Julian calendar with wrong date

I made the following program based on one of my scripts that does other calculations but the same is returning wrong dates for the Julian calendar, because March 14, 1964 falls on Friday, and 29/03/1987 falls on Saturday and 29/02/2000 falls on a second, but the same only returns days as Tuesday regardless of the date I type what is missing I do not calculate?

#include <iostream>
//Julian Date
//https://calendar.zoznam.sk/julian_calendar-sp.php?ly=1987#March
int GetDates(int day, int month, int year); // Function prototype.
int JulianDateNum(int day,int jdn,long intRes1,long intRes2,long intRes3);

int main()
{
 int month; //Declare variables
 int day;
 int year;
 int jdn=0;
 int dayOfWeek=0;

   do{
     std::cout << "\n\tPlease enter the day (dd): "; // Gets day input from user.
      std::cin >> day;

     if(day < 01 || day > 31)
     std::cout << "\n\tInvalid data! Please enter a day "
               << "less from 1-31: "; // Checks for invalid input.
    }while(day < 01 || day > 31);

  do{
     std::cout << "\n\tPlease enter the month (mm): "; // Gets month input from user.
      std::cin >> month;

      if (month < 01 || month > 12)
       std::cout << "\n\tInvalid data! Please enter a month "
                 << "from 01-12: "; // Checks for invalid input.
    }while(month < 01 || month > 12);

  std::cout << "\n\tPlease enter the year (yyyy): "; // Gets year input from user.
   std::cin >> year;

   dayOfWeek=((jdn + 1)%7); // calculates day of the week

   switch(dayOfWeek)
   {
    case 0:
    std::cout << "\n\tMonday";
    break;

    case 1:
    std::cout << "\n\tTuesday";    
    break;

    case 2:
    std::cout << "\n\tWednesday";
    break;

    case 3:
    std::cout << "\n\tThursday";
    break;

    case 4:
    std::cout << "\n\tFriday";
    break;

    case 5:
    std::cout << "\n\tSaturday";
    break;

    case 6:
    std::cout << "\n\tSunday";
    break;

    default:
    std::cout<<"\n\tDia invalido ..!!!\n\n";
   }

   std::cout << "\n\tThe Day of the week for " << day << "/" // Outputs day of week results.
             << month << "/" << year << " is: " << dayOfWeek
             << "\n\n";
  return 0; // Terminates program.
}

int GetDates(int day, int month, int year) // Function that calculates the julian date.
{
 long intRes1((2 - year / 100) + (year / 400)); // Calculation formula.
 long intRes2((int)(365.25 * year));
 long intRes3((int)(30.6001 * (month + 1)));

 int jdn(intRes1 + intRes2 + intRes3 + day + 1720994.5);

 return jdn;
}