Dates, time zones, and overall internationalization are not a programmer's friend, historically. From Julius Caesar in 45 B.C.E. (Julian calendar) through Pope Gregory the XIII in 1582 C.E. (Gregorian calendar), UTC and world-wide calendars, dates and times have shifted to ever-changing politics and new scientific understandings. Although the invention of computing and information exchange is relatively new, many concepts of date and time zone information are not ingrained in the architectures and frameworks we use every day.
Windows time zone support is sufficient, but has drawbacks and is not exposed easily in .NET (requiring registry queries and WIN32 calls).Enter the Olson Time Zone database, a public domain database of historical time zone data points. It is well maintained and used by Unix, Linux, and Java, just to name a few. It is not used by Windows nor .NET, but can be used with the PublicDomain package.
The PublicDomain package is a completely free, public domain collection of oft-needed code and packages for .NET. There is no license, so the code (or any part of it) may be included even in corporate applications. Public domain code has no authority and is provided 'AS-IS'. See this link for more details.
Getting right to the code, here is an example of using the Olson tz database through the PublicDomain DLL (installed in the GAC):
// Common usage examples:
// ======================
// Get the local computer's time zone
TzTimeZone zone = TzTimeZone.CurrentTimeZone;
// Get a common time zone
zone = TzTimeZone.ZoneUsEastern;
// Get a zone in the time zone database
zone = TzTimeZone.GetTimeZone(TzConstants.TimezoneEuropeMoscow);
// Get a TzDateTime representing the current time in Moscow.
// TzDateTime wraps a UTC value and exposes DateTimeLocal and
// DateTimeUTC version of the data for use
TzDateTime moscowLocal = zone.Now;
// All DateTime operations exist on TzDateTime, but the actual data
// is in the aforementioned properties
moscowLocal += new TimeSpan(1, 0, 0);
Console.WriteLine(moscowLocal.DateTimeLocal);
Console.WriteLine(moscowLocal.DateTimeUtc);
// Switch back to the local time zone:
zone = TzTimeZone.ZoneUsEastern;
// Get the abbreviated form of the zone, being
// daylight time sensitive. For example, for this
// zone, it will either return EST or EDT, depending
// on the date passed (DateTime.Now is used if not date
// is specified)
Console.WriteLine(zone.GetAbbreviation());
// Convert a DateTime (either local or UTC) to
// a TzDateTime, which will then carry around its
// time zone
TzDateTime localNow = new TzDateTime(zone.ToLocalTime(DateTime.UtcNow), zone);
// Now we can use this TzDateTime wrapper without ever
// losing the time zone information. If the object
// is serialized, only the UTC value is serialized (as
// well as the zone name).
Console.WriteLine(localNow.DateTimeLocal.ToString("G"));
// This provides a common data interchange and storage pattern.
// The data is always in UTC, and the TzTimeZone context
// is used to convert to local times. This is the way
// time zones should have been designed from the beginning.
Aspects of the code:
TzTimeZone extends the System.TimeZone class, providing static methods for retrieving time zone instances. Names follow the well-known Olson time zone names, such as "America/New_York", "US/Eastern", "Europe/Paris", etc. TzDateTime class primarily wraps the System.DateTime class, also storing the time zone for the date/time, and providing both a local version of the DateTime, and a UTC version. The TzDateTime class prints and serializes the UTC version by default to systematically push programmers towards using UTC dates/times. PublicDomain is created and must be installed or re-packaged with client programs. FileIOPermission for clients of the package.
TzDatabase class exists primarily at compile time for PublicDomain. When there is a new version of the Olson time zone database, the data files are updated and a script is run which reads the database, converts it to C# code, which is then placed into TzTimeZone.cs and PublicDomain is re-compiled. However, the TzDatabase can be used to get all of the database contents in objectified form, since the database is in an obfuscated form.The TzDateTime and TzTimeZone classes have been designed to make converting almost as simple as doing a large Find & Replace for DateTime -> TzDateTime, and TimeZone -> TzTimeZone. However, migration is not always as trivial. The TzDateTime constructors mimic the DateTime constructors, but also require an added parameter which specifies the time zone, or a boolean which specifies that the DateTime provided is a UTC date/time. Time zone support should be architected well, but in general, the recommendation is to create a statically initialized TzTimeZone in your code which represents the canonical time zone of your application, and use this static variable where time zones are required. This makes future modification/internationalization easier.
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