We encounter some unexpected experience in datebox component. Please see attachments datebox-nan-1.PNG and datebox-nan-2.PNG.
In datebox-nan-1.PNG, the datebox was showing NaN. And by selecting a "NaN" date, we captured datebox-nan-2.PNG.
The datebox cannot resume to normal behavior unless we reload the page. We cannot repeat it every time, it's an intermittent problem.

We create the datebox in JSP and modify the datebox by JAVA class (by autowire). Please find the code fragment below.

JSP:
<zk:datebox id="completeDateDatebox" />

JAVA:
// completeDateDatebox is an instance of org.zkoss.zul.Datebox
completeDateDatebox.setReadonly(true);
completeDateDatebox.setFormat("dd/MM/yyyy");
completeDateDatebox.setValue(aDomainObj.getCompleteDate());
completeDateDatebox.addEventListener(Events.ONCHANGE, new EventListener() {
public void onEvent(Event event) throws Exception

{ aDomainObj.setCompleteDate(completeDateDatebox.getValue()); }

});
completeDateDatebox.setConstraint("no empty: " + org.zkoss.util.resource.Labels.getLabel("error.completeDate.empty"));