Get client's timezone offset in ExpressJS

If you control the client, you can do this with client-side JavaScript.

If you don't (e.g. you're building a server-side component like an API), then you can't pull it out of the HTTP request (unless you're using sessions, but even that's not necessarily reliable).

On the upside, if it's only server-side, you shouldn't worry about it either: set all your date objects as UTC or a Unix timestamp and leave it to the client developer to handle timezones.

As others have mentioned, getting the client-side browser/OS timezone offset is not available via HTTP, so you need to send this data from the client side.

In my case I have an app that requires login...so I can store a user's preferences. The default is to use browser timezone, but the user can also configure a specific timezone to use all the time instead which is stored in their user profile (e.g. America/Dallas or Europe/Amsterdam).

I don't trust browser time to be correct. The user may have also screwed up their OS timezone setting...so timezone offset may also not be correct. However most modern OSes set timezone automatically based on Geo-IP implied location...so for many users the convenience of being able to travel around the world and login to the app and see date/time in whatever local timezone they happen to be in is worth the effort for the user experience. Users that always want to see a specific timezone can configure that and we will use that preference instead.

The way I do this is the following... on the login form add a hidden field and set the value with javascript. When the user logs in, store this timezone offset in the session. If the user hasn't set a preferred timezone, then we use this offset when rendering date/time. This means that if you have long sessions and a user can travel around between countries...they will still show the old timezone offset until the next logout/login. You can of course get this data more frequently or even on every request if you want...but for my purposes getting this on login is enough. My sessions are expired on IP address change anyway...so yeah. Of course if their session crosses a daylight savings switch then the offset won't be accurate until the next login (assuming their OS/browser TZ is correct in the first place).

Client Side

<input type="hidden" name="tzOffset" id="tzOffset">
<!-- ... -->
<script>
var tzOffset = new Date().getTimezoneOffset(),
    tzInput = document.getElementById('tzOffset');
tzInput.value = tzOffset*(-1);
</script>

Note that I multiple it by -1 because I am going to use moment.js for formatting on the express end and the offset is backwards (utc offset from local vs local offset from utc). You could also do this on the server end. You should probably validate this number as well before using it...just minimal example code here.

Server Side

Then on the server side (express) if there is a successful login, I stick that tzOffset value in the session.

Then when formatting dates in express with moment-timezone I can do something like the following. This function is a "formatter" which I can expose to pug/views and then format any random date object with the right timezone/formats for the user.

function formatDateTimeForUser (date) {
    var userTZ,     // user specified TZ like Europe/Berlin
        userTZoffset, // tzOffset we got from login form
        userDateFormat,  // user specified date format (or default)
        userTimeFormat; // user specified time format (or default)

    if (userTZ)
        return moment(date).tz(userTZ).format(userDateFormat+' '+userTimeFormat+' zz');
    else
        return moment(date).utcOffset(userTZoffset).format(userDateFormat+' '+userTimeFormat+' ZZ');
}

The ZZ format is for showing timezone in numeric offset format (relevant when we use a fixed numeric offset from client.

The zz format is for showing timezone in character format (e.g. PDT, PST, EST, etc) which is relevant when we have a timezone like Europe/Berlin instead of a fixed numerical offset.

Another approach

Push raw dates to client and do client-side formatting. Downside is less control and consistency and more js to push to the browser. moment will also run client side if you like.

I just configure my formatters server-side based on user locale and user prefs and then expose those formatters for use in my pug templates in express. So far it works pretty well for me.

Funny Story

I had a co-worker that manually set the wrong timezone on their computer and so the time was wrong. Instead of fixing the timezone they disabled network time and manually set the time to be the "correct" time.

Then they got grumpy when everyone was showing up to meetings they scheduled an hour late.

So yeah...no guarantees the client-side time or timezone offset will be correct.

The reality is, even if you have got the timezone offset, it doesn't mean you have the the correct time on client side, considering some area have "daylight saving time".

But, you could "guess" which timezone the user is in, considering the following data:

1. req.get('Accept-Language'), for which country is the user may live in now;
2. req.ip, to search in a database and find out the approximate location;
3. client-side (new Date).getTimezoneOffset(), for timezone offset;

Finally, through the calculation of the above result, your guess should be almost there. The timezone should be stored in string like 'America/New_York' but not a timezone offset number.

http://momentjs.com/timezone/ should be a good tool to deal with timezone.

After you save the "almost correct" timezone, don't forget to leave a place where user can change it.