Edit in JSFiddle

// The ui-datepicker-div style defined above getes applied on the dropdown and ensures that it gets rendered above the div with z-index of 4.
$("#datepicker").datepicker();

<input type="text" id="datepicker" />
    <br /><br />
    <div id='content'> z-index of the div is 4. The DatePicker above will dropdown above this div.</div>
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<p><font size="3"><b>Source for this<a  target="_blank"href="http://jqfaq.com/adjust-the-datepickers-z-index/"</a> JQFaq Question</b></font></p>
   <iframe id="iframe1" src="http://jqfaq.com/AdPage.html" style="width:100%;border:none;" />

#content
      {
          z-index:4 !important;        
          position:absolute;  
          height:100px;
          width:170px;
          border :1px solid #808080;
          background-color:#CCC;
          color:White;
      }
      
      #ui-datepicker-div
      {
       z-index:5 !important;
      }