Globally convert UTC DateTimes to user specified local DateTimes

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You can’t do directly what you asked for, but I will suggest some alternatives. As Nicholas pointed out, there is nothing in HTTP that would give you the time zone directly.

Option 1

  • First, decide which type of time zone data you want to work with. There are two different types available, either the Microsoft time zones that you can access with the TimeZoneInfo class, or the IANA/Olson time zones that the rest of the world uses. Read here for more info. My recommendation would be the latter, using the implementation provided by NodaTime.

  • Then determine which time zone you want to convert to. You should allow your user a setting somewhere to pick their time zone.

    • You might show a drop-down list to pick one of several time zones, or you might do something more useful, like display a map of the world that they can click to select their time zone. There are several libraries that can do this in Javascript, but my favorite is this one.

    • You might want to guess a default time zone to use, so you can be as close to accurate as possible before they pick from the list (or map). There is a great library for this called jsTimeZoneDetect. It will interrogate the browser’s clock and make a best guess assumption of what time zone it might be. It is fairly good, but it is still just a guess. Don’t use it blindly – but do use it to determine a starting point. Update You can now also do this with moment.tz.guess(), in the moment-timezone component of moment.js.

  • Now that you know the time zone of the user, you can use that value to convert your UTC DateTime values to that local time zone. Unfortunately, there is nothing you can set on the thread that will do that. When you change the system time zone, it is global for all processes and threads. So you have no choice but to pass the time zone to each and every place you are sending it back. (I believe this was your main question.) See this almost duplicate here.

  • Before you convert it to a string, you will need to also know the user’s locale (which you can get from the Request.UserLanguages value). You can assign it to the current thread, or you can pass it as a parameter to the DateTime.ToString() method. This doesn’t do any time zone conversion – it just makes sure that the numbers are in the correct position, using the correct separators, and the appropriate language for names of weekdays or months.

Option 2

Don’t convert it to local time on the server at all.

  • Since you said you are working with UTC values, make sure their .Kind property is Utc. You should probably do this when you load from your database, but if you have to you can do it manually:

    myDateTime = DateTime.SpecifyKind(myDateTime, DateTimeKind.Utc);

  • Send it back to the browser as pure UTC, in an invariant format like ISO8601. In other words:

    myDateTime.ToString("o");  // example:  "2013-05-02T21:01:26.0828604Z"

  • Use some JavaScript on the browser to parse it as UTC. It will automatically pick up the local time settings of the browser. One way is to use the built-in Date object in JavaScript, like this:

    var dt = new Date('2013-05-02T21:01:26.0828604Z');

    However, this will only work in newer browsers that support the ISO-8601 format. Instead, I recommend using the moment.js library. It is consistent across browsers, and it has better support for ISO dates, and localization. Plus you get a lot of other useful parsing and formatting functions.

    // pass the value from your server
var m = moment('2013-05-02T21:01:26.0828604Z');

// use one of the formats supported by moment.js
// this is locale-specific "long date time" format.
var s = m.format('LLLL');

The advantage of Option 1 is that you can work with times in any time zone. If you can ask the user for their timezone from a dropdown list, then you need not use any Javascript.

The advantage of Option 2 is that you get the browser to do some of the work for you. This is the best way to go if you’re sending out raw data, such as making AJAX calls to a WebAPI. However, JavaScript is only aware of UTC and the browser’s local time zone. So it doesn’t work so well if you need to convert to other zones.

You should also be aware that if you choose Option #2, you may be affected by a flaw in the design of ECMAScript 5.1. This comes into play if you are working with dates that are covered by a different set of daylight saving time rules than are currently in effect. You can read more in this question, and on my blog.

It would be so much easier if we had some time zone information in the HTTP headers, but unfortunately we don’t. These are a lot of hoops to jump through, but it’s the best way to have both flexibility and accuracy.

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