This is part of a patch written by Bruno Haible:
https://lists.gnu.org/r/bug-gnulib/2019-12/msg00192.html
* lib/xtime.h (xtime_make): Remove attempt to prevent internal
integer overflow, as it didn’t suffice. This reverts the xtime.h
part of 2018-10-12T04:46:09Z!akim.demaille@gmail.com, which I
cannot now see the need for anyway (even in cases where it works),
as the patch is helpful only when the signs of S and NS disagree,
and all callers pass nonnegative values for S and NS.
Instead, add a comment saying args should be nonnegative.