I found a solution for this little issue.
The problem is that you can't set to nullValue because in source code of RadDateTimePickerElement there is a "BypassUpdateOnLostFocus" set to true. (I think)
But anyway, you cannot set the minvalue yourself, because Text won't be refresh. And if you refresh the Text yourself (RadDateTimePicker.DateTimePickerElement.TextBoxElement.TextBoxItem.Text), you'll have bad reactions (because of mask and RadMaskTextBox hosted in)
My solution is to do this :
Add an event for each RadDateTimePicker (same event for all in your form)
|private void RadDateTimePicker_Leave(object sender, EventArgs e)
| RadDateTimePicker element = (RadDateTimePicker)sender;
| if (String.IsNullOrEmpty(element.DateTimePickerElement.TextBoxElement.TextBoxItem.Text))
| element.Value = element.NullDate.AddSeconds(1);
so, if control lost focus, you'll set to default value as well.