Function get_datetime_conversion_factor

numpy/core/src/multiarray/datetime.c:1095–1198

* Computes the conversion factor to convert data with 'src_meta' metadata * into data with 'dst_meta' metadata. * * If overflow occurs, both out_num and out_denom are set to 0, but * no error is set. */

 * no error is set.
 */
NPY_NO_EXPORT void
get_datetime_conversion_factor(PyArray_DatetimeMetaData *src_meta,
                                PyArray_DatetimeMetaData *dst_meta,
                                npy_int64 *out_num, npy_int64 *out_denom)
{
    int src_base, dst_base, swapped;
    npy_uint64 num = 1, denom = 1, tmp, gcd;

    /* Generic units change to the destination with no conversion factor */
    if (src_meta->base == NPY_FR_GENERIC) {
        *out_num = 1;
        *out_denom = 1;
        return;
    }
    /*
     * Converting to a generic unit from something other than a generic
     * unit is an error.
     */
    else if (dst_meta->base == NPY_FR_GENERIC) {
        PyErr_SetString(PyExc_ValueError,
                    "Cannot convert from specific units to generic "
                    "units in NumPy datetimes or timedeltas");
        *out_num = 0;
        *out_denom = 0;
        return;
    }

    if (src_meta->base <= dst_meta->base) {
        src_base = src_meta->base;
        dst_base = dst_meta->base;
        swapped = 0;
    }
    else {
        src_base = dst_meta->base;
        dst_base = src_meta->base;
        swapped = 1;
    }

    if (src_base != dst_base) {
        /*
         * Conversions between years/months and other units use
         * the factor averaged over the 400 year leap year cycle.
         */
        if (src_base == NPY_FR_Y) {
            if (dst_base == NPY_FR_M) {
                num *= 12;
            }
            else if (dst_base == NPY_FR_W) {
                num *= (97 + 400*365);
                denom *= 400*7;
            }
            else {
                /* Year -> Day */
                num *= (97 + 400*365);
                denom *= 400;
                /* Day -> dst_base */
                num *= get_datetime_units_factor(NPY_FR_D, dst_base);
            }