// Program 7
// This program computes the number of days between two dates.

#include <iostream>

using namespace std;

int DayCalculation (int month, int day, int year, int month2, int day2, int year2);

int main()
{
      int month;
      int secondMonth;
      int year;
      int day;
      int secondYear;
      int secondDay;
      
      cout << "Enter first dates month ";
      cin >> month;
      
      cout << "Enter second dates month  ";
      cin >> secondMonth;
      
      cout << "Enter the first dates day    ";
      cin >> day;
      
      cout << "Enter the second dates day    ";
      cin >> secondDay;
      
      cout << "Enter the first dates year    ";
      cin >> year;
      
      cout << "Enter the second dates year     ";
      cin >> secondYear;
      
      if (month >= 10 && day >= 15 && year >= 1582)
         
          cout << "The date you entered is invalid";
          
          else 
               DayCalculation ( month, day, year, secondMonth, secondDay,secondYear);
               
         return 0; 
    }
    
    int DayCalculation ( int month, int day, int year, int month2, int day2, int year2)
    
    {
         int result1 = 0;
         int result2;
         int result3;
         
         long JulianDayNumber;
         
         if (month == 1 || month == 2 )
         
         { 
           year--;
           month = month +12;
           }
           
     result1 = 2- year/100+year / 400; // test each one to make sure the right number.
     result2 = int(365.25 * year);
     result3 = int(30.6001 * (month + 1));
     
     JulianDayNumber = result1 + result2 + result3 + day + 1720994.5;
     
     cout << JulianDayNumber << endl;
     
     return 0;
     
     }

gcc -o dates dates.cpp
dates.cpp: In function `int DayCalculation(int, int, int, int, int, int)':
dates.cpp:67: warning: converting to `long int' from `double'
/tmp/ccLb60he.o(.text+0x1b): In function `main':
: undefined reference to `std::cout'
/tmp/ccLb60he.o(.text+0x20): In function `main':
: undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*)'
/tmp/ccLb60he.o(.text+0x2e): In function `main':
: undefined reference to `std::cin'
/tmp/ccLb60he.o(.text+0x33): In function `main':
: undefined reference to `std::basic_istream<char, std::char_traits<char> >::operator>>(int&)'
/tmp/ccLb60he.o(.text+0x42): In function `main':
: undefined reference to `std::cout'
/tmp/ccLb60he.o(.text+0x47): In function `main':
: undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*)'
/tmp/ccLb60he.o(.text+0x55): In function `main':
: undefined reference to `std::cin'
/tmp/ccLb60he.o(.text+0x5a): In function `main':
: undefined reference to `std::basic_istream<char, std::char_traits<char> >::operator>>(int&)'
/tmp/ccLb60he.o(.text+0x69): In function `main':
: undefined reference to `std::cout'
/tmp/ccLb60he.o(.text+0x6e): In function `main':
: undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, 

......

Compilation exited abnormally with code 1 at Wed Dec  8 18:28:49