Tính thứ ngày tháng

{Chương trình tính thứ ngày tháng}
program Hello;
uses crt;
const day = 1;
const month = 2;
const year = 3;

const monday = 0;
const tuesday = 1;
const wednesday=2 ;
const thursday = 3;
const friday =4 ;
const saturday = 5;
const sunday = 6;

type date = array[1..3] of Integer;
type month_table = array[1..12] of Byte;
var today : date;
var input_date : date;
var counter_date : date;
var day_of_week : Integer;
const days_of_months : month_table = (31, 28, 31, 30, 31, 30 , 31, 31, 30, 31, 30,31);





procedure date_to_string(_date : date);
begin
  Writeln(_date[day] ,'/' , _date[month] , '/' , _date[year]);
end;
function is_equal(date1 ,date2 : date) : boolean;
var condition1, condition2, condition3 : boolean;
begin
     condition1   := date1[day]   = date2[day];
     condition2   := date1[month] = date2[month];
     condition3   := date1[year]  = date2[year];
     is_equal     := condition1 and condition2 and condition3;
end;
function increase(date : date) : date ;
begin
  // leap year
  if((date[year] mod 4 = 0) and (date[year] mod 100 <> 0)) then
     days_of_months[2] := 29
    else
     days_of_months[2] := 28;

  date[day] := date[day] + 1;
  if(date[day] > days_of_months[date[month]]) then begin
    date[month] := date[month] + 1;
    // first day of next month
    date[day]   := 1;
  end;
  if(date[month] > 12) then begin
    date[year] := date[year] + 1;
    // first month of a year;
    date[month]:= 1;
  end;
  increase := date;
end;
begin
    Clrscr;
    {16/9/2019}
    today[day] := 17; today[month] := 9; today[year] :=2019; day_of_week := tuesday;
    // Read Input
    Writeln('Ban hay nhap ngay = '); read(input_date[day]);
    Writeln('Ban hay nhap thang ='); Read(input_date[month]);
    Writeln('Ban hay nhap nam =');   Read(input_date[year]);
    counter_date := today;
    // while counter <= input;
    Readln;
    repeat
        counter_date := increase(counter_date);
        date_to_string(counter_date);
        day_of_week := day_of_week + 1;
    until is_equal(counter_date , input_date);
    day_of_week := day_of_week mod 7;
    case day_of_week of
      monday   :  WriteLn('Thu 2');
      tuesday  :  WriteLn('Thu 3');
      wednesday:  WriteLn('Thu 4');
      thursday :  WriteLn('Thu 5');
      friday   :  WriteLn('Thu 6');
      saturday :  WriteLn('Thu 7');
      sunday   :  WriteLn('Chu nhat');
    end;
    Readln;
end.