Hi neha,

I found some mistakes in your code, I just highlighted that mistake by making those mistakes as Bold. Let me know after correction, are you able to achieve your desire output. If not I can share you some other script as well. 

Type: OnChange()

Catalog Client Script:

function onChange(control, oldValue, newValue, isLoading, isTemplate) {
if (isLoading || newValue === '') {
return;
}
var cdt = g_form.getValue('u_date_concern_reported'); //first date field
var addtime = 28; 
var addtype = 'day'; 

var gr = new GlideAjax('global.ClientDateTimeUtils');
gr.addParam('sysparm_name', 'addDateAmount');
gr.addParam('sysparm_fdt', cdt);
gr.addParam('sysparm_addtime', addtime);
gr.addParam('sysparm_addtype', addtype);
gr.getXML(ajaxResponse);

function ajaxResponse(serverResponse){
var answer = serverResponse.responseXML.documentElement.getAttribute("answer");
g_form.setValue('u_th_day', answer);  //second date field
alert(answer);
}

}

Script Include: ClientDateTimeUtils

Application: Global

Client callable:Checked

Accessible from: All application scopes

var ClientDateTimeUtils = Class.create();
ClientDateTimeUtils.prototype = Object.extendsObject(global.AbstractAjaxProcessor, {

addDateAmount: function(){
var firstDT = this.getParameter('sysparm_fdt'); //First Date Field
var addTYPE = this.getParameter('sysparm_addtype');
var addTIME = this.getParameter('sysparm_addtime'); //How much time to add
var day = new GlideDate();
day.setDisplayValue(firstDT);
day.addDays(addTIME);
return day.getDisplayValue();
},
type: 'ClientDateTimeUtils'
});