/* rexx */                                                                      
/*-------------------------------------------------------------------*/         
/*                                                                   */         
/*                        REXX Exec : ZELLER                         */         
/*                                                                   */         
/*  Description : Calculate the day of the week from a date          */         
/*                                                                   */         
/*  Created on : 12 Dec 2010                                         */         
/*  Created by : KEVIN FERGUSON                                      */         
/*             : Userid(MIT001)                                      */         
/*             : using ABBYDALE.DEVL.REXX(ZELLER)                    */         
/*                                                                   */         
/*  Called by :                                                      */         
/*                                                                   */         
/*  Calls : CLEAR, Proper                                         JUL*/         
/*                                                                   */         
/*  Change Activity :                                                */         
/*                                                                   */         
/*  MM/DD/YYYY  ID   Comment                                         */         
/*  ---------------------------------------------------------------  */         
/*  05/29/2021  ALG  Changed algorithm to correctly account for      */         
/*                   January and February dates.                     */         
/*  05/29/2021  TRC  Show calculation if required.                   */         
/*  01/11/2022  JUL  Allow for the correct date format for dates     */         
/*                   prior to 24th March 1752                        */         
/*                                                                   */         
/*                ©Copyright of Abbydale Systems LLC.                */         
/*                                                                   */         
/*-------------------------------------------------------------------*/         
day.0 = Saturday                                                                
day.1 = Sunday                                                                  
day.2 = Monday                                                                  
day.3 = Tuesday                                                                 
day.4 = Wednesday                                                               
day.5 = Thursday                                                                
day.6 = Friday                                                                  
mon.1 = January                                                                 
mon.2 = February                                                                
mon.3 = March                                                                   
mon.4 = April                                                                   
mon.5 = May                                                                     
mon.6 = June                                                                    
mon.7 = July                                                                    
mon.8 = August                                                                  
mon.9 = September                                                               
mon.10 = October                                                                
mon.11 = November                                                               
mon.12 = December                                                               
dd = 99                    /* set dummy day     */                              
mm = 99                    /* set dummy month   */                              
cal = "Gregorian"                                                /*JUL*/        
                                                                                
clear /* <-- If you don't have a clearscreen program remove this   JUL*/        
                                                                                
Say 'Do you want to see the calculation? (Reply y for trace)'    /*TRC*/        
Pull trc                                                         /*TRC*/        
do until dd < 31                                                                
   Say Day?                /* Ask for day       */                              
   Pull dd                                                                      
end                                                                             
do until mm < 13                                                                
   Say Month "(1 - 12)"    /* Ask for month     */                              
   Pull mm                                                                      
end                                                                             
Say Year (yyyy)            /* Ask for year      */                              
Pull yyyy                                                                       
monmon = mm                                                                     
entyyyy = yyyy                                                   /*ALG*/        
If yyyy < 1752 then do                                           /*JUL*/        
   cal = "Julian"                                                /*JUL*/        
end                                                              /*JUL*/        
If yyyy = 1752 then do                                           /*JUL*/        
   if mm <= 3 then do                                            /*JUL*/        
      if dd <= 24 then do                                        /*JUL*/        
          cal = "Julian"                                         /*JUL*/        
      end                                                        /*JUL*/        
   end                                                           /*JUL*/        
end                                                              /*JUL*/        
if mm < 3 then  do                                                              
   if trc = "Y" then do                                                         
      say  'Month is 'mon.monmon ,                               /*TRC*/        
      ' so subtract 1 from 'yyyy 'giving' yyyy-1                 /*TRC*/        
      say ''                                                     /*TRC*/        
      say 'Also add 12 to the month ('mm') giving' mm+12         /*TRC*/        
   end                                                                          
   mm = mm + 12                                                                 
   yyyy = yyyy - 1                                                              
end                                                                             
temp1 = ((mm+1)*13)%5                                            /*ALG*/        
temp2 = yyyy%4                                                   /*ALG*/        
temp3 = yyyy%100                                                 /*ALG*/        
temp4 = yyyy%400                                                 /*ALG*/        
if trc = "Y" then do                                             /*TRC*/        
   say '*** Calculate the complicated bits! ***'                 /*TRC*/        
   say ''                                                        /*TRC*/        
   say 'This date will use the' cal 'calendar calculation'       /*JUL*/        
   say ''                                                        /*JUL*/        
                                                                                
