c# - Get the default timezone for a country (via CultureInfo)

Question

Is there a program or a table that provides the default timezone for every country?

Yes, the US, Canada, & Russia have multiple timezones. (I think every other country has just one.) But it's better to start on the most likely if a country is known rather than just provide a list starting at GMT.

Preferably in C# but I'll take it in anything and convert to C#.

Answer 1

As identified in the comments of the question, you aren't going to be able to get a single time zone for each country. There are just too many cases of countries that have multiple time zones.

What you can do is filter the list of standard IANA/Olson time zones down to those available within a specific country.

One way to do this in C# is with Noda Time:

IEnumerable<string> zoneIds = TzdbDateTimeZoneSource.Default.ZoneLocations
    .Where(x => x.CountryCode == countryCode)
    .Select(x => x.ZoneId);

Pass a two-digit ISO-3166 country code, such as "AU" for Australia. The results are:

"Australia/Lord_Howe",
"Australia/Hobart",
"Australia/Currie",
"Australia/Melbourne",
"Australia/Sydney",
"Australia/Broken_Hill",
"Australia/Brisbane",
"Australia/Lindeman",
"Australia/Adelaide",
"Australia/Darwin",
"Australia/Perth",
"Australia/Eucla"

And if for some reason you'd like Windows time zone identifiers that you can use with the TimeZoneInfo object, Noda Time can map those too:

var source = TzdbDateTimeZoneSource.Default;
IEnumerable<string> windowsZoneIds = source.ZoneLocations
    .Where(x => x.CountryCode == countryCode)
    .Select(tz => source.WindowsMapping.MapZones
        .FirstOrDefault(x => x.TzdbIds.Contains(
                             source.CanonicalIdMap.First(y => y.Value == tz.ZoneId).Key)))
    .Where(x => x != null)
    .Select(x => x.WindowsId)
    .Distinct()

Again, called with "AU" for Australia returns:

"Tasmania Standard Time",
"AUS Eastern Standard Time",
"Cen. Australia Standard Time",
"E. Australia Standard Time",
"AUS Central Standard Time",
"W. Australia Standard Time"

If you're wondering about how reliable this data is, the country to tzid mapping is part of the IANA time zone database itself, in the zone.tab file. The IANA to Windows mapping data comes from the Unicode CLDR supplemental data. It doesn't get any closer to "official" than that.