<input id="mf" type="text" value="2009-01-01" dojoType="dijit.form.DateTextBox" promptMessage="Do it" />

<input id="mf" type="text" value="2009-01-01"   />

<script type="text/javascript">

dojo.addOnLoad(
    function(){
         var a =new dijit.form.DateTextBox({promptMessage:"Do it"},"mf");
    }
);

</script>

dojo.addOnLoad(
    function(){
         var _a =dojo.byId("mf");
        dojo.attr(_a,"dojoType","dijit.form.DateTextBox");
        dojo.parser.instantiate([_a]);
    }
);

dojo.addOnLoad(
    function(){
        dojo.parser.instantiate([dojo.byId("mf")],{dojoType:"dijit.form.DateTextBox"});
    }
);