Everyone working with .NET and TimeZones have at one point thought, why the #%&#&% can't I instantiate another TimeZone than the current one, i.e. why is it not possible to create a .NET TimeZone object in the following simple way.
TimeZone zone = new TimeZone("Europe/Copenhagen");
TimeZone zone = new TimeZone("CET");
It seems like Microsoft did not think of, or decided not to implement multi timezone support in .NET, and thats too bad, because Timezone handling, Daylight savings are just a bitch to work with.
Timezones are not so bad, since its pretty easy to identify what timezone a given country is in. There are numerous sources for this on the internet, such as http://www.timeanddate.com/worldclock/, and with a little manual work, or a little screen scraping you can have a combined country/timezone list in no time.
The real problems occurs when you want to find out whether or not a given TimeDate object is within the Daylight savings time for a given country and timezone, and in that, the timezone are of no use, since it says nothing about the daylight savings of the date.
I have tried some solutions where I try to identify the daylight savings time for the most used countries, for a few years in advance, but that won't help me with dates in the past, and is really not a good solution, since the completeness of the solution is at best lacking.
I have searched a lot of places on the internet for a proper solution, and someone have made a solution for retrieving the timezones from the windows registry combined with some dll imports fromt he win32 API, but those are not perfect either.
So having almost tossed the towel into the ring, I stumpled upon another page on the internet http://www.codeplex.com/publicdomain/, and I thought, hmm, another crappy implementation, but no, its a full .NET implementation of the tz database http://www.twinsun.com/tz/tz-link.htm. I had looked at that timezone database myself a few times, but the sheer size and format of the database was just such a daunting task, that I have never fathomed to even be able to read that into a sensible .NET format.
The .NET library called PublicDomain is very easy to work with, much intuitive, and handles timezones and daylight savings very nice.
Its as simple as:
// Get the local computer's time zone
TzTimeZone dkZone = TzTimeZone.GetTimeZone("Europe/Copenhagen");
TzTimeZone ausZone = TzTimeZone.GetTimeZone("Australia/Adelaide");
DateTime dkNow = DateTime.UtcNow; //When in danish timezone
DateTime auzNow = ausZone.ToLocalTime(dkNow);
Gracefully right :)
Too bad Microsoft did not implement this.
The good part is that they will in .NET 3.5. I'm not sure of the extend of the implementation, or how well the daylight saving rules for each country is implemented, but at least its good that Microsoft at least have started looking in this direction.