How to disable selected date range of Datebox in zk

<zk>
  <window border="normal" title="hello" apply="pkg$.TestComposer">
     
      <div>Welcome to ZK Fiddle , run it right now!</div>
     
   
    <datebox id="db11" width="150px" constraint="no empty,after 20130710,before 20130720" />
  </window>
</zk>


* constraint="no empty,after 20130710,before 20130720"
*  20130710 means 2013-07-10 and 20130720 means 2013-07-20