Excel's NetworkDays() function replacement written in PL/I

    *PROCESS MARGINS(1, 140);

     t1: proc options (main);
       dcl from char (8) date ('DDMMYYYY');
       dcl to   char (8) date ('DDMMYYYY');
       dcl holidays (7) char (8) date ('DDMMYYYY')
           init ('01012012', '06012012', '01052012', '06122012', '24122012', '25122012', '26122012');

       from = '01122012';
       to   = '31122012';

       put skip list ('Number of working days in december 2012: ' || trim(workdays(from, to, holidays)));


       (STRINGRANGE, SUBSCRIPTRANGE):
       workdays: proc (from, to, holiday_list) returns (fixed bin (31));
         dcl (from, to) char (8) date ('DDMMYYYY');
         dcl holiday_list (*) char (8) date ('DDMMYYYY') optional;

         dcl (days_from, days_to, day) fixed bin (31);
         dcl total_days fixed bin (31);
         dcl week_index fixed bin (31);
         dcl nworkdays fixed bin (31);
         dcl i fixed bin (31);
         dcl (result, holiday) bit (*) controlled;
         dcl weeks (7) bit (7) nonasgn init ('1000001'b, '0000011'b, '0000110'b, '0001100'b, '0011000'b, '0110000'b, '1100000'b);

         dcl (days, weekday, tally, substr, lbound, hbound, copy, omitted) builtin;


         days_from = days(from, 'DDMMYYYY');
         days_to   = days(to, 'DDMMYYYY');

         total_days = days_to - days_from + 1;
         if total_days <= 0 then return (0);

         allocate result bit (total_days);

         week_index = weekday(days_from);
         result = copy(weeks(week_index), total_days / 7) || substr(weeks(week_index), 1, mod(total_days, 7));

         if ^omitted(holiday_list) then
           do;
             allocate holiday bit (total_days);
             do i = lbound(holiday_list) to hbound(holiday_list);
               day = days(holiday_list(i), 'DDMMYYYY');
               if (day >= days_from) & (day <= days_to) then
                 do;
                   holiday = copy('0'b, day - days_from ) || '1'b || copy('0'b, days_to - day);
                   result = result | holiday;
                 end;
             end;
             free holiday;
           end;

         nworkdays = tally(result, '0'b);
         free result;

         return (nworkdays);

       end workdays;

     end t1;