How to Determine a Timezone by The Utc Offset

How can I determine a timezone by the UTC offset?

Short answer: you can't.

Daylight saving time make it impossible. For example, there is no way to determine, solely from UTC offset, the difference between Arizona and California in the summer, or Arizona and New Mexico in the winter (since Arizona does not observe DST).

There is also the issue of what time different countries observe DST. For example, in the US DST starts earlier and ends later than in Europe.

A close guess is possible (i.e. +/- an hour), but if you are using it to display time to users you will inevitably display the wrong time to some of them.

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Update: From the comments, it looks like your primary goal is to display a timestamp in the user's local timezone. If that is what you want to do, you should send the time as a UTC timestamp, and then just rewrite it on the user's browser with Javascript. In the case that they don't have Javascript enabled, they would still see a usable UTC timestamp. Here is a function I came up with in this question, which I used in this Greasemonkey script. You may want to tweak it to suit your needs.

//@param timestamp An ISO-8601 timestamp in the form YYYY-MM-DDTHH:MM:SS±HH:MM
//Note: Some other valid ISO-8601 timestamps are not accepted by this function
function parseISO8601(timestamp)
{
  var regex = new RegExp("^([\\d]{4})-([\\d]{2})-([\\d]{2})T([\\d]{2}):([\\d]{2}):([\\d]{2})([\\+\\-])([\\d]{2}):([\\d]{2})$");
  var matches = regex.exec(timestamp);
  if(matches != null)
  {
    var offset = parseInt(matches[8], 10) * 60 + parseInt(matches[9], 10);
    if(matches[7] == "-")
      offset = -offset;

    return new Date(
      Date.UTC(
        parseInt(matches[1], 10),
        parseInt(matches[2], 10) - 1,
        parseInt(matches[3], 10),
        parseInt(matches[4], 10),
        parseInt(matches[5], 10),
        parseInt(matches[6], 10)
      ) - offset*60*1000
    );
  }
  return null;
}

Here is a function I use on my blog to display a parsed timestamp in the user's local timezone. Again, you can tweak it to the format you want.

var weekDays = new Array("Sunday", "Monday", "Tuesday", "Wednesday",
        "Thursday", "Friday", "Saturday");
var months = new Array("January", "February", "March", "April", "May", "June",
        "July", "August", "September", "October", "November", "December");

function toLocalTime(date)
{
  var hour = date.getHours();
  var ampm = (hour < 12 ? "am" : "pm");
  hour = (hour + 11)%12 + 1;

  var minutes = date.getMinutes();
  if(minutes < 10)
    minutes = "0" + minutes;

  return weekDays[date.getDay()] + ", "
       + months[date.getMonth()] + " "
       + date.getDate()          + ", "
       + date.getFullYear()      + " at "
       + hour                    + ":"
       + minutes                 + " "
       + ampm;
}

How to get timezone by utc offset?

Yes you can do it, check this out.

final List<ZoneId> timeZoneByUtc = ZoneId.getAvailableZoneIds().stream().map(ZoneId::of)
        .filter(z -> z.getRules().getOffset(Instant.now()).equals(ZoneOffset.ofHours(-5)))
        .collect(Collectors.toList());

How to attribute time zones to a given UTC offset

In this example, the local timezone could be from North America ...

Sorry, but no. You've only determined that the offset for that point in time is UTC-6. Since it is a date in June, it could be Mountain Daylight Time in the US, but it could also be Central Standard Time in parts of Canada that don't observer daylight saving time. It could also be in Central America, such as in Belize, El Salvador, or Guatemala, or it could be in the Galapagos Islands or Easter Island.

What is the best way to calculate the timezone offset/timezone without knowing the local timezone?

You can't. From an offset alone, all you know is the offset. You cannot determine the time zone. See "Time Zone != Offset" in the timezone tag wiki.

For your scenario, the best you could hope to do with a single timestamp/offset pair would be to provide a list of time zones that might be valid for a given timestamp and offset. In some cases you'll only have one, but in many cases there will be multiple.

Now - if you have lots of data, all from the same time zone, across a long enough period of time, you might be able to filter the list further to guess at a time zone. For example, if you know you have UTC-8 offsets in January and UTC-7 offsets in July, then you know you have Pacific Time because it's the only one that has that particular combination (in recent years). However, you still can't tell if it's America/Los_Angeles (US), America/Vancouver (CA), or America/Tijuana (MX) without additional information.

However, even when you have additional information (such as the country), you may find edge cases that have ambiguous results. One such example is the following:

UTC Time:   2020-11-01T06:00Z
Local Time: 2020-11-01T01:00-05:00
Offset:     UTC-5
Country:    US

This might be Eastern Standard Time (America/New_York), or it might be Central Daylight Time (America/Chicago). Both are in effect simultaneously at this point in time due to how DST transitions operate in the US. (You can verify this here.)

Getting the client's time zone (and offset) in JavaScript

Using getTimezoneOffset()

You can get the time zone offset in minutes like this:

var offset = new Date().getTimezoneOffset();
console.log(offset);
// if offset equals -60 then the time zone offset is UTC+01

Determining the time zone offset of UTC time in .Net

is there a way for me to find out what the UTC offset is in C# given that I know the date/time and the time zone

Technically you shouldn't need to find out this information, the underlying system clock/calendar/what-have-you should take care of it. (I'm sure anybody who has worked on date-time logic will attest to the fact that it is non-trivial to say the least.)

