One of the AIX animals gave a strange result here: https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=hornet&dt=2023-10-15%2011%3A40%3A01
If you ignore the diffs due to change in column width, the interesting change seems to be: - 23:59:00-07 | 06:59:00+00 | 06:59:00+00 | 06:59:00+00 - 23:59:59.99-07 | 06:59:59.99+00 | 06:59:59.99+00 | 06:59:59.99+00 + 23:59:00-07 | 4294966103:4294967295:00+00 | 4294966103:4294967295:00+00 | 4294966103:4294967295:00+00 + 23:59:59.99-07 | 4294966103:00:00.01+00 | 4294966103:00:00.01+00 | 4294966103:00:00.01+00 But the other AIX animal 'sungazer' was OK with it. They're both on the same AIX7.1 host IIRC, both 64 bit builds, but the former is using xlc and the latter gcc. I don't immediately see what would cause that underflow on that old compiler but not elsewhere. I have a shell there (cfarm111) if someone has an idea...
