Comment by shawndumas

#include <iostream>

int main() { long long d; std::cout << "Enter a Julian Day Number: "; std::cin >> d;

    if (d < 0) {
        std::cerr << "Error: Invalid Julian Day Number (must be non-negative)." << std::endl;
        return 1; // Indicate an error to the system
    }

    const int DAYS_PER_YEAR = 365;
    const int DAYS_PER_LEAP_YEAR = 366;
    const int DAYS_PER_LEAP_CYCLE = 1461; // 4 years
    const int JULIAN_TO_GREGORIAN_THRESHOLD = 2299161; // Oct 15, 1582

    // Adjust for Julian-to-Gregorian transition
    if (d >= JULIAN_TO_GREGORIAN_THRESHOLD) {
        d += 10; // Account for dropped days
    }

    int a = d + 32044;
    int b = (4 * a + 3) / DAYS_PER_LEAP_CYCLE;
    int c = (4 * a + 3) % DAYS_PER_LEAP_CYCLE;

    int y = (b / 1460) + 1970; 
    d = (c / 4) - 365;

    if (d < 0) {
        y--;
        d += DAYS_PER_YEAR + (y % 4 == 0);
    }

    std::cout << "Gregorian Year: " << y << std::endl;
    std::cout << "Day of Year: " << d + 1 << std::endl; // Add 1 as days are typically 1-indexed

    return 0;
}