Find prime digits in range

#include <iostream>

/* run this program using the console pauser or add your own getch, system("pause") or input loop */

#include <time.h>

#include <vector>
#include <cmath>

using namespace std;

bool is_prime(long long num)
{
    for (long long i = 2; i<sqrt(num); i++) {
        if (num % i == 0) {
            // вывести, что n  не простое, так как делится на i
            return false;
        }
    }

    return true;
}


int main(int argc, char** argv) {

    std::vector<long long> numbers;

    long long start_digit = 979999999797;

    time_t start, end;

    time(&start);

    for(long long i=start_digit;i<start_digit+200;i++){

        if(is_prime(i)){
            numbers.push_back(i);
        }
    }

    time(&end);
    double seconds = difftime(end, start);


    for(long long n : numbers)  cout << n << endl;
    printf("The time: %f seconds\n", seconds);

    return 0;
}