How should I populate a list of IANA / Olson time zones from Noda Time?

Question

I am using NodaTime in an application, and I need the user to select their timezone from a dropdown list. I have the following soft requirements:

1) The list only contain choices that are reasonably valid for the present and near future for real places. Historical, obscure, and generic timezones should be filtered out.

2) The list should be sorted first by UTC offset, and then by timezone name. This hopefully puts them in an order that is meaningful for the user.

I've written the following code, which does indeed work, but doesn't have exactly what I'm after. The filter probably needs to be adjusted, and I'd rather have the offset represent the base (non-dst) offset, rather than the current offset.

Suggestions? Recommendations?

var now = Instant.FromDateTimeUtc(DateTime.UtcNow);
var tzdb = DateTimeZoneProviders.Tzdb;
var list = from id in tzdb.Ids
           where id.Contains("/") && !id.StartsWith("etc", StringComparison.OrdinalIgnoreCase)
           let tz = tzdb[id]
           let offset = tz.GetOffsetFromUtc(now)
           orderby offset, id
           select new
           {
               Id = id,
               DisplayValue = string.Format("({0}) {1}", offset.ToString("+HH:mm", null), id)
           };

// ultimately we build a dropdown list, but for demo purposes you can just dump the results
foreach (var item in list)
    Console.WriteLine(item.DisplayValue);

Answer 1

Noda Time 1.1 has the zone.tab data, so you can now do the following:

/// <summary>
/// Returns a list of valid timezones as a dictionary, where the key is
/// the timezone id, and the value can be used for display.
/// </summary>
/// <param name="countryCode">
/// The two-letter country code to get timezones for.
/// Returns all timezones if null or empty.
/// </param>
public IDictionary<string, string> GetTimeZones(string countryCode)
{
    var now = SystemClock.Instance.Now;
    var tzdb = DateTimeZoneProviders.Tzdb;

    var list = 
        from location in TzdbDateTimeZoneSource.Default.ZoneLocations
        where string.IsNullOrEmpty(countryCode) ||
              location.CountryCode.Equals(countryCode, 
                                          StringComparison.OrdinalIgnoreCase)
        let zoneId = location.ZoneId
        let tz = tzdb[zoneId]
        let offset = tz.GetZoneInterval(now).StandardOffset
        orderby offset, zoneId
        select new
        {
            Id = zoneId,
            DisplayValue = string.Format("({0:+HH:mm}) {1}", offset, zoneId)
        };

    return list.ToDictionary(x => x.Id, x => x.DisplayValue);
}

Alternative Approach

Instead of providing a drop down at all, you can use a map-based timezone picker.

Answer 2

Getting the standard offset is easy - tz.GetZoneInterval(now).StandardOffset. That will give you the "current" standard offset (it's possible for a zone to change over time).

The filtering may be appropriate for you - I wouldn't like to say for sure. It's certainly not ideal in that the IDs aren't really designed for display. Ideally you'd use the Unicode CLDR "example" places, but we don't have any CLDR integration on that front at the moment.