Function can_cast_datetime64_units

numpy/core/src/multiarray/datetime.c:1297–1334

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

 * This provides the casting rules for the DATETIME data type units.
 */
NPY_NO_EXPORT npy_bool
can_cast_datetime64_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;

        /*
         * Can cast between all units 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 1;
            }

        /*
         * 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);
            }

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

/*
 * This provides the casting rules for the TIMEDELTA data type units.