Function days_to_yearsdays

numpy/core/src/multiarray/datetime.c:196–223

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

 * and returns the year.
 */
static npy_int64
days_to_yearsdays(npy_int64 *days_)
{
    const npy_int64 days_per_400years = (400*365 + 100 - 4 + 1);
    /* Adjust so it's relative to the year 2000 (divisible by 400) */
    npy_int64 days = (*days_) - (365*30 + 7);
    npy_int64 year;

    /* Break down the 400 year cycle to get the year and day within the year */
    year = 400 * extract_unit_64(&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 number from a 'datetime64[D]' value */
NPY_NO_EXPORT int