Soni,

  Add alert in your call back function as shown below and let me know what you see.

    Here is your onChange client script on Needed By   variable:

function onChange(control, oldValue, newValue, isLoading) {

  if (isLoading || newValue == '') {

  g_form.setDisplay('title', false); // put in the variable name you want to show or hide according needed by date

  return;

  }

  var ga = new GlideAjax('ValidateNeededBy');

  ga.addParam('sysparm_name','validateDate');

  ga.addParam('sysparm_needed_by',newValue);

  ga.getXML(callBack);

  function callBack(response) {

  var answer = response.responseXML.documentElement.getAttribute("answer");

alert(answer);

  if(parseInt(answer)>=27){

  g_form.setDisplay('title', false); // put in the variable name you want to show or hide according needed by date

  }

  else{

  g_form.setDisplay('title', true); // put in the variable name you want to show or hide according needed by date

  }

  }

}