Path: utzoo!utgpu!water!watmath!clyde!burl!codas!killer!wnp
From: wnp@killer.UUCP (Wolf Paul)
Newsgroups: comp.sys.ibm.pc
Subject: Re: day of week
Message-ID: <3170@killer.UUCP>
Date: 1 Feb 88 04:19:51 GMT
References: <141900013@occrsh.ATT.COM>
Reply-To: wnp@killer.UUCP (Wolf Paul)
Organization: The Unix(R) Connection BBS, Dallas, Tx
Lines: 41

In article <141900013@occrsh.ATT.COM> fubar@squid.UUCP writes:
]
]From squid!fubar Thu Jan 28 04:22 CDT 1988 remote from occrsh
]Subject: day of week
]
]***
]*** PLEASE send replies to ihnp4!occrsh!squid!david
]***
]
]>(Square brackets denote the greatest integer function.)
]So:
]Thursday, Jan 28, 1988, should yield 4:
] N = 28   (number of day within month)
] M = 11   (Jan = 11)
] C = 19   (Hundreds w/in year)
] Y = 88   (rest of year??)
] L = 1    (1988 is leap year)
] = (N  +  [2.6M - 0.2]  +  Y +  [Y/4] +  [C/4] -   2C   -  (1+L)[M/11])  mod 7
] = (28 + [(2.6*11)-0.2] + 88 + [88/4] + [19/4] - (2*19) - (1+1)*[11/11]) mod 7
] = (28 +     [28.4]     + 88 +  [22]  + [4.75] -   38   -      2)        mod 7
] = (28 +       29       + 88 +   22   +   5    -   38   -      2)        mod 7
] = 132 mod 7
] = 6 ??
]
]What is the correct formula for this?

Seems to me the formula is ok, but your calculations are not. The greatest
integer in 28.4 is 28, not 29; in 4.75 it is 4, not 5.

Thus, the last few lines of your calculation above should read:

  = (28 +       28       + 88 +   22   +   4    -   38   -      2)        mod 7
  = 130 mod 7
  = 4

"greatest integer" is not the same as rounding up to the next highest integer
:-) ...


Wolf Paul
ihnp4!killer!wnp