This can't happen.  Although ANSI X3.159-1989 Section 3.3.5 neglects to say it explicitly, a % b is always (for integral a and non-zero integral b) strictly greater than -abs(b) and strictly less than abs(b), as may be inferred from what 3.3.5 *does* say about division and how it relates to remainder.  So you can `assert(bwlimit_next.tv_nsec < u_10e9)` if you want.