[zeller]zeller.algorithms

Let A, B, C, D denote  integer variables that have the  following values:

A  =  the  month  of  the  year,  with  March  having  the  value  1,  April  the

value  2,  .  .  .,  December  the  value  10,  and  January  and  February  being

counted  as  months  11  and  12  of  the  preceding  year  (in  which

case,subtract  1  from  C)

B  =  the  day  of  the  month  (1,  2,  3,  .  .  .  ,  30,  31)

C  =  the  year  of  the  century  (e.g.  C  =  89  for  the  year  1989)

D  =  the  century  (e.g.  D  =  19  for  the  year  1989)

Note:  if the month  is January or February, then the preceding year  is used for computation.  This is because there was a period in history when March 1st, not January 1st, was the beginning of the year. 

Let W, X, Y, Z, R also denote  integer variables.  Compute their values  in the following order using 

integer arithmetic: 

W  =  (13*A  - 1)  /  5 

X = C / 4 

Y = D / 4 

Z  =  W  +  X  +  Y  +  B  +  C  - 2*D 

R  =  the  remainder  when  Z  is  divided  by  7 

The R is what we want to know