You show me the code, I show a solution ;)
in zul
<datebox id="searchBeginDate" value="@bind(vm.loggingVmQueryObject.beginDate)" onOK="@command('filter')" />
<button onClick="@command('clearBeginDate', date = searchBeginDate)" image="/img/delete.PNG" />
in java
public void clearBeginDate(@BindingParam("date")Datebox date){
//your other code here
date.setText(null);
}