Using localStorage, Sess-io will check against a stored expiration stamp and force the user to log-out once the session has expired

Sess-io.js

// Sess-io by @ThrivingKings
// http://thrivingkings.com/read/Sess-iojs-handles-session-expiration-in-pure-JavaScript


// Date fix for good ole IE8
if (!Date.now) {
  Date.now = function() {
    return new Date().valueOf();
  }
}

// Local Storage functions for the ages
if (!window.localStorage) {
  window.localStorage = {
		getItem: function (sKey) {
			if (!sKey || !this.hasOwnProperty(sKey)) { return null; }
			return unescape(document.cookie.replace(new RegExp("(?:^|.*;\\s*)" + escape(sKey).replace(/[\-\.\+\*]/g, "\\$&") + "\\s*\\=\\s*((?:[^;](?!;))*[^;]?).*"), "$1"));
		},
		key: function (nKeyId) {
			return unescape(document.cookie.replace(/\s*\=(?:.(?!;))*$/, "").split(/\s*\=(?:[^;](?!;))*[^;]?;\s*/)[nKeyId]);
		},
		setItem: function (sKey, sValue) {
			if(!sKey) { return; }
			document.cookie = escape(sKey) + "=" + escape(sValue) + "; expires=Tue, 19 Jan 2038 03:14:07 GMT; path=/";
			this.length = document.cookie.match(/\=/g).length;
		},
		length: 0,
		removeItem: function (sKey) {
			if (!sKey || !this.hasOwnProperty(sKey)) { return; }
			document.cookie = escape(sKey) + "=; expires=Thu, 01 Jan 1970 00:00:00 GMT; path=/";
			this.length--;
		},
		hasOwnProperty: function (sKey) {
			return (new RegExp("(?:^|;\\s*)" + escape(sKey).replace(/[\-\.\+\*]/g, "\\$&") + "\\s*\\=")).test(document.cookie);
		}
	};
	window.localStorage.length = (document.cookie.match(/\=/g) || window.localStorage).length;
}

// Store the end time
localStorage.setItem('session_to', (Date.now() + 15*60000));

var page_title = document.title,
		blinking;

// Checker interval
var sessio_check = window.setInterval(function() {
	
	// Fetch the timeout stamp
	var end = localStorage.getItem('session_to');
	
	if(end) {
		
		// Set current stamp
		var now = Date.now();
		
		// Expired
		if(now>=end) {
			
			// Clear interval
			sessio_check = window.clearInterval(sessio_check);
			sessio_check = null;
			
			// Clear storage for other tabs
			localStorage.setItem('session_to', 0);
			
			// Redirect to logout
			window.location = '/logout';
		
		
		// Nearing the expiration	
		} else if( (end-now)/60000 <= 1) {
			
			blinkTitle(page_title, 'Session expiring...');
		} else {
			
			if(blinking) {
				
				// Reset title
				document.title = page_title;
			
				// Clear the blinking
				blinking = window.clearInterval(blinking);
				blinking = null;
			}
		}
	}
}, 100);

// Blink the title every second
function blinkTitle(oT, nT) {
	
	if(!blinking) {
	
		blinking = window.setInterval(function() {
		
			if(document.title==oT) {
				document.title = nT;
			} else {
				document.title = oT;
			}
		}, 1000);
	}
}