Steps to Reproduce

    <timebox onChange="Clients.log(String.valueOf(event.getValue()) + self.value)"/>

update the value -> blur
clear the value -> blur
observe the event and component values logged

Current Result

clearing the time results in 12:00am instead of NULL

Expected Result

enable clearing the time value, to set a null value at server side

Debug Info

this worked in 8.6.1

Root Cause

Timebox.coerceFromString doesn't calculate the emptyCount, correctly missing to clear the value

https://github.com/zkoss/zk/blob/v8.6.2/zul/src/archive/web/js/zul/db/Timebox.js#L164-L184

possible side effect of ZK-4252 (8.6.2)

Workaround

by adding the following script an empty input can be detected -> returning an empty value

    zk.afterLoad('zul.db', function() {
    	var xTimebox = {};
    	zk.override(zul.db.Timebox.prototype, xTimebox, {
    		coerceFromString_ : function(value) {
    		    if(/\d/.test(value)) {
    			    return xTimebox.coerceFromString_.apply(this, arguments);
    		    }
    		    this._changed = false;
        		return;
    		}
    	});//zk.override
    });//zk.afterLoad