I got it to work. I had to go into includes/class_dm_user.php and modify this line:
Code:
(!$this->adminoverride AND $this->registry->options['reqbirthday'] AND ($birthday['day'] <= 0 OR $birthday['month'] <= 0 OR $birthday['year'] <= 0))
And change $birthday['year'] <= 0 to $birthday['year'] < 0
If you enter nothing for year during registration, it defaults to 0, so changing that code will allow nothing to be entered for the year.