Time zone confusion and implementation hints

Thomas KIPP trwk76 at gmail.com
Thu Jul 1 20:29:48 UTC 2010

Hi Yves,

There was an effort made a couple of years ago on CodePlex, implementing the

Here's the link: http://publicdomain.codeplex.com/



On Thu, Jul 1, 2010 at 9:22 PM, Yves Goergen <nospam.list at unclassified.de>wrote:

> Hi,
> I'm currently implementing an API for the tz database that should look
> like .NET's TimeZoneInfo class to replace that in my application. The
> API includes:
> * Convert a date and time from UTC to a time zone
> * Convert a date and time from a time zone to UTC
> * Return the base UTC offset of a time zone for a date and time
> * Return the save offset of a time zone for a date and time
> * Determine whether a date and time is ambiguous in a time zone
> * Determine whether a date and time is invalid in a time zone
> It will come with a code generator that "compiles" the text files into
> C# code so that there's no need to deploy additional files with the
> application. The API's internal data structures are very similar to the
> tz database records, these are already coded.
> Currently I'm stuck because of the flexibility of the tz rules. These
> can define transition times either in local, standard or universal time.
> Additionally, the caller of my API passes a date and time that is either
> local or universal. All this needs to be matched correctly. The right
> rule needs to be selected and there can be ambiguous or invalid times
> all along the way. It's not only hard to implement, it's also hard to
> describe, it's very confusing not to have a common base [zone] you can
> hold on to.
> I have now read about any manpage and code file from the tzcode
> distribution, starting with zic.c and going backwards in the list. I
> think the file localtime.c does the actual conversion but it's hard to
> read because it contains only few comments and uses short all-lowercase
> global variable names. I couldn't follow it to its core where the magic
> happens.
> I have analysed .NET's TimeZoneInfo implementation with .NET Reflector
> but it's not half as mighty as what the tz database can express. I also
> looked at the C# library ZoneInfo, but it uses an incompatible license,
> reads the text files at runtime and last but not least is quite inexact
> around transition times which it inacceptable for a calendar
> application. Also I think it's a good idea to write my own API for it
> because I'll need a Java port later as well, working on the exact same
> data.
> Could somebody assist me in this, please? Are there helpful thoughts to
> use when implementing such a programme? Right now my biggest problem is
> the transition times and in consequence finding the right rule to use
> for a given date and time.
> --
> Yves Goergen "LonelyPixel" <nospam.list at unclassified.de>
> Visit my web laboratory at http://beta.unclassified.de
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