You should override both the 'formatter' and 'parser' functions.
<script>
$.fn.datebox.defaults.formatter = function(date){
	var y = date.getFullYear();
	var m = date.getMonth()+1;
	var d = date.getDate();
	return (d<10?('0'+d):d)+'.'+(m<10?('0'+m):m)+'.'+y;
};
$.fn.datebox.defaults.parser = function(s){
	if (!s) return new Date();
	var ss = s.split('.');
	var m = parseInt(ss[1],10);
	var d = parseInt(ss[0],10);
	var y = parseInt(ss[2],10);
	if (!isNaN(y) && !isNaN(m) && !isNaN(d)){
		return new Date(y,m-1,d);
	} else {
		return new Date();
	}
};
</script>