ps etime lxc fedora

hi,all in the fedora lxc, the etime no value. root@fedora-22:~# ps ax -o pid,etime,command PID ELAPSED COMMAND 1 00:00 /sbin/init 149 00:00 /usr/lib/systemd/systemd-journald 201 00:00 /usr/lib/systemd/systemd-logind 233 00:00...