalid = ModelState.IsValid;
   etc.

The Person object has a property BirthDate, type DateTime. When i enter some invalid data in the form, say 'blabla' which is obvious not a valid Datetime, it fills all the (other) Person properties with the correct data and the BirthDate property with a new blank DateTime. The bool isvalid has the value 'false'. So far so good.

Then i do this:

return View(p);

and in the view i have this:

<%= Html.TextBox("BirthDate", String.Format("{0:g}", Model.BirthDate)) %>
<%= Html.ValidationMessage("BirthDate", "*") %>

Ant there it comes: i EXPECTED the model to contain the new, blank DateTime because i didn't put any new data in. Second, when the View displays something, it must be a DateTime, because Model.BirthDate can't hold anything but a DateTime. But to my surprise, it shows a textbox with the 'blabla' value! (and the red * behind it)

Which ofcourse is nice because the user can seee what he typed wrong, but how can that (blabla)string be transferred to the View in a DateTime field?