Getting the current GMT world time

How can i get the current time? (in JavaScript)

Not the time of your computer like:

now = new Date;
now_string = addZero(now.getHours()) + ":" + addZero(now.getMinutes()) + ":" + addZero(now.getSeconds());

But the real accurate world time?

Do i need to connect to to a server (most likely yes, which one? and how can i retrieve time from it?)

All the searches I do from google return the (new Date).getHours().

Edit:

I want to avoid showing an incorrect time if the user has a wrong time in his computer.

You can use JSON[P] and access a time API:

(The code below should work perfectly, just tested it...)

function getTime(zone, success) {
    var url = 'http://json-time.appspot.com/time.json?tz=' + zone,
        ud = 'json' + (+new Date());
    window[ud]= function(o){
        success && success(new Date(o.datetime));
    };
    document.getElementsByTagName('head')[0].appendChild((function(){
        var s = document.createElement('script');
        s.type = 'text/javascript';
        s.src = url + '&callback=' + ud;
        return s;
    })());
}

getTime('GMT', function(time){
    // This is where you do whatever you want with the time:
    alert(time);
});

First, to get the accurate GMT time you need a source that you trust. This means some server somewhere. Javascript can generally only make HTTP calls, and only to the server hosting the page in question (same origin policy). Thus that server has to be your source for GMT time.

I would configure your webserver to use NTP to synchronize its clock with GMT, and have the webserver tell the script what time it is, by writing a variable to the page. Or else make and XmlHttpRequest back to the server when you need to know the time. The downside is that this will be inaccurate due to the latency involved: the server determines the time, writes it to the response, the response travels over the network, and the javascript executes whenever the client's cpu gives it a timeslice, etc. On a slow link you can expect seconds of delay if the page is big. You might be able to save some time by determining how far off from GMT the user's clock is, and just adjusting all the time calculations by that offset. Of course if the user's clock is slow or fast (not just late or early) or if the user changes the time on their PC then your offset is blown.

Also keep in mind that the client can change the data so don't trust any timestamps they send you.

Edit: JimmyP's answer is very simple and easy to use: use Javascript to add a <script> element which calls a url such as http://json-time.appspot.com/time.json?tz=GMT. This is easier than doing this yourself because the json-time.appspot.com server works as a source of GMT time, and provides this data in a way that lets you work around the same-origin policy. I would recommend that for simple sites. However it has one major drawback: the json-time.appspot.com site can execute arbitrary code on your user's pages. This means that if the operators of that site want to profile your users, or hijack their data, they can do that trivially. Even if you trust the operators you need to also trust that they have not been hacked or compromised. For a business site or any site with high reliability concerns I'd recommend hosting the time solution yourself.

Edit 2: JimmyP's answer has a comment which suggests that the json-time app has some limitations in terms of the number of requests it can support. This means if you need reliability you should host the time server yourself. However, it should be easy enough to add a page on your server which responds with the same format of data. Basically your server takes a query such as

http://json-time.appspot.com/time.json?tz=America/Chicago&callback=foo

and returns a string such as

foo({
 "tz": "America\/Chicago", 
 "hour": 15, 
 "datetime": "Thu, 09 Apr 2009 15:07:01 -0500", 
 "second": 1, 
 "error": false, 
 "minute": 7
})

Note the foo() which wraps the JSON object; this corresponds to the callback=foo in the query. This means when the script is loaded into the page it will call your foo function, which can do whatever it wants with the time. Server-side programming for this example is a separate question.

Why don't you send the time with every page? For example somewhere in the html:

<span id="time" style="display:none;">
    2009-03-03T23:32:12
</span>

Then you could run a Javascript while the site loads and interpret the date. This would reduce the amount of work the network has to do. You can store the corresponding local time and calculate the offset every time you need it.

You could use getTimezoneOffset to get the offset between the local date and the GMT one, and then do the math. But this will only be as accurate as the user's clock.

If you want an accurate time, you should connect to a NTP server. Because of the Same Origin Policy, you can't make a request with JS to another server then yours. I'd suggest you to create a server-side script that connects to the NTP server (in PHP, or whatever language you want) and return the accurate date. Then, use an AJAX request to read this time.

A little addition to Mr. Shiny and New and James answers

Here is a PHP script which you can place on own server and use instead of json-time.appspot.com

<?php
header('Content-Type: application/json');
header("Expires: Tue, 01 Jan 1990 00:00:00 GMT");
header("Cache-Control: no-store, no-cache, must-revalidate, max-age=0");
header("Cache-Control: post-check=0, pre-check=0", false);
header("Pragma: no-cache");

$error = "false";
$tz = $_GET['tz'];

if ( !in_array($tz, DateTimeZone::listIdentifiers())) {
   $error = 'invalid time zone';
   $tz = 'UTC';
}

date_default_timezone_set($tz);

?>
<?php echo htmlspecialchars($_GET['callback'], ENT_QUOTES, 'UTF-8' ); ?>({
 "tz": "<?php echo $tz ?>",
 "hour": <?php echo date('G'); ?>,
 "datetime": "<?php echo date(DATE_RFC2822); ?>",
 "second": <?php echo intval(date('s')); ?>,
 "error": "<?php echo $error; ?>",
 "minute": <?php echo intval(date('i')); ?>
})