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,
    );
  },
});