Hi:

It turns out that the Wikipedia web page:  
http://en.wikipedia.org/wiki/Zeller's_congruence
has a typo in the equation.  It shows the Month term to be 26*(M+1)/10 
which is equivalent to 13*(M+1)/5 <- WRONG!

The web pages about Zeller at:  
http://www.merlyn.demon.co.uk/zeller-c.htm#OIW
Under the title "Other Implementations for Day-of-Week" gives a concise 
explanation of Zeller's method.  Note the month term is:  (13*M-1)/5.
The problem is with the parenthesis.  Interestingly changing the -1 to 
+1 makes no difference.   (Zeller months are numbered 1 to 12 with 
March=1 Feb =12, where Jan & Feb are in the prior year.  The Zeller year 
is a 2 digit year.)
The month term can be put into a table with entries of: nop, 2, 5, 7, 
10, 12, 15, 18, 20, 23, 25, 28, 31
and that reduces to nop, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4, 0, 3.  I added 1 
to all of those to get Saturday =0.
It's now working for Feb 28 and the following day in years 1992, 2000, 
2004, 2006, 2100.

Have Fun,

Brooke Clarke

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