When trying to set the position of the constraint on a datebox with 'after_start', 'after_end', 'before_start' or 'before_end'

<datebox constraint="no empty, before_start"/>

it throws an Exception

org.zkoss.zk.ui.UiException: Not a date: _start. Format: yyyyMMdd
	at org.zkoss.zul.SimpleDateConstraint.parseDate(SimpleDateConstraint.java:148)

Whereas the positions 'start_after', 'start_before', 'end_after' or 'end_before' works.

The problem is in the parseConstraint method of org.zkoss.zul.SimpleDateConstraint, where its checks if the constraint start with 'before'/'after', then tries to parseDate the end of the constraint.

 else if (constraint.startsWith("before")) {
			_end = parseDate(constraint.substring(6));
			return 0;
		} else if (constraint.startsWith("after")) {
			_beg = parseDate(constraint.substring(5));
			return 0;
		}

A solution would be to check if the constraint is a position 'before_', 'after_', ... or to check if the end of the constraint starts with ''.