How to find difference between two dates in ServiceNow?

Client Script:

function onChange(control, oldValue, newValue, isLoading) {
    var strt = g_form.getValue("start_date");
    var end = g_form.getValue(end_date);
    var ajax = new GlideAjax('DateCalculattion');
    ajax.addParam('sysparm_name','dateCal');
    ajax.addParam('sysparm_strt',strt);
    ajax.addParam('sysparm_end',end);
   ajax.getXMLAnswer(getDateDiff);
    }
     function getDateDiff(answer) {
        g_form.setValue("date_diff", answer);
    }

Name:DateCalculattion

Client callable:checked,client:checked

var DateCalculattion = Class.create();
 DateCalculattion.prototype = Object.extendsObject(AbstractAjaxProcessor, {
 dateCal: function() {
 return gs.dateDiff(this.getParameter('sysparm_strt'),this.getParameter('sysparm_end'), false);
 }
 });