How to detect user's timezone?

I need to know what time zone is currently my users are in based on their IP or http header.

I got many answer regarding this issue, but i could not understood those answer. Some said use -new Date().getTimezoneOffset()/60 (from here). But what does it mean?

I have a date_default_timezone_set("Asia/Calcutta"); in the root of my (index.php) page. So for this I have to get the timezone dynamically and set it in place of Asia/Calcutta.

Solution 1:

To summarize Matt Johnson's answer in terms of code:

<script type="text/javascript" src="//ajax.googleapis.com/ajax/libs/jquery/2.0.0/jquery.min.js">
</script>
<script type="text/javascript" src="//cdnjs.cloudflare.com/ajax/libs/jstimezonedetect/1.0.4/jstz.min.js">
</script>
<script type="text/javascript">
  $(document).ready(function(){
    var tz = jstz.determine(); // Determines the time zone of the browser client
    var timezone = tz.name(); //For e.g.:"Asia/Kolkata" for the Indian Time.
    $.post("url-to-function-that-handles-time-zone", {tz: timezone}, function(data) {
       //Preocess the timezone in the controller function and get
       //the confirmation value here. On success, refresh the page.
     });
  });
</script>

Solution 2:

Time zone information of the browser is not part of the HTTP spec, so you can't just get it from a header.

If you have location coordinates (from a mobile device GPS, for example), then you can find the time zone using one of these methods. However, geolocation by IP address is not a great solution because often the IP is that of an ISP or proxy server which may be in another time zone.

There are some strategies you can use to try to detect the time zone, such as using jsTimeZoneDetect library, which is a great starting point, but imperfect enough that you can't just rely on that alone. If you're using moment.js, there's a built in function in moment-timezone called moment.tz.guess() that does the same thing.

The idea of using JavaScript's getTimezoneOffset() function is flawed in that you are not getting a time zone - just a single offset for a particular date. See the TimeZone tag wiki's section titled "TimeZone != Offset".

However you look at it, ultimately you have to decide on one of two approaches:

• Ask the user to tell you their time zone, from some sort of drop-down list or map-based timezone picker control.

OR

• Only send time to the browser in UTC, and use JavaScript on the browser to convert to whatever local time zone the user might have their computer set to.

I discuss this in more detail (from a c# perspective) in this answer.