Delphi 5 - Date Field Validation and Trapping

I have a date input field that I want to validate that the contents REALLY are a date :-).

So the OnExit event reads:

procedure TForm1.Purchases_Postings_DateExit(Sender: TObject);
begin
     Try
        StrToDate(Form1.Purchases_Postings_Date.text);
     Except On EConvertError Do
        Begin
                ShowMessage('Please enter a valid date.');
                Exit;
        end;
     end;
end;

Where Form1.Purchases_Postings_Date.text is the input field.

However before this gets called I get the message:

Project TEST.EXE raised exception class EConvertError with message '99/99/9999' is not a valid date. Process stopped. Use stop or run to continue.

And 99/99/9999 is a suitibly suicidal bogus date.

With thanks.