--progress-timestamp is supposed to make -P report a Unix Epoch time stamp,
for easy correlation with the entries in other log files (like the postgres
server log file using %n).
But that broke in this commit:
commit 1d63f7d2d180c8708bc12710254eb7b45823440f
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Date: Mon Jan 2 13:41:51 2017 -0500
Use clock_gettime(), if available, in instr_time measurements.
The commit before that one changed pgbench to make it tolerate the change
in clock, but it overlooked --progress-timestamp.
Cheers,
Jeff