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From: Wilhelm Spickermann <jwos.20.unbenutzbar@spamgourmet.com>
Newsgroups: comp.lang.ada
Subject: Re: Inspiration for a better calendar package?
Date: Thu, 18 Dec 2008 22:48:52 +0100
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Date: 2008-12-18T22:48:52+01:00
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Dmitry A. Kazakov wrote:

> Where is any problem? Except that the algorithm is not good. It
> should be (still the case 2):
> 
> delay 60.0 * 60.0 * 24.0 * 25.0 * 3; -- come close enough to
>                                      --  judge 
> delay until Time_Of (Year =>..., Month =>..., Day =>...); 
> -- do things
> exception
>   when Time_Error =>  -- watch your fingers lady!

One problem is, that the error message may come 75 days after
typing here. 

Trying to solve this problem lead me to "wall time". If we do all
the checks possible early and also take into account the given
time zone and then store the result in a time zone independent
way into some opaque type, which can later be transformed
further -- then this opaque type is just the proposed "wall
time".

Another problem is, that the result type and also the operation
of Time_Of may depend on the Operating System configuration. In
most OS I know, there is no possibility to give an error message
about missing leap second information. The case of a missing
leap second information is just handled as the case of no leap
second. And adding leap seconds or for instance changing a Unix
from "posix" to "right" localtime will bring the times to jump
around even if everything was given in UTC.

Wilhelm