Need help with DateTime D4 / MS SQL 7.0

In a table I have date-field that is not duty (the birthday is not
allways known).
How can I realize this in a form, because DateTimePicker not allows
NULL.
Can anyone show me a workaround to this problem, i.e. a Mask for a
string-field, which allows NULL and verifies that the date is correct,
if there is one ?

Thanx (sorry for bad english)
Micha