Function can_cast_timedelta64_units

numpy/core/src/multiarray/datetime.c:1342–1384

* This provides the casting rules for the TIMEDELTA data type units. * * Notably, there is a barrier between the nonlinear years and * months units, and all the other units. */

 * months units, and all the other units.
 */
NPY_NO_EXPORT npy_bool
can_cast_timedelta64_units(NPY_DATETIMEUNIT src_unit,
                          NPY_DATETIMEUNIT dst_unit,
                          NPY_CASTING casting)
{
    switch (casting) {
        /* Allow anything with unsafe casting */
        case NPY_UNSAFE_CASTING:
            return 1;

        /*
         * Only enforce the 'date units' vs 'time units' barrier with
         * 'same_kind' casting.
         */
        case NPY_SAME_KIND_CASTING:
            if (src_unit == NPY_FR_GENERIC || dst_unit == NPY_FR_GENERIC) {
                return src_unit == NPY_FR_GENERIC;
            }
            else {
                return (src_unit <= NPY_FR_M && dst_unit <= NPY_FR_M) ||
                       (src_unit > NPY_FR_M && dst_unit > NPY_FR_M);
            }

        /*
         * Enforce the 'date units' vs 'time units' barrier and that
         * casting is only allowed towards more precise units with
         * 'safe' casting.
         */
        case NPY_SAFE_CASTING:
            if (src_unit == NPY_FR_GENERIC || dst_unit == NPY_FR_GENERIC) {
                return src_unit == NPY_FR_GENERIC;
            }
            else {
                return (src_unit <= dst_unit) &&
                       ((src_unit <= NPY_FR_M && dst_unit <= NPY_FR_M) ||
                        (src_unit > NPY_FR_M && dst_unit > NPY_FR_M));
            }

        /* Enforce equality with 'no' or 'equiv' casting */
        default:
            return src_unit == dst_unit;
    }
}

/*
 * This provides the casting rules for the DATETIME data type metadata.