Function get_days_from_date

python/pyarrow/src/arrow/python/datetime.cc:104–155

Calculates the days offset from the 1970 epoch.

// Calculates the days offset from the 1970 epoch.
static int64_t get_days_from_date(int64_t date_year, int64_t date_month,
                                  int64_t date_day) {
  int64_t i, month;
  int64_t year, days = 0;
  int64_t* month_lengths;

  year = date_year - 1970;
  days = year * 365;

  // Adjust for leap years
  if (days >= 0) {
    // 1968 is the closest leap year before 1970.
    // Exclude the current year, so add 1.
    year += 1;
    // Add one day for each 4 years
    days += year / 4;
    // 1900 is the closest previous year divisible by 100
    year += 68;
    // Subtract one day for each 100 years
    days -= year / 100;
    // 1600 is the closest previous year divisible by 400
    year += 300;
    // Add one day for each 400 years
    days += year / 400;
  } else {
    // 1972 is the closest later year after 1970.
    // Include the current year, so subtract 2.
    year -= 2;
    // Subtract one day for each 4 years
    days += year / 4;
    // 2000 is the closest later year divisible by 100
    year -= 28;
    // Add one day for each 100 years
    days -= year / 100;
    // 2000 is also the closest later year divisible by 400
    // Subtract one day for each 400 years
    days += year / 400;
  }

  month_lengths = _days_per_month_table[is_leapyear(date_year)];
  month = date_month - 1;

  // Add the months
  for (i = 0; i < month; ++i) {
    days += month_lengths[i];
  }

  // Add the days
  days += date_day - 1;

  return days;
}

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