How about a different approach: who cares about time? (You're not going to reliably sync the system clock with JavaScript.)
Instead, use a Node server with socket.io to synchronize when your clients advance the slideshow. Instead of the clients deciding when to advance, the server tells them to.
This approach comes with the added bonus of being able to manually fiddle with the slideshow while it's running. In the example that follows, I've added a Next button that causes all connected clients to immediately advance to the next slide.
app.js
var express = require('express')
, app = express.createServer()
, io = require('socket.io').listen(app)
, doT = require('dot')
, slide = 0
, slides = [
'http://placekitten.com/700/400?image=13',
'http://placekitten.com/700/400?image=14',
'http://placekitten.com/700/400?image=15',
'http://placekitten.com/700/400?image=16',
'http://placekitten.com/700/400?image=1',
'http://placekitten.com/700/400?image=2',
'http://placekitten.com/700/400?image=3',
'http://placekitten.com/700/400?image=4',
'http://placekitten.com/700/400?image=5',
'http://placekitten.com/700/400?image=6',
'http://placekitten.com/700/400?image=7',
'http://placekitten.com/700/400?image=8',
'http://placekitten.com/700/400?image=9',
'http://placekitten.com/700/400?image=10',
'http://placekitten.com/700/400?image=11',
'http://placekitten.com/700/400?image=12',
];
app.listen(70); // listen on port 70
app.register('.html', doT); // use doT to render templates
app.set('view options', {layout:false}); // keep it simple
doT.templateSettings.strip=false; // don't strip line endings from template file
app.get('/', function(req, res) {
res.render('index.html', { slide: slide, slides: slides });
});
app.post('/next', function(req, res) {
next();
res.send(204); // No Content
});
setInterval(next, 4000); // advance slides every 4 seconds
function next() {
if (++slide >= slides.length) slide = 0;
io.sockets.emit('slide', slide);
}
views/index.html
This file is processed as a doT template.
<!DOCTYPE html>
<html>
<head>
<title>Synchronized Slideshow</title>
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
<script src="/socket.io/socket.io.js"></script>
<script>
var curslide = {{=it.slide}}; // the slide the server is currently on.
$(function() {
$('#slide' + curslide).css('left',0);
$('#next').click(function() {
$.post('/next');
});
});
var socket = io.connect('http://localhost:70');
socket.on('slide', function(slide) {
$('#slide' + curslide).animate({left:-700}, 400);
$('#slide' + slide).css('left',700).show().animate({left:0}, 400);
curslide = slide;
});
</script>
<style>
#slideshow, .slide { width:700px; height:400px; overflow:hidden; position:relative; }
.slide { position:absolute; top:0px; left:700px; }
</style>
</head>
<body>
<div id="slideshow">
{{~it.slides :url:i}}
<div id="slide{{=i}}" class="slide"><img src="{{=url}}"></div>
{{~}}
</div>
<button id="next">Next ></button>
</body>
</html>
Copy these two files into a folder, then run
$ npm install express socket.io dot
$ node app.js
and navigate to http://localhost:70 in several different windows, then see the magic.
