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From: Rick Maffei <due.to.spam@none.provided>
Newsgroups: comp.lang.ada
Subject: Re: Zeller's Algorithm
Date: Tue, 30 Jul 2002 12:17:02 -0500
Organization: Lockheed Martin Corp, Valley Forge PA
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Paul A Storm wrote:
> 
> Help!
> 
> I am trying to find a Ada implementation of Zeller's algorithm that
> works!  I have found several on the web
> but none of them have worked right.
> 
> For example I tried this one from
> http://www.adapower.com/alg/zeller.html  Inputting June 19 2002 returns
> Wednesday, correct.  Inputting June 19 1999 produces Sunday, incorrect.
> That's a Saturday. Inputting
> February 3, 2002 produces Friday, incorrect.  That's a Sunday.  What
> gives?
> 
> I have had similar weird results using other similar Ada Zeller versions
> I've found on the web.
> 
> I'm wondering if maybe there's a difference in rounding down.  Perhaps a
> 83 vs. 95 peculiarity?
> I don't think so.  It produces compilation errors using the -gnat83
> switch.
> 
> I'm running gnat 3.13a on Sun 2.6
> 
> Argh!  Anybody got any ideas on how to fix this or know of an Ada
> implementation of Zeller's algorithm
> that works?
> 
> Paul Storm

While reviewing the latest Risks Digest, I came across the following
item titled "Possible day-of-week error" concerning the Zeller
algorithm:

http://catless.ncl.ac.uk/Risks/22.18.html#subj10.1

Just thought it may be of interest in this thread.

Rick Maffei