How to warn user that their session will timeout soon and how to log them out once it expires

logout.js

var logoutUrl = "https://test-swhv.ehawaii.gov/swhv-wikiflow/logout.php";
var refreshUrl = "https://test-swhv.ehawaii.gov/swhv-wikiflow/refresh.html";

function displayTimeOutWarningMessage() {
    var thirtyMinutes = 1000 * 60 * 30;
    var fifteenMinutes = thirtyMinutes / 2;
    var lastActivity = new Date().getTime();
    var checkTimeOut = function() {
        var currentTime = new Date().getTime();
    	if (currentTime >= lastActivity + fifteenMinutes) {
    	    var time = moment().format("MM/DD/YYYY hh:mm A");
    	    var warningMessage = "[" + time + "] You have been inactive for more than 15 minutes, and your session will time out in less than 15 minutes. ";
    	    warningMessage += "Click OK to continue using this web application, or click Cancel to logout.";
    	    if (confirm(warningMessage)) {
    	    	if (new Date().getTime() >= lastActivity + thirtyMinutes) {
    	    	    alert("Your session has expired. You will now be logged out.");
    	    	    window.location.href = logoutUrl;
    	    	} else {
    	            $.ajax({
    	                url: refreshUrl,
    	                type: "GET"
    	            });
    	            lastActivity = new Date().getTime();
    	            window.setTimeout(checkTimeOut, 1000);
    	    	}
    	    } else {
    	    	window.location.href = logoutUrl;
    	    }
    	} else {
            window.setTimeout(checkTimeOut, 1000); // check once per second
        }
    }
    checkTimeOut();
}

$(document).ready(function() {
    displayTimeOutWarningMessage();
});