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From: James Giles <JGiles@cris.com>
Subject: Re: language wars (results 13 September) last posting
Date: 1996/09/18
Message-ID: <3240AB89.267D@cris.com>#1/1
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to: James Giles <JGiles@cris.com>
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 comp.lang.ada,comp.lang.apl,comp.lang.basic,comp.lang.c,comp.lang.fortran,comp.lang.perl.misc,comp.lang.pl1,comp.lang.rexx,comp.lang.pascal.misc,comp.lang.smalltalk
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Date: 1996-09-18T00:00:00+00:00
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> > [...]
> > Skeptics are invited to write a program to do the calculation for
> > themselves.
> 
> Assuming that *you* have done so, please have the courtesy to run it
> 2800 years and note that the 13th is equidistributed in weekdays.
> Then you can come back and apologise.  You have to run it more than
> 400 years because you have to go through a complete cycle of exceptions
> to the century rule.

I find that I'm the one to apologise.  And i do.

The issue that confused me was the irrelevant statement the 12
wasn't divisible by 7.  That has nothing to do with it.  The issue
is entirely a matter of what day of the week a year begins on.
During the century, the days are equidistributed (and will be over
this next century change as well).  But, because of the 400 year
rule, 4 centuries *is* commensurate with 7 days.  This means that
centuries begin on one of four different days of the week: Sat,
Mon., Wed., or Fri..  That asymmetry is sufficient to distort
the distribution of days for 13ths of months.

J. Giles
Ricercar Software