#include <stdexcept>

// Gregorian Calendar started on October 15, 1582

const long gregorianStart = (31L * (10 + 12L * 1582) + 15);

long greg2jul(
  int day,
  int month,
  int year
) {
  // From "Numerical Recipes in C", adapted

  if (year == 0) throw std::domain_error("There is no year 0");

  // There is no calendar year 0.  As a consequence, e.g., the year 10
  // BC of the calendar is the year -9 of the Julian system.  The
  // formal parameter "year" of this procedure is the *calendar* year,
  // positive (AD) or negative (BC).

  long jy = (year < 0 ? year + 1 : year);
  long jm;

  // Months before March are considered the last months of the
  // previous year

  if (month > 2) jm = month + 1;
  else {
    --jy;
    jm = month + 13;
  }

  long result = day + 1720995L;
  result += static_cast<long>(365.25 * jy);
  result += static_cast<long>(30.6001 * jm);

  if ((31L * (month + 12L * year) + day) >= gregorianStart) {
    long cent = static_cast<long>(0.01 * jy);
    result += 2 - cent;
    result += static_cast<long>(0.25 * cent);
  }

  return result;
}