time ¶POSIX specification:
https://pubs.opengroup.org/onlinepubs/9799919799/functions/time.html
Portability problems fixed by Gnulib:
gettimeofday and timespec_get
on some platforms:
glibc 2.31 or newer on Linux,
FreeBSD 14.0,
AIX 7.2, native Windows.
Namely, in the first 1 to 2.5 milliseconds of every second (or, on AIX and
Windows, in the first 5 milliseconds of every second), time returns
a value that is one less than the tv_sec part of the return value of
gettimeofday or timespec_get.
Portability problems not fixed by Gnulib: