When creating a dialogue with jQuery that contains a Datepicker text input the Datepicker calendar will appear below the dialogue due to the dialogue’s z-index being higher.  The easiest universal way to work around this is to include a one liner in the open event function of the initial dialogue call.

$('#dialogue').dialog({
    modal: true,
    open: function() {
        $("#ui-datepicker-div").css("z-index", $(this).parents(".ui-dialog").css("z-index")+1);
    }
});