Path: utzoo!yunexus!maccs!cs3b3aj
From: cs3b3aj@maccs.McMaster.CA (Stephen M. Dunn)
Newsgroups: comp.lang.pascal
Subject: Re: Day-of-week algorithm, please!
Summary: Some years such that x mod 4 = 0 aren't leaps
Message-ID: <2394@maccs.McMaster.CA>
Date: 11 Apr 89 05:17:39 GMT
Article-I.D.: maccs.2394
References: <19040@adm.BRL.MIL> <1780001@hpcc01.HP.COM>
Reply-To: cs3b3aj@maccs.UUCP (Stephen M. Dunn)
Organization: McMaster U., Hamilton, Ont., Can.
Lines: 27

In article <1780001@hpcc01.HP.COM> azarian@hpcc01.HP.COM (Randy Azarian) writes:
...
>  total := total + + (year - 1) div 4;

   But watch out, because not every year such that year mod 4 = 0 is a leap
year!  The strangeness in the pattern occurs on century years, but I'm not 
sure which way round it is.  It is either that years divisible by 400 are
_not_ leap years, or that any century year is not a leap year _except_
for years divisible by 400.

   Confused?  It's either that 1900, 2100, 2200, 2300, 2500 are leap years and
2000,2400 aren't (first case, above), or that 2000, 2400 are leap years and
1900, 2100, 2200, 2300, 2500 aren't.

   However, I'm not sure which way it is.  If it's the first way, your
function will only work for another 10 years or so.  If it's the second
way, your program's fine for another 110 years.

   Does anyone know which way it is?  (Just to satisfy that part of my
brain that killed the cat)  If I were at home, I could look it up in a
book on my bookshelf, but I'm here at school, so I'm in the dark.

-- 
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! Stephen M. Dunn, cs3b3aj@maccs.McMaster.CA ! DISCLAIMER:           !
! This space left unintentionally blank - vi ! I'm only an undergrad !
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