Function days_to_yearsdays

python/pyarrow/src/arrow/python/datetime.cc:159–193

Modifies '*days_' to be the day offset within the year, and returns the year.

// Modifies '*days_' to be the day offset within the year,
// and returns the year.
static int64_t days_to_yearsdays(int64_t* days_) {
  const int64_t days_per_400years = (400 * 365 + 100 - 4 + 1);
  // Adjust so it's relative to the year 2000 (divisible by 400)
  int64_t days = (*days_) - (365 * 30 + 7);
  int64_t year;

  // Break down the 400 year cycle to get the year and day within the year
  if (days >= 0) {
    year = 400 * (days / days_per_400years);
    days = days % days_per_400years;
  } else {
    year = 400 * ((days - (days_per_400years - 1)) / days_per_400years);
    days = days % days_per_400years;
    if (days < 0) {
      days += days_per_400years;
    }
  }

  // Work out the year/day within the 400 year cycle
  if (days >= 366) {
    year += 100 * ((days - 1) / (100 * 365 + 25 - 1));
    days = (days - 1) % (100 * 365 + 25 - 1);
    if (days >= 365) {
      year += 4 * ((days + 1) / (4 * 365 + 1));
      days = (days + 1) % (4 * 365 + 1);
      if (days >= 366) {
        year += (days - 1) / 365;
        days = (days - 1) % 365;
      }
    }
  }

  *days_ = days;
  return year + 2000;
}

// Extracts the month and year and day number from a number of days
static void get_date_from_days(int64_t days, int64_t* date_year, int64_t* date_month,