If you are in a systemd machine, you can use a monotonic timer
The timer unit /etc/systemd/system/shutdown_after_an_hour.timer
[Unit]
Description=shutdown after an hour
[Timer]
OnBootSec=1h
[Install]
WantedBy=timers.target
The timer unit /etc/systemd/system/shutdown_after_an_hour.service:
[Unit]
Description=shutdown after an hour
[Service]
ExecStart=/sbin/poweroff --force --no-wall
Type=oneshot
It is enabled via
# systemctl enable shutdown_after_an_hour.timer
Its status (particularly how much time is left before the shutdown) is available via
# systemctl list-timers shutdown_after_an_hour.timer
It will work on the next reboot, it is not useful to systemctl start it during the session when it is created as it will either not work (because it was not triggered during the reboot) or shut down the machine right away if past one hour. I have never tested that as I think of it.