Ideally, any time you need to convert a time zone, you have three things:

1. A DateTime object representing the value to be converted.
2. A TimeZoneInfo object representing the known current format of the DateTime object (in this case UTC).
3. A TimeZoneInfo object representing the target time zone.

Given those things, any time you want to display the DateTime to a localized user, you'd use something like TimeZoneInfo.ConvertTime to perform the conversion. This should ideally be done as close to the UI as possible so that backing logic is always consistently performed in UTC.

How can I get the UTC offset from a timezone

I would recommend not to use tzdb directly, but rather a date library that deals well with timezones. In particular, I found Luxon to be good for this, see their documentation about timezone handling. To get the offset, you just create a DateTime with the desired timezone, then get its .offset:

const dateInZone = DateTime.fromISO("2022-10-23T21:10:56Z", {
  zone: "Europe/Stockholm"
});
console.log(dateInZone.offset)

Alternatively, create a Zone instance and get its .offset() at a particular timestamp:

const zone = new IANAZone("Europe/Stockholm");
console.log(zone.offset(Date.parse("2022-10-23T21:10:56Z")));

Calculate the UTC offset given a TimeZone string in JavaScript

There is no function in ECMAScript (ECMA-262) that can perform the operation you requested. This is simply because standard ECMAScript does not know anything about time zones other than that of the local computer, and UTC.

However, in browsers that support the ECMAScript Internationalization API (ECMA-402), and fully support the IANA time zone database identifiers, you can hack together a function like this:

function getTimezoneOffset(d, tz) {
  const a = d.toLocaleString("ja", {timeZone: tz}).split(/[/\s:]/);
  a[1]--;
  const t1 = Date.UTC.apply(null, a);
  const t2 = new Date(d).setMilliseconds(0);
  return (t2 - t1) / 60 / 1000;
}

This will work in current versions of Chrome, and perhaps in a few other places. but it is certainly not guaranteed to work everywhere. In particular, it won't work in Internet Explorer browsers of any version.

Example usage (in Chrome):

getTimezoneOffset(new Date(2016, 0, 1), "America/New_York") // 300
getTimezoneOffset(new Date(2016, 6, 1), "America/New_York") // 240
getTimezoneOffset(new Date(2016, 0, 1), "Europe/Paris") // -60
getTimezoneOffset(new Date(2016, 6, 1), "Europe/Paris") // -120

A few things to note about this particular function:

• Like I mentioned, It's not going to work everywhere. Eventually, as all browsers catch up to modern standards it will, but it won't currently.

• The date you pass in will indeed affect the result. This is due daylight saving time and other time zone anomalies. You can pass the current date just with new Date(), but the result will change based on when you call the function. See "time zone != offset" in the timezone tag wiki.

• The results of this function are the same as Date.getTimezoneOffset - in terms of minutes, with positive values being West of UTC. If you are working with ISO8601 offsets, you'll need to convert to hours and invert the sign.

• The function relies on the time zone formatting functionality of the toLocaleString function. I picked the 'ja' culture, because the date parts were already in the correct order for the array. This is a hack indeed. Ideally there would be an API that would let you access time zone information without binding it to a locale when formatting. Unfortunately, the designers of this particular API have made the mistake of associating time zone with locale. This is a mistake that's been made in a few other APIs from various languages, and unfortunately was carried into JavaScript here.

  Restated plainly: The only time zone functionality in ECMA-402 is to apply a time zone when formatting a string, which is a design flaw, IMHO.

• There's a bug in my example usage section above, that exemplifies part of why this API is a mistake. Specifically, there's no way to specify a time zone when you create a Date object. The Jan 1st and July 1st dates I pass in are created in the local time zone, not in the time zone specified. Therefore, the output may not be exactly what you expect near a transition. This could be hacked even more to work around this problem, but I will leave that as an exercise to you.

Again - Though this answer satisfies the criteria asked for, as there are no external libraries involved, I strongly recommend against using this in any production code. If you're planning on doing anything important with this, I'd use one of the libraries I listed here.

How to calculate time zone UTC offset for specified date and time with NodaTime (given daylight)?

Fundamentally, you need DateTimeZone.GetUtcOffset, which accepts an Instant.

If the DateTime value always has a Kind of Utc, you can use Instant.FromDateTimeUtc. If it might have a different Kind, you'll need to work out more detailed requirements.

Next you need an IDateTimeZoneProvider to map the time zone ID into a DateTimeZone. That might be DateTimeZoneProviders.Tzdb, or it might be one you've injected somewhere for testability.

Once you've got the Instant and the DateTimeZone, you can call GetUtcOffset to get an Offset. You can convert that back to a TimeSpan, but I'd actually encourage you to avoid using DateTime and TimeSpan as far as possible in your app - if you can use the Noda Time types everywhere within your codebase and only convert between those and the BCL types at boundaries (e.g. database access) you'll find you need to do a lot less of this work.

But if you really need a TimeSpan, the method would look like this:

TimeSpan GetTimeZoneOffset(string timeZoneId, DateTime dateTimeUtc)
{
    Instant instant = Instant.FromDateTimeUtc(dateTimeUtc);
    DateTimeZone zone = DateTimeZoneProviders.Tzdb[timeZoneId];
    Offset offset = zone.GetUtcOffset(instant);
    return offset.ToTimeSpan();
}

Note that the IDateTimeZoneProvider[string] indexer will throw an exception if the time zone isn't found in that provider. If you want to handle this a different way, use IDateTimeZoneProvider.GetZoneOrNull() and check whether the result is null or not.

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