When select a value, the 'onSelect' event will be triggered, which can be used to constrain the min or max value in datebox.
<input class="easyui-datebox" data-options="onSelect:onSelect"></input>
function onSelect(value){
var d = $.fn.datebox.defaults.parser('11/15/2012');
if (value<d){
$(this).datebox('setValue','11/15/2012');
}
}