   say 'The INTeger value of (('mm'+1)x13)/5) =' temp1,          /*TRC*/        
       'Name it V1'                                              /*TRC*/        
   say '                       mm + 1 =' mm + 1                  /*TRC*/        
   say '                   (mm + 1)13 =' (mm + 1)*13             /*TRC*/        
   say '   Int value of (mm + 1)*13/5 =' temp1 '   so V1='temp1  /*TRC*/        
   say ''                                                        /*TRC*/        
   say 'The INTeger value of year/4 =' temp2 'Name it V2'        /*TRC*/        
   say '          Int value of 'yyyy'/4 =' temp2 '   so V2='temp2/*TRC*/        
   say ''                                                        /*TRC*/        
   if cal = "Gregorian" then do                                  /*JUL*/        
   say 'The INTeger value of year/100 =' temp3 'Name it V3'      /*TRC*/        
   say '        INT value of year/100 =' temp3 'so V3='temp3     /*TRC*/        
   say ''                                                        /*TRC*/        
   say 'The INTeger value of year/400 =' temp4 'Name it V4'      /*TRC*/        
   say '        INT value of year/400 =' temp4 'so V4='temp4     /*TRC*/        
   end                                                           /*JUL*/        
   v1 = temp1                                                    /*TRC*/        
   v2 = temp2                                                    /*TRC*/        
   v3 = temp3                                                    /*TRC*/        
   v4 = temp4                                                    /*TRC*/        
   say ''                                                        /*TRC*/        
   say '*** End of complicated bits! ***'                        /*TRC*/        
   say ''                                                        /*TRC*/        
   If cal = "Gregorian" then do                                  /*JUL*/        
   say 'Add day + V1 + Year + V2 - V3 + V4 =' temp1,             /*TRC*/        
   'Name it TOT1'                                                /*TRC*/        
   say '   'dd' +' v1 '+' yyyy '+' V2 '-' V3 '+' V4 '=' temp1    /*TRC*/        
   say ''                                                        /*TRC*/        
   end                                                           /*JUL*/        
   else do                                                       /*JUL*/        
   say 'Add day + V1 + Year + V2 + 5 =' temp1,                   /*JUL*/        
   'Name it TOT1'                                                /*JUL*/        
   say '   'dd' +' v1 '+' yyyy '+' V2 - 5 '=' temp1              /*JUL*/        
   say ''                                                        /*JUL*/        
   end                                                           /*JUL*/        
   TOT1 = temp1                                                  /*TRC*/        
end                                                              /*TRC*/        
If cal = "Gregorian" then do                                     /*JUL*/        
   temp1 = dd+temp1+yyyy+temp2-temp3+temp4                       /*ALG*/        
end                                                              /*JUL*/        
else do                                                          /*JUL*/        
   temp1 = dd+temp1+yyyy+temp2 + 5                               /*JUL*/        
end                                                              /*JUL*/        
/*                                                                              
if yyyy <=  1752 then do                                                        
  if trc = "Y" then do                                           /*TRC*/        
  Say 'If the date is before 24th March 1752 then add 11 to TOT1'/*TRC*/        
   say ''                                                        /*TRC*/        
end                                                              /*TRC*/        
  temp1 = temp1 + 11                                                            
end                                                                             
if yyyy =  1752 then do                                                         
  if mm <= 3 then do                                                            
     if mm = 3 then do                                                          
        if dd <= 24 then do                                                     
           temp1 = temp1 - 11                                                   
        end                                                                     
     end                                                                        
     temp1 = temp1 + 11                                                         
  end                                                                           
end                                          */                                 
temp4 = temp1%7                                                                 
temp2 = temp4*7                                                                 
temp3 = temp1-temp2                                                             
if trc = "Y" then do                                             /*TRC*/        
   say 'Calculate the remainder (MOD) of' TOT1,                  /*TRC*/        
   'divided by 7 =' temp3                                        /*TRC*/        
   say ''                                                        /*TRC*/        
   say 'The remaining number corresponds to the day of the week' /*TRC*/        
   say ''                                                        /*TRC*/        
   say 'In this case the remaining number is' temp3              /*TRC*/        
   say ''                                                        /*TRC*/        
   say 'Which corresponds to a 'Proper(day.temp3)                /*TRC*/        
   say ''                                                        /*TRC*/        
end                                                              /*TRC*/        
say Proper(day.temp3) dd Proper(mon.monmon) entyyyy              /*ALG*/        
exit                                                                            
/*-------------------------------------------------------------------*/         
/*                           End of ZELLER                           */         
/*-------------------------------------------------------------------*/