Determine a user's timezone

The most popular (==standard?) way of determining the time zone I've seen around is simply asking the users themselves. If your website requires subscription, this could be saved in the users' profile data. For anon users, the dates could be displayed as UTC or GMT or some such.

I'm not trying to be a smart aleck. It's just that sometimes some problems have finer solutions outside of any programming context.

There are no HTTP headers that will report the clients timezone so far although it has been suggested to include it in the HTTP specification.

If it was me, I would probably try to fetch the timezone using clientside JavaScript and then submit it to the server using Ajax or something.

JavaScript is the easiest way to get the client's local time. I would suggest using an XMLHttpRequest to send back the local time, and if that fails, fall back to the timezone detected based on their IP address.

As far as geolocation, I've used MaxMind GeoIP on several projects and it works well, though I'm not sure if they provide timezone data. It's a service you pay for and they provide monthly updates to your database. They provide wrappers in several web languages.

All the magic seems to be in

visitortime.getTimezoneOffset()

That's cool, I didn't know about that. Does it work in Internet Explorer etc? From there you should be able to use JavaScript to Ajax, set cookies whatever. I'd probably go the cookie route myself.

You'll need to allow the user to change it though. We tried to use geo-location (via maxmind) to do this a while ago, and it was wrong enough to make it not worth doing. So we just let the user set it in their profile, and show a notice to users who haven't set theirs yet.

If you happen to be using OpenID for authentication, Simple Registration Extension would solve the problem for authenticated users (You'll need to convert from tz to numeric).

Another option would be to infer the time zone from the user agent's country preference. This is a somewhat crude method (won't work for en-US), but makes a good approximation.

There can be a few ways to determine the timezone in the browser. If there is a standard function that is available and supported by your browser, that is what you should use. Below are three ways to get the same information in different formats. Avoid using non-standard solutions that make any guesses based on certain assumptions or hard coded lists of zones though they may be helpful if nothing else can be done.

Once you have this info, you can pass this as a non-standard request header to server and use it there. If you also need the timezone offset, you can also pass it to server in headers or in request payload which can be retrieved with dateObj.getTimezoneOffset().

1. Use Intl API to get the Olson format (Standard and recommended way): Note that this is not supported by all browsers. Refer this link for details on browser support for this. This API let's you get the timezone in Olson format i.e., something like Asia/Kolkata, America/New_York etc.

Intl.DateTimeFormat().resolvedOptions().timeZone

2. Use Date object to get the long format such as India Standard Time, Eastern Standard Time etc: This is supported by all browsers.

let dateObj = new Date(2021, 11, 25, 09, 30, 00);

//then 

dateObj.toString() 

//yields

Sat Dec 25 2021 09:30:00 GMT+0530 (India Standard Time) //I am located in India (IST)

Notice the string contains timezone info in long and short formats. You can now use regex to get this info out:

let longZoneRegex = /\((.+)\)/;
dateObj.toString().match(longZoneRegex);

//yields

['(India Standard Time)', 'India Standard Time', index: 34, input: 'Sat Dec 25 2021 09:30:00 GMT+0530 (India Standard Time)', groups: undefined]

//Note that output is an array so use output[1] to get the timezone name.

3. Use Date object to get the short format such as GMT+0530, GMT-0500 etc: This is supported by all browsers.

Similarly, you can get the short format out too:

let shortZoneRegex = /GMT[+-]\d{1,4}/;
dateObj.toString().match(shortZoneRegex);

//yields

['GMT+0530', index: 25, input: 'Sat Dec 25 2021 09:30:00 GMT+0530 (India Standard Time)', groups: undefined]

//Note that output is an array so use output[0] to get the timezone name.

There's no such way to figure the timezone in the actual HTML code or any user-agent string, but what you can do is make a basic function getting it using JavaScript.

I don't know how to code with JavaScript yet so my function might take time to make.

However, you can try to get the actual timezone also using JavaScript with the getTzimezoneOffset() function in the Date section or simply new Date().getTimezoneOffset();.

I think that @Matt Johnson-Pints is by far the best and a CanIuse search reveals that now it is widely adopted:

https://caniuse.com/?search=Intl.DateTimeFormat().resolvedOptions().timeZone

One of the challenges though is to consider why you want to know the Timezone. Because I think one of the things most people have missed is that they can change! If a user travels with his laptop from Europe to America if you had previously stored it in a database their timezone is now incorrect (even if the user never actually updates their devices timezone). This is also the problem with @Mads Kristiansen answer as well because users travel - you cannot rely on it as a given.

For example, my Linux laptop has "automatic timezone" turned off. Whilst the time might update my timezone doesn't.

So I believe the answer is - what do you need it for? Client side certainly seems to give an easier way to ascertain it, but both client and server side code will depend on either the user updating their timezone or it updating automatically. I might of course be wrong.